Probably the most frequent care question we get when someone purchases a houseplant is: “How often should I water it?” And the answers tend to be “keep it evenly moist” or “water it when the soil feels dry”, or, “stick your finger in the soil and water it when it feels moist about an inch down from the top.” Or something along those lines. “When it needs it,” may sound flip but that’s the real answer.
So, how do you know “when it needs it?”
Big Picture. You may think that houseplants come from a jungle environment where it’s always humid, shady, and wet. But plants that live in those conditions don’t adapt well to the dry air and cooler temperatures of the average home. They grow better in greenhouses or terrariums. Plants grown as houseplants originate from all sorts of environments, and if you take the time to Google your latest acquisition, you’ll get clues to the care; for example, many tropical plants, like weeping figs, rubber plants, and palms are trees that are perfectly happy growing in full tropical sun. Furthermore, in many parts of the tropics there are distinct wet and dry seasons, where plants may go for weeks or months without rain, so many of our houseplants are adapted to tolerate drying out between watering. So reading up on where a plant comes from can be useful.
More potted plants are killed by overwatering than underwatering. If you just forget to water, it’s pretty obvious—plant wilts and dies. Bad parent. But how can overwatering be a problem? Plants need a balance between having moisture in the soil and air in the soil. Plant roots need oxygen just like people do. When you overwater, a couple of things happen: the spaces in the soil that should be filled with air are filled with water; so no oxygen. You also create an environment that promotes certain fungi that attack and kill the roots.
How can one overwater?? Potting mixes are designed to hold a proper ratio of water and air. There is no real dirt in potting soil because it doesn’t drain well in a pot—it holds too much water, not enough air. Potting “soils” are mostly composed of an organic material of some sort, like peat moss, with perlite or vermiculite, or sometimes little beads of styrofoam, to keep the peat from packing too tightly. The goal is for the potting soil to absorb water when water is applied, yet drain well after watering so that air spaces are available for the roots. When you water, gravity pulls the surplus water from the top of the soil to the bottom; the more porous the soil, the better it drains. One reason to repot houseplants periodically is that the organic component of the soil decays and turns to muck; it packs tighter and there are smaller air spaces resulting in poorer drainage and less air for the roots.
So far so good, as long as the container has a hole in the bottom for excess water to drain out. If it came wrapped in foil or cellophane, remove that when you water and put it back on in say, 20 minutes. Don’t let the plant sit in a saucer of water. Don’t pot directly into a decorative pot (jardinière) with no drainage holes; set your plant in its draining pot into the decorative pot, preferably sitting on top of an overturned flower pot or something similar to keep it off the bottom of the jardinière. The pot containing your plant has to drain after watering or you are just asking for trouble.
The plant on the left has rotten roots, whereas the plant on the right has healthy roots.
An uplifting experience: how do I determine that it’s time to water? Weight, weight—don’t tell me! Potting soils are very light weight when dry, and heavy when wet, and it’s easy to tell the difference. If the plant is small enough, just pick it up. If it’s heavy or heavyish, let it go another couple of days before you water. When it gets light, it needs water. You want it to get light, but not so light that the plant wilts. If the plant is wilting, and the pot is heavy, then you have overwatered, a root rot has set in, and the plant is probably destined for a new career as compost. Watering a wilting plant that already is in soggy soil just makes things worse. By the way, I water most of my plants in the kitchen sink; that way, the excess water drains away and then I put them back in their saucers and back in the window. Easy.
If the pot is light and the plant is wilting: then it really does need water. If you really let it go and the soil gets very dry, the soil will shrink and pull away from the side of the pot, and it may even become water repellent. When you water, the water just runs down the sides, between the inner edge of the pot and the shriveled up soil ball. You will have to put the pot in a saucer or bowl of water for an hour or two, or water repeatedly so that the soil absorbs the water and swells back up again. If the plant hasn’t reached what in horticulture is called “the permanent wilting point” it should rehydrate and stand up strong. Don’t be surprised if you have some browning on the edges of the leaves from the stress. You may also have some leaves turn yellow a week later as a delayed stress response. Just trim them off.
What If you can’t lift the pot because it’s too heavy? Then you are back to knowing what the plant needs in terms of watering frequency from your research or discussions with our houseplant staff. But the literal rule of thumb is, feel the soil, and dig down a little. Wait until the top inch or so of soil is dry before watering. It may be two or three weeks between watering for most large floor plants. By the way, I read somewhere once that you can’t really feel soil moisture per se, what you are feeling is a temperature difference: damp soil is cooler.
Heavy, man. When I shop for potting soils, I lift the bag. I find that potting soils that are heavy are better for outdoor containers—sun, wind—you need something that doesn’t dry out too fast. Lighter weight soils are better for indoors. Our staff will advise depending on your needs. And of course, there are special situation soils for some of the oddball plants–orchids, succulents and so on–for plants that have special watering needs beyond the scope of this article.
I have one or two pot-bound ferns that need water a couple of times a week. I check everything else about once a week, but I don’t water everything each week. Just when they need it.
by Larry Hurley, Behnke horticulturist