If there were a scientific study on houseplant survival—a big ask since no one makes any money when houseplants survive—I’d guess the biggest killer is the way they’re watered.
Well, that, and being tossed in the trash.
Everyone understands that plants need water, and that “overwatering” is a danger as well. But that leaves a wide range of judgment calls about how much is right, from submerging plants in an aquarium and leaving them parched.
Not only do plants’ water needs vary by species, they also vary with temperature, light, the plant’s size and the rate of lush new growth. The soil’s ability to absorb and hold water, as well as the container volume, also affect how quickly plants dry out.
It’s next to impossible to prescribe a specific amount or frequency for watering plants without tracking an impossibly long list of variables. It’s more realistic to set plants up to tolerate a wider range of moisture levels, and learn how to see the signs a plant needs water.
For that, it’s good to understand the mechanism that injures plants when they’re too wet or dry.
Why indoor plants die from overwatering
In some of the wettest climates on Earth, the land gets up to ten meters of precipitation per year and springs to life with dense, lush misty forests covered in moss and vines. Overwatering is no concern to wild plants in the Pacific Northwest or tropical rainforests—they thrive in the constant drizzle of rain and mist. Yet if you remove those plants from the forest and put them in pots on a windowsill, they might be vulnerable to overwatering and rot. For all but the most drought-adapted species, the problem is simpler than you might think: it’s not that they die of over-hydration—they drown.
The most visible organisms on Earth are plants and animals, and there are big differences between the two kingdoms in terms of respiration. Animals’ bodies deliver oxygen to all their cells by circulating blood, which gets its oxygen from the lungs or gills. So enormous, metabolically-active creatures like hippos and whales can wallow in water constantly as long as they periodically poke their nostrils above the surface to breathe. That’s not true for plants, which lack an oxygen transport system in their sap, so they have to absorb oxygen directly into all their tissues from outside. Tree trunks, flowers, stems and leaves are covered in tiny openings—stomata—that exchange gasses with the outside air. The roots, which sometimes have stomata and sometimes absorb oxygen directly through their much thinner, more delicate outer skin, can only grow in soil that has holds oxygen. When roots reach lower layers of soil, too dense or damp or deep to exchange gasses with the air, they stop growing downward and instead spread horizontally through the fertile shallows.
Submerged roots can sometimes gather enough dissolved oxygen to survive in water, particularly if the roots are in clear water with few microbes. Hydroponic growing techniques use movement—the water is constantly circulating—to keep dissolved oxygen in the water. Even cuttings stuck in a jar can sometimes root, if the species is amenable to that (and as long as there are no rotting leaves or stems in the water, which is fatal for reasons that will soon be clear). But getting enough oxygen to pass through waterlogged soil is another matter, especially when there is decaying organic there.
Soil is full of bacteria and fungi, consuming bits of carbon and scavenging oxygen to fuel their metabolic needs. A healthy soil biome is an important component of the ecosystem, and when in balance it helps plants thrive. But bacteria and fungi are more tolerant of low-oxygen zones than plants’ roots are, and if soil oxygen levels drop low enough that roots start to die, those dead roots immediately become food for an additional flush of microbial life that causes the oxygen-starved zone to expand.
This is not usually an issue in wild soils, which naturally shed water downhill or let it drain down into the earth. In fact, rain carries dissolved oxygen into soil and pulls additional air down into the voids between soil particles as the water table drops. In a container, though, standing water crowds out voids for gas exchange and can block oxygen from reaching roots. After a few days without an oxygen supply, hypoxia can spread and kill off the entire root system. Then, the top of the plant will wither and wilt as if it weren’t being watered at all.
Drown-proofing indoor plants
The simplest way to protect containerized plants from drowning is to make sure the container has holes in the bottom to let excess water drain, pulling fresh air in behind it. A layer of gravel or sand at the bottom of a container doesn’t cut it; these only create stagnant voids rather than circulating air. To oxygenate the whole pot or container to prevent it from killing roots, there needs to be an opening for water to drain out at the lowest point, pulling fresh air all the way through. A shallow tray, only allowed to hold standing water for a day or two at a time, can catch excess water without creating dead zones, but a deep tray allowed to hold standing water for longer periods is risky. Coarse potting mix with perlite or vermiculite also helps oxygenate soil. A tropical houseplant will tolerate heavy watering in a container as long as excess water is allowed to drain and air seeps in.
