Winter grays

…and not one of the fifty shades everyone was talking about.

It has now been three weeks in a row that we haven’t seen the sun here. According to the weather forecast, this isn’t going to change soon. I think the longest I can remember an overcast stretch here is two weeks, so three (with at least another on the way) is getting a bit dull (no pun intended). In late January I’d expect the days to just begin to lengthen, but in these conditions it’s hard to tell.

The garden has also put me on a new path, or rather, reintroduced me to an old one. After fighting with the hardpan clay under my property for years, I finally had a lightbulb moment last summer: make it work for me rather than against me. I played around with ceramics a lot as a child, and the clay under my backyard is actually pretty pure. I have more free time in winter (no garden), so I’ve been processing clay to make it workable. Hopefully when the weather warms up and I can be messy outside, I can build a few things for the garden, then take them somewhere to be fired. My very own terracotta pots and saucers, how nice that would be.

(Most potters will tell you that working with clay is quite therapeutic, but I suspect most modern potters don’t have to break their backs to dig it out of the ground, then thump it around either dry or in water so they can sieve the larger particles out by hand…)

I need to make friends with someone who has a stone grinder that I can use on all the shale in my garden as well. Shale is just compressed clay, and usually fairly soft as stone goes, so it would be wonderful to get rid of all the shale piled up in various corners that way. I could be wrong about this, but I seem to remember reading that a lot of clay used in the ceramics industry these days comes from pulverised shale.

Anyway, most of the clay I have is gray. As above, so below?

Winter houseplants 16: a few common diseases and Conclusion

Following from the previous post.

A few common houseplant diseases include:

Powdery mildew: this appears as a thin layer of powdery white fuzz on the leaves and stems, which later dry up and go brown. It likes to attack new leaves, which then grow distorted. Unusually for a fungus, this one actually takes hold when the conditions are warm and dry, but then spreads when things get more humid or moist.

Botrytis/grey mould: this looks just like the latter name says: a light, slightly fluffy mass of grey filaments growing on the surfaces of leaves, flowers, and stems. In addition to poor air circulation, it’s also encouraged by overfeeding and overwatering the plant.

Sooty mould: another one that’s what it says on the tin. It looks like patches, or a dusting, of tiny black specks. Unlike the others, this one doesn’t really affect the plant directly. In fact, it’s a symptom of something else: one of the pests I mentioned in the previous posts. Aphids, scale insects, and mealybugs suck sap from the plants and excrete a thick, sugary liquid called honeydew. This rains down on the plant parts below and creates a shiny, sticky buildup; this is what the sooty mould is growing and feeding on. Treat to get rid of the pests, then wipe off the sooty mould and honeydew with a damp cloth. Leaving it will only encourage more, and when it gets too thick it will inhibit photosynthesis. If the plant doesn’t have those pests, check the plants around it.

These three are among the most common and most cosmopolitan houseplant diseases. Individual plant species may be affected by their own particular ones; for example black spot on roses (which has happened on roses I’ve brought inside).

For people starting seedlings indoors, another fungal condition often rears its ugly head, called damping off. This is when a seedling suddenly develops a “soggy” weak spot on its stem, droops, and dies. There’s nothing you can do about it except prevent it by using clean materials, good air circulation, and perhaps a light sprinkling of sulphur or cinnamon.

Finally, I’ll talk about bacterial leaf spot, especially common on orchids. These look like little sunken chocolatey brown dots on the leaves that expand into larger spots or streaks, often with yellow haloes. I find spraying with hydrogen peroxide (from the pharmacy) helps greatly, but if it gets bad, then prune and burn.

I think that’s it for this series of posts. If it seems that I’ve repeated myself sometimes, well, I did. I’ll excuse that, however, by emphasising that any living thing is affected by the components of its environment as a whole, not as unrelated factors working in isolation. Moisture, soil, humidity, the container – these things all work in tandem to affect the plant’s health directly, and indirectly by influencing the plant’s resistance (or susceptibility) to disease and pests. Only light works independently of those factors, but it too is affected by (and affects) the plant’s surroundings.

Winter houseplants 15: dealing with disease

Following from the previous post.

