Protecting Garden Plants from Punishing Heat and Drought

As gardeners, we choose plants that seem like a good fit based on our experience, or the recommendations of people like friendly nursery managers or neighbors with green thumbs and great gardens. In a predictable world, that’s fine. But when nature takes a sharp left turn into uncharted territory, one option is to choose hardy native varieties that already have an affinity for a specific region. They are less likely to develop problems for reasons other than unseasonable weather and may be in a better position to survive. This is becoming a popular trend, so there’s plenty of information about native species for different areas of the country. Another is to prefer drought- and heat-tolerant cultivars.

Let’s look at some of the symptoms plants exhibit when they’re not getting enough water or are experiencing heat stress. Then we’ll explore several ways to avoid problems.

Signs of Drought or Heat Stress in Plants

High heat and drought often crop up simultaneously, or drought will follow a prolonged period of higher-than-normal temperatures. Both conditions can have a negative impact on photosynthesis and other plant functions. Keeping a gardening log will help when closing in on the most likely or pressing diagnosis. Look for these early signs of stress on your plants when it’s extremely hot and dry:

• Wilting leaves and stems

• Dropped leaves, blossoms, or immature fruits

• Leaves that turn yellow or appear burnt along their margins

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• Slow or stunted growth

• Curled or rolled leaves

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• Bolting (life cycle escalation from heat stress)

• Blossom-end rot

You can see some of these symptoms are generic and can have more than one cause. Making the right judgement call usually involves observing several factors operating in concert. It bears repeating that keeping a garden journal can be very helpful.

The good news is many popular and important plant varieties do have ways to protect themselves when water is scarce, or temperatures become damaging and can recover after limited exposure to adverse conditions. For example, to conserve moisture, some can take up carbon dioxide at night rather than during the heat of the day or can store water reserves in fleshy leaves or roots. Others develop more robust root networks to search for water. Research into developing heat and drought-tolerant cultivars is ongoing, which will likely result in better choices at the garden center going forward.

Now, let’s get to the bad news. Some plants, shrubs, and even trees have a very limited ability to cope with heat or drought stress, especially during life stages like flowering or fruiting. In the case of fruiting plants, a specimen may appear to recover and look fine until the fruits themselves fail to mature, become stunted, develop split skins, or exhibit other symptoms. It can be difficult or impossible for these plants to recover and perform normally after even one bout of extreme heat or water deprivation.

How Much Water Is Enough?

Plants that receive insufficient water are unable to perform photosynthesis efficiently and will begin to show signs of stress. So, how much water is enough? A common rule of thumb for food and flowering plant maintenance recommends adding one inch of water to the garden, slowly, once a week. If you’re confused, you’re in good company. It’s hard to imagine even a dedicated gardener scurrying around with a ruler in hand, trying to measure water depth before all that precious moisture soaks into the soil. So, let’s refine the recommendation to make it more intuitive: When watering, plants will benefit most from one inch of surface water within easy reach of their roots over a one square foot area. This is a bit more than half a gallon of the wet stuff (0.623 gallons.), distributed around the base of each plant.

If your plants aren’t receiving adequate water as defined above, either from soaking rain or ancillary watering, it should alert you to potential problems before symptoms appear. Avoiding a problem is always preferable to dealing with it down the line. Of course, different plants vary in their water needs, so always take the precaution of checking the environmental preferences for the plants you put in the garden and consider soil composition and other important factors in your strategy.

Suggestions for Minimizing Drought and Heat Stress in the Garden

You can attack plant stress issues caused by high heat or drought from several different angles, but none is more effective than planning ahead. Yes, this isn’t too helpful if your garden is in trouble today, but it is great advice for next year.

Choose the right plants

Choose appropriate plants and provide them with an optimal growing environment based on their specific needs. When in doubt, err on the side of caution and select plants with drought and heat resistance, native varieties. Many online seed sites categorize their offerings based on one or several search criteria that can help you choose the best candidates. If you don’t have a crystal ball and want more weather information before you plan your next garden venture, try checking out the National Weather Service’s Climate Prediction Center online. For something closer to home, you can also drill down on data for your backyard, or close to it, at worldclimate.com.

Be generous with soil amendments

Don’t be scared to use soil amendments designed to improve fertility and increase water retention. Ideal garden soil is loamy, consisting of a mixture of clay, sand, and decomposing organic materials that provide plenty of nutrients and retain enough, but not too much, moisture.