Managing soil microbes
Plants from drier climates, such as cacti, aloes and other succulents, are more vulnerable to drowning. In addition to death from lack of oxygen, their roots face another threat: having evolved in dry soil, they don’t have the same level of natural resistance to the bacteria and fungi that grow in wet soil. If the level of water-loving bacteria and fungi in the soil builds up too high—even if the soil is still relatively oxygenated—pathogens can infect the roots and rot them out, or begin spreading up the stem and rotting the entire plant. This problem is even worse when arid-climate plants, used to the intense sun of the desert, are growing in the relatively dim indirect light of indoor spaces and become light-starved. With less energy to spend fending off disease, they’re more vulnerable to rotting when they’re waterlogged.
There’s no way to completely prevent disease organisms from reaching roots. Soil-borne fungi and bacteria are everywhere: their spores fill the air, settle on all surfaces, and attach themselves to stems and roots. Entire ecosystems of bacteria, slime molds, fungi, microscopic animals, viruses and amoebas live in every cubic inch of wild soil. There are far too many soil organisms to count—there are millions, and probably billions, of distinct varieties on Earth, each with its particular set of favorite conditions. Although the number of species is greatest in wilderness areas and mature garden soil, containerized plants still have a diverse array of microbes among their roots.
With so many species to deal with, and more coming in on every puff of air or grain of dust, we mostly trust nature to do its thing. By growing plants in conditions they’re naturally adapted to, we promote the best microbial environment for each type of plant. That means choosing the right substrate, and allowing soil to alternate between wet and dry to promote healthier roots.
Some types of plants call for coarse, sandy soil with more inorganic particles like perlite or bits of stone—with less food for decomposers, inorganic soils carry a lower microbial load (though they are far from sterile) and are better for plants that are at high risk for rot. Beyond that, alternate deep and thorough watering, which allows plant tissues to swell and store water, with letting the soil surface dry out. Microbes will multiply when the soil is wet, but the dry spells bring their populations back down before they become a threat. The wet-dry cycle replicates natural precipitation patterns, and plants are well-adapted to it, as long as you provide enough water with each cycle, and let the excess water drain out.
Underwatering
Compared to overwatering, underwatering is more straightforward. Living plant tissues are up to 90 percent water—thin cellulose cell membranes enclosing a soup of molecules that maintain life. While woody tissue is rigid on its own, leaves and stems need enough water to keep their cells pressurized and maintain their structure. If stored water drops below a crucial threshold, cells will lose pressure, wilt and die.
Not only do plants need water to stay alive—the same way animals need water or they will die of dehydration—plants use up water molecules when they photosynthesize. In direct figures, six water molecules combine with six carbon dioxide molecules to make a single molecule of glucose, which can be burned as energy, or attached in chains to make cellulose, lignin or other long-term structures as the plant grows. But under drought stress, plants close their stomata to limit evaporation and end up shutting down photosynthesis. Due to evaporative losses, it ultimately takes hundreds of water molecules to make a single molecule of sugar, and a chronically drought-stressed plant will stop growing.
Wilting, yellowing, dropping leaves or a lack of growth are all visual signs a plant might need more water. Plants should be watered enough to saturate the soil, until water starts to come out of the bottom of the container and fill the tray, and watered again when the soil is dry. Additionally, potting media that becomes bone dry can contract and lose its capacity to accept new water unless it is soaked for a few minutes or hours. If plants are being watered but still show signs of drought stress, check to see if the soil is still dry to the touch after being watered. If so, you may need to set it in a bowl or bucket for a few hours to let it swell, then remove it to let it drain.
Summary
- Many houseplants can tolerate more water as long as they are in a container with drainage holes in the bottom that allow oxygen to penetrate the soil.
- Putting sand or gravel in the bottom of a pot does not make up for a lack of drainage holes.
- Cacti, succulents and other drought-tolerant plants should be planted in a course or sandy potting mix that dries faster and has less organic material to reduce the amount of decay microorganisms.
- When you water, water plants generously—enough to saturate the soil until water pools in the tray.
- After watering a plant, let the soil dry on the surface before you water it again. A wet-dry cycle is natural and healthy for plants.
- If bone-dry potting media is letting water run though without soaking it up, it needs to be set in a deep bowl or bucket of water for a few hours until it begins holding water again, then allowed to drain.