Moving on to diseases. I’ll be blunt: plant diseases, especially indoors, can be difficult to control and even harder to eradicate. Prevention is definitely better than cure here.

Most common winter houseplant diseases are caused by fungi. Fungi are generally encouraged by still, moist air, so good circulation goes a long way toward keeping your plants healthy. As I said in a previous post about humidity, maintaining good humidity is a balance: too dry and the plant suffers; too moist and you risk fungi taking hold.

I don’t want to give the impression that fungi in general are bad. In fact the vast majority of them are beneficial or inconsequential. It’s just that we notice the bad ones more and take the good ones for granted. The same goes for bacteria: it’s not about how much are present so much as which ones. If you constantly use antibacterial products on everything in the house, you’re actually doing yourself a bad turn in the long run. (As a biologist, I have quite a long rant about this, but I’ve strayed from the topic at hand…)

You can improve air circulation by not crowding your plants together. I know (from experience) that if you have a lot of houseplants and limited space, the temptation is to jam as many into the window or under the grow-light as can fit. Well, you have to do what you have to do. Individual plants (depending on the general growth habit of the species) can be pruned to have a more open, airy shape, and this also benefits air circulation.

In addition to good air circulation, fungi can be controlled by spraying with baking soda and water or dusting with powdered sulphur. Baking soda, however, is a prophylactic: it helps prevent fungi from taking hold, but won’t do anything to cure the problem. Sulphur and garlic spray can both help to control fungi once they’ve shown up, but even these aren’t always reliable, especially if the problem is bad. I’ve also heard good things about cinnamon. However, spraying and dusting are not always practical in the house.

Often, the best way to get rid of a disease is to prune out the affected parts and burn them. Do not put the prunings into your compost bin/heap; if burning is not practical, then put them in your municipal green waste pickup and let the city deal with it. Such facilities can kill disease spores; backyard composters and worm bins usually don’t. The same goes for anything that was affected by disease and fell off the plant. With houseplants, I would recommend pruning and prevention rather than dusting or spraying.

Use clean, sharp tools to prune (standard advice), but make sure to sterilise the blade(s) after every cut, and I mean every cut. Dip the blades in rubbing alcohol and then pass them through a flame. A bleach solution works too, but it needs to soak for fifteen minutes to sterilise the blades, so…that’s kinda boring. Leave that for when you’re finished. Don’t just prune out the bits that look diseased; prune well into healthy growth to maximise the likelihood of removing affected tissue.


Winter houseplants 14: some common pests

Following from the previous post about dealing with houseplant pests.

Some common indoor pests include:

Aphids: little green, yellow, red, orange, or black critters that suck the sap out of the plant and cause wilting and internal drying of the plant. Female can reproduce parthenogenically (they clone themselves without any need of the males), so their numbers can increase ridiculously quickly. They like to cluster around the tenderer parts (new shoots, leaves, buds) and I find it immensely satisfying to run my finger and thumb up the stem to crush them (before spraying). Outdoors, ladybirds and hoverflies are the classic aphid predators, but these are not practical to use in a home; greenhouse keepers will probably find them far more useful. Spray with soap or garlic.

Whitefly: these look like tiny little white moths, smaller than a pinhead. In medium to bad infestations, you might see a cloud of them flying off if you disturb the plant. Little white flecks stuck to the undersides of the leaves are another symptom (I’m not sure what these actually are; I think they’re either dead whiteflies or possibly the remnants of moulting). Spray with soap or garlic every two or three days for at least two weeks; they lay eggs in the soil that will survive, so you have to get the adults as they complete their life cycle. You can also get sticky traps for whitefly.

Red spider mite: tiny red arachnids (mites are arachnids); they do spin webs that one could be forgiven for mistaking for those of real spiders. They are also capable of parthenogenesis. Symptoms include stippled yellowing leaves, a general sickly look to the plant, and the webs. Remove the webs, burn them, and spray the plant with soap. They generally take hold when humidity is very low, so increasing the humidity can help control future infestations.

