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I suppose there’s not too much to do in the garden at the moment, now that the initial flurry of digging over and spring sowing and spring cleanup is over (roses are finally pruned!). I intend to get rid of more of the front lawn and turn it into flower beds, and that will begin soon. I did sow some Belgian endives and some sorrel seeds a few days ago (about a month late, but oh well). Now that spring is finally well under way, more things are sprouting and I can take stock of what I can divide or uproot and pot up for the upcoming Master Gardener plant sale. Other than that (which I tend to categorise as ‘pottering about’), there’s the daily process of bringing things in and out of the house to be hardened off or (re)acclimitised to outdoor life.

While tedious, this is important. Plants raised indoors are accustomed to a very different set of conditions than outside. Animals are used to sudden changes and can cope with some very drastic ones with no problems, but plants live on a different system and time scale. This is at least partly because most animals are mobile, and most plants are not (except as seeds). If you simply take an indoor-raised plant and stick it outside, it will probably die, or at least suffer a lot. The reverse is also true.

Why? Well, three main things: temperature, light, and humidity. Indoors, temperatures are fairly stable, while outdoor temperatures can fluctuate wildly. However, during the main growing season, temperature fluctuation tends to be small and gradual. A seedling raised in a 19C house that’s suddenly cast out into, say, 13C spring weather (going down to 5C at night) will not be at all happy.

Light is also a main factor, as it usually is with plants. Often, what seems a nice brightly-lit room to humans functions as no better than shady outdoor conditions for plants. Plants used to low-light conditions that are plunged into full glaring sunshine will quickly be scorched. Furthermore, ultraviolet radiation (UV), an inherent component of sunlight, is also key. Artificial lighting usually doesn’t produce much UV, and, importantly, UV only partially travels through glass (specifically, UVA). So even a plant growing in a sunny window won’t be used to normal outdoor UV levels. If you’re wondering why this is important, consider this: UV is mainly what causes colours to fade and bleach in the sun, and what can potentially cause skin cancer in humans. Plastics are naturally degraded by long-term exposure to UV; any plastic item that is intended for outdoor use must have an UV inhibitor added to its composition.

Finally, humidity. I find this to be one of the most overlooked factors in houseplant care. The average Canadian house in winter is a dry one. Central heating produces warm, dry air with a relative (and absolute) humidity that is surprisingly lower than that outside. Air conditioning in summer also makes the air dry; this in fact is one of its operating principles. Some plants, especially tropical ones, need a high humidity, and even those native to low-humidity environments, such as the Mediterranean, will find indoor humidity (or lack thereof) not to their taste. In this case, however, the problem is more with plants coming in rather than plants going out.

So hardening off of seedlings, or acclimitisation of mature plants, is necessary unless you want to kill them. There’s really nothing complicated or mysterious about it. Three or four weeks before you want them to go outside permanently, or for summer (and this will depend on the conditions the individual species can tolerate), start them off gently. Place them outside in a shady location for just a few hours at first. Every few days, increase the amount of sunlight and the amount of time spent outdoors. A cold frame or an unheated greenhouse is ideal for starting the process, but not necessary. It may be tedious, but it’s not difficult.

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