July 2020 » Botanish

It’s hot! It’s barely mid July and we’ve already had some 100-degree days in the Denver area. The hottest time of year here is the last week of July and first week of August, so we’ll be struggling with these temperatures for some time. Some parts of the world are, of course, even hotter. Heat stress forces plants against the upper limit of their adaptations, and they may require intervention.

Signs of heat stress in plants

When the weather is hot, water evaporates much more quickly from leaves and from the soil. This is especially true in low humidity, which is even harder on plants. Plant tissues that lose too much moisture will wilt, scorch or die back.

Symptoms of heat stress overlap with symptoms of drought stress, but there are some differences. Because of the way heat peaks in the middle of the day, plants suffer from repeated assaults rather than a gradual scarcity of water.

Wilting

The most immediate sign of heat stress in plants is wilting. When wilting is related to heat, it is notable that plants can wilt even when the soil is moist. The roots are still taking up water, but they can’t keep up with the rate of evaporation from foliage. The short-term solution is to wet the soil even more thoroughly to increase the absorption rate. You can further ease the strain on roots by mulching with wood chips, straw or leaves to cool the soil.

Leaf curl

A sunflower plant with permanently curled leaves from heat stress — This sunflower has curled leaves caused by damage from heat and drought. The deformation resembles damage from pesticides or certain infections, but in this case it began during a heat wave and didn’t affect nearby plants with a better water supply, suggesting hot weather was the culprit.

When dehydration occurs repeatedly or lasts for days, it can change the way young leaves develop. Cells on the margins of a developing leaf can’t get enough water to expand, while leaf centers develop normally. The uneven expansion forces leaves to crinkle or curl as they mature, and sometimes the deformation is permanent. This type of drought stress is especially common on fast-growing plants like sunflowers, tomatoes and peppers. (Note: viruses and other diseases can also show up as leaf curl. But if the problem began in abnormally hot weather and leaves are not discolored or dying, the likely cause is heat stress). The typical solution to leaf curl is to water plants more regularly.

Scorched leaf margins

Ohio buckeye tree with scorched leaves — Ohio buckeye trees are particularly prone to scorching during hot days, in which the margins of the leaves dry out and die. Although the damage is unsightly, this young tree will survive and produce healthy foliage in the spring.

Severe or repeated heat stress can scorch mature leaves by dehydrating and killing the outer edges of the leaves. Scorched leaves will not improve and the plant won’t look better until it grows new shoots. Mulch, deeper watering, and shade protection on atypically hot days will reduce the incidence of scorch. Resist the temptation to cut off damaged leaves; they don’t look good, but they still photosynthesize and that helps the plant to grow new, undamaged leaves. Meanwhile, the dead, scorched tissue helps to shade and protect lower foliage from further damage.

Sunscald

Unlike wilting and scorch, which are primarily caused by dehydration, sunscald appears when plant tissues are killed by bright sunlight. In the same way that ultraviolet light burns human skin, high-energy photons from the sun damage the chlorophyll and DNA in plant cells beyond the plant’s ability to repair them. Scalded tissue is permanently bleached white or tan. It’s especially common on plants that have recently been moved or brought home from the nursery. To prevent sunscald, move plants to a brighter setting by acclimating them gradually with shade, part shade, light shade and finally full sun. That allows leaves to produce compounds that absorb excess light or free radicals and will prevent sunscald from occurring.

Thermal burn

Leaves can burn on exceptionally hot days, especially when air temperatures are above 100 degrees. Foliage in direct sunlight, or plant tissues in contact with rocks, gravel or metal reach lethal temperatures. Plants are literally cooked to death: the absolute maximum temperature plant cells of any species can tolerate is 140 degrees, while 100-degree temps can kill cells if they go on for hours and the plant is not acclimated to heat. Thermal burn on leaves resembles frost damage. Stems through solar-heated rocks or gravel can be killed from the base, cutting the plant off from the roots. On extremely hot days—the kind of heatwave that happens once every few years—only shading can protect foliage and it’s impossible to prevent some damage. Water and mulch soil with organic material to prevent thermal burns on stems. To protect plants from further damage, leave burnt leaves in place to provide shade until the heat wave is passed.