Mealybug: these look like tiny greyish-whitish-pinkish pillbugs, often covered with white fluff and/or having two long tails (a particular species). If there are just a few, try swabbing them with rubbing alcohol (or crushing them…); otherwise spray with soap.

Scale insect: these appear as small scales or shells of various shapes and colours stuck to the stems and leaves. These are in fact mature females that live inside the scales, sucking the juices from the plant. If there are just a few, scrape them off gently with your fingernails and swab the area with rubbing alcohol. Less mature females have softer shells that can be penetrated by soap. Larger infestations might warrant pruning the affected parts off and burning them. You can also try spraying with rubbing alcohol first (test first to make sure it doesn’t damage the plant), then with soap, or a mixture of a spoonful of vegetable oil to a cup of water. For long-term control, soap solution will kill males and immature females, which are mobile and soft, but you’ll probably never see or notice them.

Fungus gnats: these aren’t too pestiferous really; they’re more of an unsightly nuisance than a real threat to healthy houseplants. They’re little black flying or walking insects that might be mistaken for fruit flies at first glance. The larvae feed on soil fungi and plant roots, but seldom to a harmful extent unless the infestation is really bad. Sometimes they can be a symptom of overwatering. They’re probably most dangerous to seedlings, because they can spread fungi on their feet – such as the sort that causes damping off. I’ve never found it necessary to control these by the usual methods; my carnivorous Pinguicula do a nice job of it. They’re also attracted to water, in which they have a tendency to drown. Oddly enough, once I left some leftover sesame seed oil in a saucer on the kitchen counter and the next day there were a bunch of fungus gnats drowned in it. Whether they were attracted to the oil itself, or the shiny surface, or something else, I couldn’t say. I really like sesame oil, so I don’t care to waste any by conducting tests.

Winter houseplants 13: dealing with pests

Following from the previous post.

Most indoor pests are insects or arachnids and can be controlled by the many insecticidal soaps on the market now. Actually, ordinary soapy water will do, but you must be sure to use soap and not household detergent. Soap (technically itself a type of detergent) and other detergents have different chemical activities and different effects. To confuse matters, sometimes soaps are marketed as detergent and vice versa. Dishwashing liquids, shampoos, some liquid hand soaps, and most modern laundry detergents/soaps are detergents. Bar soap is soap, and the old-fashioned washing crystals are soaps as well.

Soaps work by dissolving the waxy coating that insects secrete to prevent water loss through their exoskeletons, so they dehydrate. Detergents don’t do that. However, detergents are better surfactants, so a bit added to soapy water can help the liquid to coat the plant. I suppose a detergent solution might work by engulfing and suffocating the insect, but don’t depend on it.

If you make your own soap solution, beware of making it too strong or it can burn the plant. If you’re worried about this, test it first on a single leaf or small section of the plant (leave it on for a couple days to see if the leaf goes brown and/or shriveled). It should, however, be strong enough that the water feels soapy to the touch.

Do not use soap on ferns, cacti/succulents, or conifers.

Garlic spray, made by mashing garlic cloves and soaking them in oil for a few days, then mixing this with water (yes, I know oil and water don’t mix) and a little detergent, works very well on some pests, but not others. Test for damage to the plant first and dilute if necessary.

Spraying your plants indoors can be a messy job, so you might want to put the plant in the shower or bathtub before you start. Be sure to spray every part of the plant: under the leaves, in the cracks and joints of the stems, all the little crevices between the flowers – everything. Spray until the point of drenching; you want to see the stuff dripping off. You will probably need to do this several times at intervals of two or three days. It’s not a bad idea to wash the plant off the next day by spraying with clean water, but it isn’t usually necessary.

Another way depends on identifying the exact species of pest you have. If you can do this, you might be able to order special sticky traps. These traps are loaded with pheromones that attract the (adult) pests, which get stuck. Since these traps are usually hung on or around the plants, they are generally only available for pests that fly at some point (I could be wrong about this; haven’t looked into them for a while).

Winter houseplants 12: pests and diseases

You know, I was going to end this series with the last post before it occurred to me to address houseplant afflictions. Bet you’re sorry it did.