Consider using primed seeds

Primed seeds are good if you live in drought country. These special seeds have been partially germinated through limited hydration and then dried, which puts them in a kind of suspended animation. When rehydrated, they germinate more quickly and reliably than untreated seeds. This makes them good candidates where drought is a problem.

Increase the spacing between plants

This helps reduce competition for water and other resources, while improving air flow. Plants spaced further apart will also have more access to soil nutrients.

Make use of available afternoon shade

Shade can be provided by buildings and hardscape elements, like walls and fences. If your vegetable plot is perfect in May but sizzling hot on August afternoons, relocating it to an area of afternoon shade in your home’s shadow will protect plants during the hottest part of the day. It will also help reduce their water demand.

Employ a layered planting scheme

Use either a layered planting scheme or an aspect of intercropping, where smaller, more delicate plants are positioned to enjoy some shade from their larger neighbors. This is the time to survey your garden and scope out the best exposures for the plant varieties you favor. Next year, you can put that knowledge to good use.

Use smart water management practices

One option is to install a timed, drip irrigation or comparable water delivery setup. Another is to add a water harvesting system, like one or a series of rain barrels. If you’re paying your utility company more for water these days, consider how nice it would be to get some free, courtesy of Mother Nature.

If you do plan on using harvested water, check the regulations for your state first. You can find out more at the National Conference of State Legislatures website. Some states reward water harvesters, while others are working to penalize them. If the water you plan to harvest is slated for your vegetable patch, avoid watering directly on plant leaves, or opt for a unit that includes an ultra-violet light or other approach to bacteria control.

Sow seeds earlier in the season

This will reschedule important developmental stages in your plants so they move up on the calendar, hopefully maturing before the higher temperatures become destructive. This can be a good strategy, although it is limited by the earliest frost-free date for your location. If it’s been a while since you visited the USDA Plant Hardiness Zone Map for your area, now is a good time. The map was redrawn in 2012, and most areas have shifted half a zone higher. If your zone has changed, you may have more latitude about planting times, and you might even be able to expand your plant repertoire a little.

Sow seeds directly in the garden whenever possible

Direct seeding may be risky and not something you’re used to, but it could pay dividends. The theory here is that directly sown seedlings will adapt more readily to the actual environmental conditions and be better prepared to survive any heat or drought stress down the road. If you’ve ever hardened off indoor germinated seedlings for life in the garden, you get the idea. In a nutshell, tougher young plants become tougher mature plants.

Water during the morning or evening hours

At night or early in the day, there is less risk of moisture loss through evaporation. This saves water and makes it easier to evaluate the amount of water available to your plants.

Practice deep watering

If you throw a little water around the garden when you get home from work at night, covering plants lightly but consistently, you encourage their roots to stay close to the surface where water is plentiful. That’s fine until water becomes scarce in July, and plants aren’t positioned to access water reserves deeper in the soil. Think of deep watering as an exercise in plant training. Watering to a depth of an inch, as we explained earlier, trains plant roots to burrow down where there’s a better chance of finding adequate moisture on dry days. Soil at a greater depth is also cooler. Heat stress in plants isn’t just about air temperature. Increased soil temperature plays a role, too. Deeper roots are more effective roots.

Add Mulch

Mulch will enhance the soil’s ability to retain moisture and remain cooler during high-heat days. Mulch also reduces stress on plants with shallow root systems, and eases competition for water resources by limiting weed growth. Choose an organic mulch like dried leaves or straw that will help improve your soil's ability to retain water for next season’s challenges.

Employ shade cloth

Shade cloth will provide partial protection to plants during the brightest, hottest part of the day. Like a row cover product, shade cloth is made of polypropylene or polyethylene, and can be screwed into or otherwise secured to a simple wood or PVC frame. Once assembled, a shade cloth structure can also be used to extend the fall growing season, so it’s a garden two-for-one.

Don’t allow soil to dry out completely

Dry soil becomes porous and this limits its ability to retain moisture. This affects dwell time—the amount of time water from a good soaking is available to thirsty plant roots as it transits to the nearest aquifer.

Consider using a plant growth regulator

Try find a plant growth regulator containing abscisic acid, which can assist in helping plants control their responses to drought stress.