24 Jul

Heat & Drought Stress

By Chelsea Ruiz

Summer’s intense heat and humidity can take a toll on your garden. With conditions changing quickly, now’s the time to watch for signs of
stress. Are your plants showing symptoms of heat or drought? Here's what to look out for.

Summer Stress Signs with REAL garden examples!
 
As we experience a very hot and humid summer, it’s important to check for signs of drought and heat stress in your plantings. This summer, the heat and humidity rolled in fast, and some plants may be experiencing stress. What does heat and drought stress look like? What should we be looking for?
 
Symptoms of plants experiencing heat or drought stress
  • Flagging
This is the earliest symptoms. When soil gets dry, plant leaves may begin to wilt, especially at the tips of their stems. If you wiggle or touch the leaves or stem, it feels limp. In the trade, we refer to this as ‘flagging’. Flagging occurs when the leaves can’t pull adequate moisture from the soil, and the cells lose their turgor pressure, or fullness of water, causing the leaf to feel soft. Confusingly, some plants begin to flag in heat and humidity, even if the soil has adequate moisture. Usually, this is because there is a lot of moisture in the air, making it difficult for moisture to be transpired from the ground through the plant out through the stomata of the leaves. Hydrangeas are well known for their tendency to droop in the middle of the day but recover as cooler evening temperatures return. If you are unfamiliar with a new plant, try feeling the soil moisture to check if it’s dry and the plant needs water. If you notice flagging, it’s essential to provide irrigation to maintain the plant's health. Otherwise, the later steps begin to appear.
  • Greying of green leaf color
When the plant has been flagging for a while, we enter the world of ‘permanent wilt’. The intense UV light deteriorates the green chlorophyll in the leaves, making the foliage appear more “grey” or less intense in color than it was before the drought and heat stress. If you’re noticing greying of the foliage, we’re getting into dangerous territory, and the plant needs supplemental watering. Once a plant enters permanent wilt, its physical appearance begins to degrade, and more drastic steps will be necessary to get the plant to survive, let alone look attractive.
  • Yellowing or browning of the leaves
This is when the plant is at its max for UV light tolerance, and the leaf cells are burning up. The leaf color bleaches, turning yellow, and if exposed to too much light, it begins to burn and turn brown. If the tips of the plants are turning brown, it’s usually a sign of a lack of moisture at one point, and the tips are fried. But if the middle of the leaf turns yellow and then brown, sometimes with holes in the foliage, that’s too strong of light, and the plant is burning. If this is the case, the plant requires more shade. We’ll address this later on in the post. Sometimes, increased supplemental soil moisture offsets sunlight stress, but sometimes it doesn’t. Observe plant stress tolerance levels and make an action plan depending on site conditions and available resources.
 
What’s more confusing is the browning and rotting of leaves due to high humidity. This is especially noticeable in drought-tolerant, fuzzy plants in high-humidity regions such as Nepeta, Salvia nemorosa, Calamintha, Monarda punctata, lavender, or Stachys byzantina. What to do if this is your case will be touched on later.
 
  • Lower leaf drop
If the leaves are perky but you’re seeing the lower leaves of the stem turn yellow and then drop off, it could be two opposite issues. Problem One - drought stress in the past, and the plant loses older, less productive leaves as a way of jettisoning the cargo to lighten the load and make it through the tough times. If this occurs, there’s usually nothing to do this season besides providing supplemental irrigation going forward when required. But lower leaf drop can also be Problem Two - too much water, especially early in the season, and if there is overcrowding in the bed. If you received a lot of rainfall earlier in the season or the irrigation systems were overwatering, and the plants are growing close together, the lower leaves may be rotting due to excess moisture, heat, and insufficient airflow. Both problem one and problem two are just what the plant will look like for the rest of the season, but changes may need to be made to avert these problems in the future.
  • Aborting flowers
As stated earlier, plants will jettison the cargo to stabilize the ship during tough times, and reproductive activities fall into the less valuable category. If a plant is severely stressed by heat and a lack of water, it will abort its flowers, either by not producing them in the first place, slowing down the production of more, or even worse, they will simply drop off. Tomatoes in the South are legendary for this, but it can also happen to any flowering plant, especially if it's been planted on the edge of its tolerance limits. Increasing supplemental irrigation and providing light afternoon shade can help restore a plant’s flowering abilities but so can returning cool evening temperatures.
  • Splayed or opening of habit
In times of water stress, as the leaves lose water pressure, the foliage may flop over with the decrease in leaf rigidity. After this happens, the plant habit opens up, or splays, and the stems permanently fix in this position for the rest of the growing season, even after enough soil moisture returns. A good trim or a cut back to the ground can encourage a flush of new foliage, depending on the plant species and time of year.
 