Yellowing leaves

Brugmansia showing yellow leaves from heat stress — This Brugmansia (angel’s trumpet) is showing signs of moderate heat stress, triggering the plant to drop its older leaves. Yellowing is a natural part senescence (natural aging and death) and the fact that the plant has time to selectively drop certain leaves means that the stress is not as damaging, but is occurring repeatedly or over a long period of time. Although the stress is mild, it does mean that growth is being slowed significantly and the plant may even decrease the amount of live tissue so that the roots can better support the canopy.

It’s normal and healthy for plants to drop older leaves and grow new ones, and plants do so regularly. Older leaves don’t produce as much energy for the plant, and plants regularly recycle their canopies. When leaves turn yellow before they fall, they are dying in a controlled process that usually doesn’t harm the plant. However, during times of severe stress, yellowing can accelerate and many or most leaves may drop.

Spider mites

Spider mites are barely visible to the naked eye, clinging to the sides or undersides of leaves and sucking the fluids out of plant cells. Mite damage appears as small, yellow dots that can expand and fuse together as the mites proliferate. Plants sensing this damage can drop all their leaves in an attempt to starve the mites out or make them go away. Mite-damaged perennials usually survive, but the damage can set them back for a season or severely stunt their growth. Smaller plants or annuals may die. Mites increase their activity in hot weather, and can overwhelm and kill plants in a severely drought-stressed garden. Water and humidity lower spider might activity. Insecticides, which kill beneficial predatory insects, can exacerbate the problem in outdoor gardens. The long-term solution is to water, mulch, and make sure gardens are full of heat-tolerant species planted at a density that the water supply can support.

Shock or early dormancy

Extreme heat stress can force a plant into dormancy as a last-ditch effort to survive. Trees will occasionally do this if they are transplanted in summer. Many cool-season grasses and lawns can go into and out of dormancy easily. Other plants will be left weakened or traumatized when they finally begin to regrow. Some plants are unable to go dormant at all, and just die. The solution is to water, and adapt your garden to drought so that it doesn’t happen again. If plants go dormant easily, it may be better to withhold water and let them stay dormant until cooler weather comes.

Succulents are trendy, beautiful, and the most ornamental category of plants with their wildly distinct colors and shapes. They give off the impression of foolproof plants that thrive on abuse and neglect with simple, one-size-fits-all needs.

A succulent container garden on a patio in San Francisco highlights the diverse colors and textures among succulent plants.

That’s not quite the case.

Succulent enthusiasts’ common refrain is to “think of the desert;” give them lots of warmth and little water. Just don’t let them get cold, or freeze. That’s what their native habitats in the wild are like, right?

That’s not necessarily true either! I’ll explain.

What are succulents?

“Succulence” is really a very general term. It describes thick, fleshy organs that appear on different plants regardless of the plant family they come from. Succulent plants aren’t all related to each other: spiky-leafed succulents like aloe and agave are monocots, just as lilies or palm trees are monocots. Cacti, sedums and jade plants are dicots, the same as sunflowers or rhubarb. Succulence can describe succulent leaves (like an echiveria), succulent stems (like a barrel cactus), or succulent roots (like a phalaenopsis orchid).

Various types of succulent plants can include succulent leaves, succulent stems or succulent roots. — “Succulence” is a descriptive term for thick, fleshy plant organs that store water. Plants with succulent organs come from many plant families and are not closely related. The term can refer to succulent leaves (top row), succulent stems (bottom left) or succulent roots (bottom right).

Where do succulents come from?

Succulents are not all desert plants! Some are, which explains why they’ve adapted to store water through dry spells. A saguaro cactus, for example, is prepared to take advantage of a late-summer torrential thunderstorm that floods the Sonoran Desert with rain. The cactus will swell rapidly with water, and very slowly draw down those reserves through one or more years of drought.

