Following from the previous post. In fact, I’ll repeat the very end of it here.
For some plants, especially those that aren’t growing much and are kept in constant conditions, you can work out a more-or-less consistent routine of when to water and how much. In general, though, the best way to know when to water is to use your eyes and your brain (and possibly a finger).
Keep track of how much and when last you watered something (easier said than done if you have a lot of houseplants). Look at the plant, and look at the soil – is the plant showing signs of water stress (see below); is the soil still moist or is it dry? Is it a plant that needs a little or a lot of water in general? Then act accordingly.
Signs that a plant is thirsty are wilted leaves and flopping stems; gone too far the leaves will turn brown and crispy and fall off. Some plants have other symptoms; for example spider plants (Chlorophytum comosum) will also go from vibrant green to a slightly greyer tinge.
Ironically, overwatering results in identical symptoms, at least for the visible growth. Remember that plants aborb water from the soil through their roots. Overwatering causes the roots to die and rot, which means the plant gets less water. It’s not too uncommon that someone sees their overwatered plant wilting and assumes it needs more water – so they give it more water, which makes the problem even worse.
It also pays to know your plant (observation observation observation). Plants don’t have personalities as we think of them, but sometimes they behave as though they do. For example, some plant species fold and droop their leaves when it gets dark – so that by itself is not a sign of a watering problem.
There are two main ways to water houseplants: top watering and bottom watering. Both are pretty much what it says on the box. Top watering means you add water from the top, and it is better to add it at soil level than dousing the leaves and stems with water. The latter case is not only messy, but a lot of it can be lost to evaporation or splashing rather than going to the roots. Far more importantly, water that sits on the top growth for long can encourage fungal diseases, especially in low air circulation. And fungal diseases are difficult to control once established.
Bottom watering means water taken up from the bottom of the pot. Most plant pots have (and should have) a hole in the bottom so excess water can drain through; this hole also provides a way for water to enter. Pouring water into a saucer or other container that the pot stands in is bottom watering.
For most mature plants, which method you choose doesn’t matter too much. What is more important is that each watering be thorough. Having said that, most people find top watering to be more convenient. Remember to add a little more water for plants in porous containers; in such cases you literally need to water the pot as well as the plant.
A pot that’s been allowed to dry out too much is best “rehabilitated” by bottom watering. One of the ironies of soil/potting mix is that when it gets really dry, it becomes hydrophobic, i.e. it repels water rather than soaking it up. It might also contract on itself and pull away from the sides of the pot, especially if it contains clay. When the soil is too dry, water poured from the top either runs off the surface and down the cracks, or just straight through, and the soil absorbs very little. By bottom watering, you give the soil a chance to rehydrate itself gradually – depending on how bad it is, it could take an hour or more. If it is that bad (and the plant is still alive…), you might find it more useful to bottom water using a part-filled bucket rather than a saucer.
Of course, if the container has no drainage holes (an undrained container), you will need to top water. I would only recommend these for aquatic plants, or possibly bog plants – and these seldom make easy houseplants. Personally, though, I would grow those in drained containers, but sitting in a larger container of water. I find it makes for easier management in the long run. Um…unless they’re floating plants, in which case you’d need an undrained container.