Anyway, pests and diseases – well, what’s the difference? Both are caused by other organisms attacking the plant. In practical terms, plant diseases are caused by microorganisms (bacteria, viruses, fungi) and pests are, well, pests – you can see them with the naked eye (assuming you have reasonable vision), at least at some stage in their life cycles. Some people might count nutritional deficiencies as diseases as well: scurvy in humans, for example, is a Vitamin C deficiency. For plants, that’s more of a problem with the soil and fertiliser, so I won’t go into it here.

I can’t possibly talk about every single disease or pest that could affect every single houseplant – I certainly don’t know about every single houseplant, much less everything that attacks them. Even if I did, that would fill an entire set of books.  I’ll just go into the ones that are most common on houseplants here. Much of it also applies in greenhouses.

Pests and diseases indoors can sometimes seem to be much more of a problem because of the isolated nature of the environment. Outdoors, many of these things are controlled to a manageable extent by natural predators or simply the living conditions; indoors, those factors are not present. Annoyingly, they are often brought into the house when you bring the plants inside for overwintering; they may not be apparent because they were kept under control by outdoor factors and then suddenly they boom into a problem. Some of them spend part of their life cycles in the soil and remain hidden there for a time.

As is usual, prevention is better than cure. When you start thinking of bringing the plants inside, take a good look at them for signs of anything wrong. If you find diseases, prune off the affected plant parts and if possible burn them. A good drench of insecticidal soap or solution is a good idea, but it must reach every single part of the stems and leaves and it won’t reach things hiding in the soil. A hard pruning (which is a good idea for a lot of plants at this point) can make this a lot easier. If a plant is due for repotting and a complete soil change, think about doing so now.

Prevention also depends on keeping plants healthy in other respects: plants that are weak, overwatered, underwatered, overfed, underfed, or otherwise not given conditions to their liking, are more susceptible to pests and diseases. Just like humans, really.

Even if you do all you can, don’t beat yourself up if later something bad does show up. Pretty much all of the common pests and diseases have stages that are very difficult to see or find, much less eradicate.

Once you do recognise something amiss, try to isolate the plant while you treat it so as to minimise the likelihood of the problem spreading. On the other hand, by the time you notice it, the problem probably already has spread…


Winter houseplants 11: temperature and fertiliser

I’ll conflate these two topics in one post because I really don’t have much to say about either in the context of winter houseplants. And I seem to prefer long posts…

I’ll go out on a limb and claim that most people probably keep their houses/apartments somewhere around 18 or 19 °C in winter, give or take a few degrees (I keep mine at 19°). For the vast majority of houseplants, even most tropicals, this is tolerable. Only a few will demand significantly higher temperatures just for survival. Even I wouldn’t suggest you should heat your home to the point it becomes uncomfortable or uneconomical for the sake of a few extra-fussy houseplants.

If you do keep such plants (and right now I can’t even think of any…at least, any that are legal around here…), you’ll probably have to fuss around with lamps, space heaters, Wardian cases, and the like.

It is important, though, to keep the temperature fairly constant. Plants don’t appreciate hot or cold draughts any more than we do, so try not to put them where cold air blows in from the front door, or under/over an air vent.

As for fertilising, well, don’t. Houseplants slow down their growth in winter and they just don’t need the feeding. Chemical fertilisers especially could build up in the soil to toxic levels. At most, fertilise at half strength and double intervals; if in doubt, err on the side of stinginess.

Organic fertilisers are much gentler and if you have any left over from summer, are a better bet if you feel you must fertilise in winter. There is limited space in a container to add compost or manure, of course, so liquid forms are easier to use. Compost tea, manure tea (neither is as bad as they sound), and comfrey liquid (which is a lot worse than it sounds) are good ones at any time of year. Just dilute them more in winter.

Take this with a grain of salt, of course – if a particular plant IS growing away nicely and steadily in winter, then a bit of judicious fertilising might well be in order. And if you’re trying indoor salad crops (arugula does pretty well in a sunny window, for example), a bit of feeding for them wouldn’t be unreasonable either.

Winter houseplants 10: soil and potting media


Well, the winter soil requirements for houseplants aren’t any different from any other season. As usual, choose a growing medium that suits the needs of the plant.