  • Disease
Environmental stressors weaken the plant and make it more susceptible to disease. When the temperatures are warm and consistently moist, black spot may be prevalent in some species. In periods with low airflow around foliage and oscillation between drought stress and soil moisture flooding, various species of mildew run rampant. Becoming familiar with which plants are susceptible to which diseases and taking steps to prevent them is the best way to manage disease outbreaks in the landscape. When disease is present, a very powerful spray program is required to control problems. This can have knock-on adverse effects on the environment. We here at North Creek strongly encourage prevention over treatment for disease in the landscape.
What to do now
  • Work out a supplemental irrigation system for the most at-risk plants for the rest of the season - this will not be the first or the last of the heat and humidity. If you’re noticing plants struggling with moisture issues, we do not recommend moving them now. Transplant shock, water stress, and heat increase the chances for plant loss. Wait till this fall to transplant them to a new location. Until then, supplement their moisture needs on an as-needed basis. Morning overhead watering, spot watering, or laying drip hose will tide the plant over till more permanent solutions can be implemented.
  • Remove dead foliage - if the foliage is brown from rot or leaf drop, maintenance teams can remove it and compost it. When an entire stem is brown and dead, and we’re still on the hook for weeks more of high humidity, it may be best to cut the stem to the ground and compost it. While we’re all for the look of gentle decay in winter, the high humidity and heat will rot the stem, which can smother neighbors, causing a section of rot and a forlorn garden come late August. If the foliage is brown/dead from disease, do not put it in the compost (most folks' compost does not cook hot enough to kill disease, and the compost will spread it to other plants).
  • Splayed plant care - Some plants respond well to a trim to encourage better habit after falling open or can be cut back to the ground to reflush, depending on the plants. Usually, attempting to cage a plant after the splaying will result in an ungainly appearance with very noticeable staking. Asters, Vernonia, Amsonia, Heliopsis, and Solidago are some of the plants that tolerate a trim if the plant spreads open. A trim of 1/3rd to 1/2th will encourage new growth. Be warned, it can also remove flowers and delay flowering, so consider this when determining if a trim works for your location. Some plants can be cut back to the ground after flowering and will reflush with new growth for an attractive, lush look for the rest of the summer, like sedges, Siberian iris, or daylilies.
What to plan for later
 
Folks may get tired of hearing us say this, but we’ll say it again- it’s important to choose the right plant for the right place. If site conditions have changed over time, plants may need a close look and re-evaluation of the space to assess if they are still suitable. Gardens and outdoor spaces are not static. We continuously reconsider our trial beds, evaluating what plants are thriving, what plants are not meeting expectations, and if there are environmental changes (washout from unexpectedly large storms, a tree dying, shade going away, high temperatures for prolonged periods). This is all a part of the process. Once the challenges of the space have been assessed, an action plan is developed.
  • Fried yellow/brown foliage each summer - plan on relocating the plant in fall to another spot with afternoon shade or more consistent soil moisture and replace with a more suitable plant.
  • Foliage dropped or melted away - the plant requires increased airflow around it. Give it greater space by thinning companions to increase airflow. If this is a consistent issue that the client does not welcome, new plant material that matches the conditions of the site may need to be selected.
  • Plant flops open - utilize different maintenance techniques to manage floppy plants. Preventative spring staking is one option for species that don’t respond well to foliage trims (Hydrangea, Iris, Liatris). An early summer trim can prevent late-season splaying, such as giving a Chelsea Chop to asters, Vernonia, Amsonia, Phlox, Helenium, Heliopsis, Helianthus, and Eupatorium. A third, more permanent, option is to replace large, splaying varieties with selections with smaller, more mounded habits with thicker stems less inclined to splay open.
  • Drought dieback but still alive - plan on transplanting in the fall to a new location, or this plant may need to be on the chopping block, for it's no longer suitable to the space
  • Foliar disease runs rampant - if that specific plant must remain, plan on setting up a supplemental irrigation system for that plant to help deliver consistent moisture to minimize stress and prevent disease outbreaks. If that specific plant is not a dealbreaker, research a more disease-resistant variety and replace the specimen.
Getting Through This

Hot and muggy times call for cool heads. The middle of summer is perfect to take a look at the performance of perennials in the garden and see if the plants are growing well in their location. Observation, experience, and selective zhuzhing can go a long way to getting a space to make it through a tough season. Tend to what needs tending, clear what needs clearing, accept things out of your control, and plan for tomorrow. It’s the gardener’s way.

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