In contrast, Sempervivums (commonly known as hens-and-chicks) are native to rocky alpine zones, thriving on exposed rock faces or gravely soils where the escape the competition of taller plants. Their succulent leaves allow them to survive where thin soil doesn’t hold much moisture, but they need periodic rain or snowmelt to recharge their small reserves. They continue photosynthesizing happily through winter and can tolerate deep freezes in cold climates. They’re also tolerant of part shade growing in the cracks of rocks, but can bake to death in very high heat.

Sempervivum (hens-and-chicks) come from rainy environments, using their water-storing leaves to survive in thin soil.

Other succulents thrive in mild coastal areas, where rain may be scarce but nightly fog or mist is a valuable source of moisture. A large number of popular succulents come from Mediterranean climates, such as South Africa, where winter rain offers a lush growing season, and then the plants hunker down in a semi-dormancy through summer’s dry heat. Finally, there are many tropical rainforest succulents—such as Epiphyllum cacti hybridized from cacti that root in the bark of rain-drenched trees. They enjoy frequent rain, but don’t do well with waterlogged soil.

Rhipsalis cacti are just one of many types of succulents that are adapted to humid, tropical rainforest conditions.

So you can see that these plants come from diverse conditions. With that in mind, when you bring home a new or unusual succulent it’s best to do some reading on that specific species to avoid the risk of failure. But there are still some general rules you can follow for a diverse collection of succulents.

General succulent needs

Fast-draining soil

Planting succulents in loose, fast-draining soil or potting media helps them oxygenate their roots and maintain a healthy root system. “Succulent potting mix” is high in sand, perlite, chunky organic material or small stones.

Drench-and-dry watering cycle

Succulents survive dry spells by storing water in their tissues. When you do water, you don’t need to be stingy. Give them enough water that they can completely fill their reserves and swell to full size. Saturate the soil until some water drains from the container into the tray; that could mean, in a one-gallon container of gritty soil, that the soil can hold a quart of water or more. (It’s OK to leave standing water in the tray for a short time to make sure the soil is absorbing water, but for most succulents, you’ll want to dump out any remaining water that is still in the tray after a few hours.) For a healthy, vigorous plant growing in warm temperatures, soil moisture can be used up quickly. But don’t water again until the soil’s surface is dry to the touch.

Give your succulents enough sun

It’s inevitable that you’ll occasionally mess up and give plants a little more water than they like, but a healthy plant that is receiving plenty of light can defend itself against rot. Indoors, that often means the plant needs to be directly in window, close to the glass, with at least some direct sun. When plants start to appear pale in the center or “stretch out” on long, soft stems, that means they’re becoming etiolated. Plants do this to try to climb over whatever is blocking the light, and it can indicate poor health. It can be hard to re-adapt a severely etiolated plant to full sun. It’s better to avoid the problem completely by keeping them in a very bright location.

Don’t overfertilize

Succulents typically grow slowly and therefore don’t need a lot of fertilizer. That’s especially true in the winter, or if the plant is indoors, where fertilizer can trigger lanky, weak growth.

Move your succulents carefully and sparingly

Succulents grow slowly and hang on to their leaves and branches for a long time. That means it’s harder for them to adjust to changing light levels, or turn their leaves to face light. They need more time to adapt to new conditions than other plants. Resist the urge to move them around or rotate them often—do so sparingly. If you move plants outdoors, it’s vital to harden them off in shaded areas before exposing them to full sun.

A sunburned — A Cotlydon orbiculata plant (“pig’s ear”) with sunburn from being moved into full sun too quickly.

Summary

Don’t assume that all succulents like to be very dry. Some come from more humid climates and prefer more water—you may need to identify the particular succulent species to know what it likes best.

In general, succulents resent standing water and will prefer a fast-draining soil mix. A container with drainage is a must.

Water succulents enough for the tissues to stay plump—wrinkly or withered leaves are a sign of drought stress. Each time you water, do so generously, then let the soil begin to dry before you water the plant again.

As houseplants, succulents generally need to be kept in brighter areas, close to windows, and especially ones that get direct sun.

Monthly Archives: July 2020

Dealing with a summer-stressed garden