I say “soil” but soil as such is usually not considered a great choice for potted plants. Certainly the potting mix manufacturers want you to believe that. There is an element of truth to it, however.

Soil from the garden tends to get too compacted in a pot after a while. Plant roots need oxygen, and water needs spaces between soil particles in order to penetrate. When the soil in a pot settles too much, the plant can suffer. Also, if you use soil from the garden, you can bring all sorts of unwanted guests into the house with it. If your soil is naturally sandy, you could get away with it; clay soil is just the opposite and will start to compact very quickly.

Outside, frost action in winter and the activity of soil fauna (worms, insects) naturally keep the soil aerated. Earthworms don’t do well in enclosed spaces (composting worms are different) and most people don’t want insects skittering about their houses. As for frost action, well, if you keep your potted plants in a place where frost hits them, they’re not really houseplants, are they?

The addition of lots and lots of organic matter more-or-less solves the problem – compost, well-rotted manure, and leafmould are all good choices for most houseplants. Orchid mixes contain lots of large pieces of bark, and cactus/succulent mixes contain various components to keep the mix lean and free-draining. Perlite, vermiculite, and crushed pumice are inorganic minerals that can also keep the mix light.

I personally don’t believe that peat (or peat moss) is harvested in a responsibly sustainable manner, so I avoid using it, or mixes based on it, when possible.

Having said all that, there are ways you can get away with using topsoil or even garden soil in containers. As I said above, adding various materials slows down or even prevents compaction; in effect you make your own potting mix. If you intend to repot the plant on a regular basis (at least once a year) with fresh soil, that essentially makes the compaction problem irrelevant. There are also a few plants that don’t seem to be bothered by compaction at all. People as gardening-besotted as I am often make their own potting mixes based on what they have at hand (or can get), but many people will find it easier to just buy the manufactured stuff.

A potting mix doesn’t need to contain soil at all – a lot of the prepared ones are soilless. Pure garden compost, or compost mixed with leafmould, perlite, or vermiculite, is an excellent potting medium, IF the plant likes a lot of fertility and moisture. Other plants that prefer their growing media to be drier or poorer (or both) include succulents, carnivorous plants, and the various Mediterranean herbs (thyme, sage, oregano, lavender, rosemary). Don’t put those in a rich, fertile soil – they will not do well.

Remember that just as containers don’t hold a great deal of water, the soil in them has a limited amount of fertility. Most containerised plants need to be fertilised regularly during the growing season, some more than others, and a few not at all. However, during winter most houseplants slow down and fertilising should be reduced or stopped entirely.

Winter houseplants 9: increasing humidity


There are various ways to humidify the air in a house. Possibly the most obvious is to use a humidifier; make sure yours is of the right capacity for the size of the room it’s in. Many if not most furnaces can be outfitted with a device that remoistens the air after it’s been heated, and this has the advantage of humidifying the entire house. People living in apartment buildings, however, might not have this option. Indoor fountains are also a good way to introduce moisture into the air, especially the ones that create mist.

There are also low-tech options and they can be surprisingly effective. A deep saucer filled with stones and then topped up with water is a classic, tried and true method. It is so effective I find I have to refill mine (a 10-inch saucer) every two or three days. For maximum effect you would ideally have one under each plant. I place mine on a stand over the air vent nearest the plants. Failing all else, shallow pans of water will suffice, but do make sure that pets and children (and adults for that matter) don’t make it a hassle.

Cooking helps, believe it or not, if it’s done regularly. I always recommend that people cook for themselves rather than eating out. It’s usually healthier and cheaper, it’s a useful skill, and it helps to humidify and warm the house. A lot of people say they’re too busy, or stressed from work when they come home, to cook, but it can be quite calming and therapeutic if you give it a chance. It doesn’t mean you have to become a pretentious foodie and it doesn’t have to be a gourmet production.

Speaking of cooking, kitchens and bathrooms tend to be the most humid rooms in the house. If they have enough light and space, they’re actually pretty good places to keep houseplants – if you don’t mind having them there.

