“When/how much/how often should I water my house plants?” It’s the question asked the most by gardeners who are just getting their feet wet, so to speak.
There’s no way around the following deceptively simple answer, though it’s not the one they want to hear:
When they need it. Which is generally less often than you’d think.
Unfortunately no magic formula exists, whatever so-called gardening experts, plant sellers and well-intentioned fellow gardeners advise. The internet and gardening literature are full of well-intentioned advice to “water potted plants every three or four days”, “every one to three weeks”(!), “water outdoor containers daily”, “give your vegetable garden an inch of water per week” and so on. Such specific advice is generally useless and can be actively harmful.
Factors that should be taken into account when deciding when/if to water:
the type of plant
its stage of growth
the type of growing medium
the size of the container (if pot-grown)
ambient temperature and humidity
the time of year
Obviously a cactus needs less water and less frequently than, say, a wax begonia. Even then there isn’t a set of rules to be followed all the time. A cactus plant in active growth (out-of-doors in the summer, for instance) will need more water than one in dimmer light and cooler temperatures indoors over the winter. There are succulents that tolerate and enjoy frequent rainfall during the warm months, but would be at risk of rotting and dying if given that much water during their winter rest.
Actively growing seedlings need more careful attention to adequate watering than a mature plant. Let your tomato seedlings go dry for a day and that could be the end; a full-grown plant might get a bit droopy after several days without rain, but will perk up quickly after a shower.
A pot-bound plant needs more water than the same size plant in a large container. In fact, if the pot is too big, the plant can’t take up all the water in that large a soil mass and the risk of root rot and plant death increases. On the other hand, when up-potting rapidly growing plants like many annuals, ignore advice to slowly increase the size of the pot, say from a four-inch to a five-inch pot. It’ll rapidly get root-bound. You can jump a pot size or two, for instance to a 6-incher, and you and the plant will be happier.
Here in no particular order are other tips I’ve found useful in determining when to water plants.
Make sure as much as possible that the soil is freely draining. This means no standing water long after a heavy rain. If indoors, use a lightweight soilless mix, generally one that’s peat-based, amended with commercial potting soil, vermiculite or perlite in proportions that result in good water retention but allow the excess to drain off rapidly. Many successful formulas exist. I tend to use what’s handy for indoor and container plants; Pro-Mix gives good results if I don’t feel like mixing my own batch. When you pick up a bag of soilless houseplant mix it should feel lighter than you expect. Avoid bagged “topsoil” and even most “potting soil” sold in garden centers as a sole medium for growing house plants in pots. It’s bound to be too heavy and water retentive.
Get to know the visual difference between soil medium that’s starting to dry out. For one thing, the surface is a lighter color. Pick up pots. If a pot feels lighter, it may be time to water. Above all, stick a finger in the pot or into the soil bed to feel what it’s like a few inches down. Frequently, the surface will look dry but the soil surrounding most of the root ball will still be moist. Outdoor mulch is your friend, to retain moisture in the soil, keep it cooler in hot summer weather and retard weed growth.
Learn the early signs of water stress. A plant that needs water will frequently show it before there are obvious signs of wilt - the foliage may look duller or subtly less turgid. A plant that has been overwatered often paradoxically will wilt. That’s because the roots have been deprived of oxygen. Pouring more water onto the plant just makes matters worse. Wilting in the presence of excess water is a phenomenon also seen outdoors, for instance on a hot summer day after a heavy thunderstorm. The sun comes back out and bakes the waterlogged plant, which wilts. Usually such a plant will recover after awhile, so don’t panic.
When you water, don’t sprinkle a small amount of water on the surface every day. Give the plant a good soaking when it needs it. This advice also goes for plants in dormancy, such as potted trees. A sprinkling doesn’t get down deep into the root ball, and the result may be the plant stressing or dying due to lack of water. If you’re thirsty, does a tablespoon of iced tea suffice? Nope.
Advice you’ll commonly hear is to never let a plant sit in a saucer of water. This isn’t necessarily valid. Water-loving plants (for instance cannas and many carnivorous plants) appreciate wet feet. And others are more tolerant of such conditions than you might think. I’ve left potted palms and other large foliage specimens in saucers of water containing a couple of inches of water outdoors during the summer and it saves me some watering time as well as lowering the chance of adverse effects due to lack of water. And when going away on vacation (no more than a week to ten days) I commonly let plants sit in trays containing water so they’ll hold over till I get back. Just be careful not to do this with certain plants such as most succulents, and to allow the pots to dry out almost completely after you return, assuming the soil is still damp at that point.
Some house plants i.e. orchids are sold with tags or little booklets suggesting you water by putting a few ice cubes on top of the soil every so often. This advice is probably well-intentioned as a means of preventing overwatering, but overall is, for lack of a better word, nuts. Are tropical plants adapted to having a near-constant trickle of icy water down to their roots? Of course not. Skip the ice cubes. The water you use should be room temperature or a bit warmer. Same goes for water used to mist a plant’s foliage.
Everyone’s forgetful once in a while and a prized plant wilts. This is less of a calamity than having a plant that keels over due to being waterlogged. Sometimes you can rescue a plant that’s wilting or starting to rot due to excess water by removing it from its pot, cutting off dead or dying parts and repotting in fresh, rapidly draining medium. Or you can take stem cuttings.
One piece of dubious garden wisdom involves never never watering plants late in the evening, or at least to never get water on the leaves just before dark or after nightfall. Nature happily ignores this advice, as evidenced by perfectly healthy gardens which are frequently rained on overnight. Even house plants can be safely watered just before bedtime (theirs and yours). Just don’t overwater and overcrowd plants, avoid using icy cold water and aim for good air circulation.
Warnings can be found online about watering plants with tap water, due to the presence of chlorine, chloramine (increasingly used in municipal water treatment to kill pathogens since it breaks down more slowly than chlorine), fluoride and other alleged Scary Chemicals in the water. While rainwater in a pristine wild setting may be somewhat better for plants than city tap water, such a difference is likely to be far outweighed by other cultural factors. The level of chlorine in municipal water, unless it causes it to reek like an overtreated swimming pool, is unlikely to harm plants, as is the fluoride added to prevent tooth decay. Some people such as cannabis growers have become quite alarmist about added fluoride, pointing ominously to lists of plants said to be harmed by it. Spider plants are said to suffer tip browning due to fluoride, though this effect is also seen with inadequate watering and fertilizer burn (my impression is that spider plants develop tip browning if you just look at them the wrong way). The general rule for chemicals is that the dose makes the poison, and I haven’t seen convincing evidence that fluoride at levels added to tap water is at all harmful to the vast majority of plants we grow, or to people for that matter. So invest in that fancy water filtration system if you want, but don’t expect obvious benefits. Note that if you want to minimize chlorine/chloramine exposure to seedlings, there are relatively inexpensive tap water conditioners sold to aquarists that are said to remove these agents and can be adapted to horticulture.
I hope these tips are helpful. Above all, when it comes to watering less (often) is generally better, and use your sensitive fingers to get into the soil and feel what it’s like down in the nether regions invisible to the naked eye.
The matter of when and how to water garden plants is another issue, deserving its own separate analysis. Coming soon…