December 2020 » Botanish

How much and how often to water indoor plants

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.

Although containers with no drainage can be interesting or attractive, there’s a risk that too much water will over-saturate the soil and drown the roots.

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.

A container with drainage holes and a shallow tray is all it takes to drastically reduce a houseplant’s risk of overwatering, allowing you to water more frequently and keep the soil moist for faster growth. This plant is a Senecio rowleyanus, or ‘string of pearls.’

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

A brugmansia (angel’s trumpet) wilts in its container.

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.

Growing amaryllises as houseplants that bloom every year

One of my earliest memories of being excited about plants comes from the heirloom amaryllises my grandpa had packed on every windowsill in their house. He kept them outside on a table or bench each summer, bringing them inside before the weather got cold. They bloomed in scattered succession from November to March. The antique bulbs were inherited from his mother—a non-hybrid species variety with salmon, daylily-sized flowers on curved stems that pale in comparison to the treelike poles bearing clusters of dinner-plate blossoms you find for sale in department stores each year leading up to Christmas. The heirloom has its charm, though, and is hard to find now except in rare specialty shops at a high price.

I was so excited about my grandpa’s amaryllises that he gave me one when I was early elementary school age and it was my first experience caring for an indoor plant. I eagerly propagated it into a small collection. In our embarrassment of riches, though, our family took the antique bulbs for granted. Some were tossed intentionally when they developed scale infestations or mealybugs, others were left out by accident past the first frost and transformed into sorrowful piles of soggy slime. Priority went to newer and more varied kinds of houseplants. I started collecting other types of hybrid amaryllises, cross-pollinating them and creating experimental hybrids with their seeds, and found myself short on space. I didn’t take any plants with me when I went off to college, and after my grandpa passed away in 2017, the last of his heirloom amaryllis bulbs froze to death in a cold snap that penetrated so deep it froze and killed the bulbs that had been stored in pots in their garage.

Sometimes, we don’t appreciate things until they’re almost gone. The amaryllis collection gradually dwindled down to a single specimen now siting on the floor next to the sliding glass door at my parents’ house, infected with a mosaic virus that stunts its growth. In healthier form the variety produces a constant supply of offsets, and my grandpa used to save them all and pot them up to form his vast collection of clones.

A note on names: amaryllis is a misnomer

The flowers we all know as amaryllis are technically not amaryllises at all, but hippeastrums. True amaryllises are temperamental plants native to of South Africa, a genus containing only two species, one fairly common—Amaryllis belladonna—and another extremely rare and yet-uncultivated offshoot only recently identified as a separate species. They are best suited for growing outdoors in dry-summer Mediterranean climates, such as coastal California, where they multiply rapidly and are so resilient that they’ve come to litter abandoned farmhouse plots and hug the neglected corners of parking lots. In contrast, these true amaryllises rot in hot humid climates and are very difficult to master indoors.

True amaryllis, Amaryllis beladonna, is a separate genus from which the popular houseplant has taken its common name. Amaryllis is native to South Africa, whereas the commercial Hippeastrums sold as amaryllis are native to South America.

True amaryllises are commonly referred to as naked ladies or surprise lilies. (And as if the naming confusion wasn’t complicated enough, those common names also apply to the separate but visually near-indistinguishable species Lycoris squamigera, which tolerates colder winters and the humid continental weather of the eastern U.S.) But the genus Hippeastrum, with hundreds of species that have been hybridized to countless colorful cultivars, is much more diverse and widely commercialized than Amaryllis. So it gets credit for stealing all the attention, and the common name.

Most people treat their amaryllis/hippeastrum as a throwaway seasonal decoration, like a poinsettia, enjoying it for a week or two of blooms and then tossing it out to buy a new one the following year. But some people are tempted to try to save their plants, and find them difficult. The plants are drought-tolerant and can hibernate for a long time so they’re close to invincible when it comes to keeping them alive in some form, but the problem is that amaryllis/hippeastrums prefer a lot more light than is available to most houseplants. Struggling to achieve reblooms, people have come up with an array of amaryllis care myths that only complicate the task of growing a healthy plant. In truth, amaryllis/hippeastrums are pretty simple and low-maintenance when put in the right spot.

Getting amaryllises to rebloom

Hippeastrum is a tropical genus native to equatorial parts of South America, and as such, it doesn’t experience much seasonal variation. It has the ability to go dormant in drought, but dormancy isn’t necessary for any purpose. The plant will rebloom in winter or spring on a windowsill without intentional forcing, although, if you have any particular concern about the timing, you can force a brief dormancy by withholding water in fall and re-introducing watering six weeks before you want the plant to bloom. Because dormancy takes time away from photosynthesis, and forces the bulb to sacrifice its existing foliage and invest energy into regrowing it later, I prefer not to force dormancy; flowers typically show up at any time January through May in the popular hybrids and that’s fine with me.

Size is everything

With every five to seven leaves, the amaryllis/hippeastrum plant produces an embryonic flower bud that hangs out inside the bulb until the right conditions occur for the inflorescence to develop. The increasing day length just after the winter solstice will set things in motion, although sometimes just moving the plant to a new spot, or other triggers—intentional or idiosyncratic—will set it off. The only thing that really matters in making flowering possible is growing a big, fat healthy bulb with the energy stores necessary to support the enormous blooms. Lacking a sufficient bulb size, the embryonic flowers will eventually self-abort.