Smaller plants can be kept in or under large transparent jars or containers. Remember to lift the covers off for a few moments every couple days or so in order to let fresh air circulate in. Large plastic soda/pop bottles with the bottoms cut off make good covers, especially since you can leave the cap off to provide ventilation. Beware of scorching if you leave these in direct sun; the covers (especially glass ones) can focus sunlight to a damaging degree. Wardian cases, cloches, and terraria are great ways to deal with low humidity.

Some plants, especially citrus and many tropicals, appreciate being sprayed or misted with water on a daily (or even twice daily) basis while the air is dry. If you occasionally do this to the point of the water beading and running off, it has the benefit of washing off any dust that settles on the plant. Also, as the water evaporates it increases the humidity in some small way. However, you have to balance misting against two things. One is the mess that it potentially creates, especially if you do it to the point of dripping; watch out if you have nice hardwood or parquet or carpeted floors. The other, and more important in my eyes, is that if the water sits on the leaves for too long, it can encourage fungal diseases to take hold.

Humidity for houseplants can really be a balancing act. You want it to a level so that plants will survive comfortably, but not so high that it encourages diseases or damages your house. Getting it high enough to damage the house if you use central heating, however, is unlikely.

I’ve never lived in a house that didn’t use gas furnace central heating, so I don’t know how other heating systems affect humidity in winter. If anyone reading this has some such experience with other heating methods, I wouldn’t mind some insight.

Winter houseplants 8: humidity in the house


I think humidity is a reasonable topic to follow watering. For some reason, it doesn’t get as much attention as other factors such as light and water, but it can have a huge impact on houseplants.

There are two measures of humidity: absolute and relative. Absolute humidity is a simple measure of the amount of water vapour present in the air. Relative humidity is a comparative measure of how much water vapour is present vs. the amount of water vapour the air is currently capable of holding. The capacity for water vapour changes according to such factors as temperature and atmospheric pressure. Both measures are expressed in percentages. The same sample of air could have an absolute humidity of, say, 3% and a relative humidity of 90%. Weather reports generally give relative humidity.

Most newer urban and suburban buildings in developed temperate-climate countries these days use central heating in winter. Well, they do around here. I think. And central heating is very good at stripping the moisture from the air. Plants like a certain level of humidity in their environment, although most will eventually adapt to lower humidity (up to a point) if they survive long enough. Even many plants from naturally dry regions often dislike the arid air of a centrally-heated house. On the other hand, most succulents barely notice, being naturally adapted to the extremely dry air of a desert.

When I was much younger, I learned a hard lesson about acclimatising plants when bringing them in for winter. I wanted them to stay outside and growing for as long as possible and only thought about bringing them in when temperatures got too cold. By then the heating would already be on and I couldn’t understand why many of the plants would suddenly drop most of their leaves while they were still green. I eventually figured out it was the (lack of) humidity, but not before I lost a citrus bush and a couple of miniature roses and store-bought bonsai. Now I bring them inside well before the heat goes on, even if the weather is fine. Experience teaches which plants are bothered more.

A random note about bonsai: despite how they are depicted on television and in movies, they are NOT houseplants. However miniaturised, they are trees and need to be outside, and deciduous trees need a dormant stage during winter. Any given bonsai should be kept indoors for only a couple days out of any week and only about five days in any month (or better yet, not at all). The only time you keep bonsai in the house for extended periods is to overwinter tender species, and those usually need special measures to keep them healthy.

It can be difficult to regulate humidity in the house, but it’s well worth it for both humans and plants. For most of us, it’s more pleasant and healthier to breathe somewhat moist air and it reduces chapped lips and dry skin. Moist air feels warmer than dry air, even when it isn’t. In fact, drying the air was one of the basic principles behind early air conditioning.

As far as plants go, the air can’t be too humid, but that probably won’t be a problem. Arid-climate plants will be just fine in moist air. Stagnant moist air can encourage fungal diseases, however, so if you manage to make your house, or at least your plant spaces, very humid, good air circulation is important. Anyone with a greenhouse can tell you that. And you wouldn’t want it humid enough to make the room damp, even if you could.