Large, onion-shaped amaryllis bulbs are likely to be able to bloom when the dormant buds are triggered by changes in light. While the largest and possibly the second largest bulb in this pot will bloom, the multiple of offsets are too small and will need another year or two to grow.

Different cultivars of amaryllis/hippeastrum bear different sized bulbs, from the size of a shallot in dwarf varieties to a hefty grapefruit in the giant ones. Given the vast genetic range in size, an easy way to tell whether your amaryllis bulb is on track to rebloom is the bulb’s shape. If it’s fat and squat like an onion, that means it’s swelling rapidly, and is likely close to the maximum size and will have the strength to bloom. If it’s more slender like a leek or daffodil bulb, it has probably regressed to a juvenile phase and needs more time to build reserves.

The brightest light builds the biggest bulbs

And for this, the biggest challenge most indoor gardeners have with amaryllis bulbs is giving them enough light. They are at their best outdoor plants in warm climates, where they can naturalize in light shade. When grown at northern lattitudes, they perform best in a greenhouse, sunroom, or a south-facing window. Unless your home is uncommonly well-lit, your best bet is putting the plant directly on the windowsill or as close to it as possible since natural light diminishes dramatically even a couple feet from the glass. Or, you can place the plants outside in partially-shaded areas for summer (ideally in areas sheltered from too much wind or foot traffic). But beware: the straplike amaryllis leaves are prone to kinking or breaking, and when moved, the plant likes to let its existing foliage die off and produce new leaves, which uses precious energy. So you’re most likely to get a large, healthy bulb if you find an ideal spot and leave the plant there without moving it at all for at least 6 months at a time.

Water, soil and fertilizing amaryllis bulbs

Amaryllises are relatively forgiving when it comes to water. It’s good to water them thoroughly for a while and then let them dry out periodically, but avoid leaving them so dry that they begin to drop leaves. In bright summer light and in full leaf, one of the larger plants can use more than a quart of water per week, and in the lush vegetative period it’s OK to keep the soil continually moist. Occasionally, it’s good to let the soil surface become dry to the touch to avoid rot.

Most commercial indoor potting blends are OK for amaryllis, but if the soil is too dense or peaty I like to add perlite, or mix standard potting media with succulent mix. The container should be at least 3 times the diameter of a full-sized bulb, and have drainage holes. (Pots without drainage holes are doomed to failure, which is unfortunate when so many amaryllis grow kits come with pots with no drainage—perhaps the growers’ insurance that customers will have to come back for a new purchase next year?)

Amaryllis should be planted with two thirds of the bulb above the soil surface and should not be repotted often; their thick succulent roots are an expensive investment and disturbing them will force them to regrow, reducing the bulb’s size. They’re more likely to rebloom when rootbound and it is OK to combine bulbs in a container or allow bulbs to coexist with their own offsets. If you choose to force dormancy, it is possible to do so by withholding water or moving the plant into darkness without removing the bulb from its pot. If over the years the soil begins to lose volume or lose its ability to hold water, it’s possible to lift the plant and add soil without completely exposing the root system, although plants will be fine if repotting is limited to every 3-5 years.

Finally, amaryllises are a plant that enjoys a regular feeding, with dilute, well-balanced fertilizer, particularly in the fast-growing vegetative period in summer. That’s when you can really capitalize on the long days and bright light and grow a very large bulb. Avoid fertilizing them in the months leading up to blooming season, since nitrogen will stimulate long stems that are more prone to breaking. To avoid excessive accumulation of fertilizer salts, once or twice a year you can set the container in the sink and flush it thoroughly, letting water drain through the bottom of the pot, then try to return it to the same position it was growing in previously so it doesn’t behave as though it has been moved.

Bringing it home

Some amaryllis cultivars are hardier than others—apparent when identical windowsill conditions produce bulbs of vastly different sizes. In my experience, red or orange-flowered varieties tend to be more vigorous and light pink or white plants are more temperamental, often failing to rebloom when others would. But I don’t know if there’s anything substantial behind that pattern or if it’s just a coincidence. Regardless, it’s more than possible to keep a collection of amaryllis plants alive as houseplants indefinitely, for reliable repeat blooms, as long as you have the light and space. It seems like a shame to toss a perfectly healthy bulb that just finished blooming, and often department stores and clearances will put remaining bulbs on a deep clearance sale just after the season ends with Christmas. If you find yourself tempted, give it a try!

Summary

Amaryllises can be easy plants to grow, but need more light than most houseplants to grow large enough to bloom.

Keep amaryllises in a sunroom or south-facing window, as close to the glass as possible, to give them brighter light.

If you decide to put amaryllises outside for the summer, choose a sheltered, lightly-shaded spot. Be sure to bring them back in well before frost.

A dormancy period is not necessary to get amaryllises to rebloom. It may be easier to grow a larger plant if you don’t force them to go dormant, and avoid pruning off leaves while they are still green.

The natural blooming season for amaryllises indoors is late winter or early spring.

The main factor determining whether an amaryllis can bloom is the size of the bulb. A fat, onion-shaped bulb is a good sign that the plant has enough stored energy, while a thin, slender bulb is likely in a juvenile stage.

Amaryllises need a container with drainage holes in the bottom to thrive.

Try to move the plant as little as often so that the plant doesn’t drop its leaves.

Applying a dilute, well-balanced fertilizer in summer is helpful to promote lots of leafy growth that enlarges the bulb.

Water amaryllis bulbs deeply and thoroughly, then let the soil begin to dry out on the surface before watering again. They can survive long periods without water, but will do better with regular water.