The most favorable temperature for plant growth is generally 70 degrees to 85 degrees. Above that many plants need help to thrive in our yards.
Plants cool themselves by a process called transpiration. They draw water from the ground into their roots. It travels up their stems and into their leaves where specialized cells open to release the water as vapor. It is typically a plant’s first line of defense against heat.
In extreme heat it is largely up to us as gardeners to provide the next line of defense: mulch and water.
Mulch is critically important because it moderates soil temperature and helps soil retain moisture. Two to three inches control weeds which compete for water and take nutrients intended for our preferred plants. Mulch also prevents erosion caused by heavy downpours. It is a workhorse in the garden.
In hot weather leaves lose water at a much faster rate than roots absorb it. Even with adequate water in the soil, heat may cause a plant to wilt. As the day cools and overnight roots catch up with water uptake needs. Wilt disappears until the heat of the next day.
Water in morning and evening, not during the heat of the day. It is easier for plants to take up water when they are not under stress from the sun. In addition, during cooler parts of the day more water will be absorbed into the soil instead of evaporating.
Hand watering is effective because it delivers the right amount of water to each target plant. Drip watering can supply water any time of day without getting leaves wet. Drip hoses which are typically buried under mulch do not lose water to evaporation. Both hand watering and drip irrigation are more efficient than overhead sprinklers.
Many annuals have shallow root systems which easily dry out in summer heat and sun. Make the most of rain and irrigation by creating a small moat around the plants to collect water and channel it to plant roots.
Some plants exhibit a shorter bloom period; others stop flowering and setting seed in response to hot temperatures. Avoid fertilizing to restart blooms. Roots’ ability to absorb nutrients diminishes during high heat; fertilizer serves only to stress plants.
Liquid fertilizers feed plants directly instead of supporting the soil. Top dress ornamentals and side dress vegetables with organic matter to help hold moisture and feed the soil. That will benefit plants now and after the weather cools.
When nights are over 70 and days over 86 degrees tomatoes produce flowers; however, the flowers do not pollenate. They drop off instead of setting fruit. Peppers and eggplants also fail to pollenate. Squash, melons, cucumbers, pumpkins drop blossoms, too. Over 90 degrees beans stop flowering. Don’t fertilize. In many cases it encourages more leafy growth than the roots can support.
Heat-resistant varieties will continue to produce fruit during high temperatures but expect a lower yield.
Pick orange tomatoes; they won’t turn red in high heat. Allow them to finish ripening inside or in the shade.
Water garden crops regularly. When the temperature drops the plants should start setting fruit again.
Consider using temporary sun protection some for plants. Shade cloth can be positioned on just one side of a plant to block the intense afternoon sun. A wooden trellis may be as much shade a plant needs during the hottest part of the day.
Maintain your lawn at a height of at least three inches. That allows grass to provide enough shade to help retain moisture in the soil.
It may seem exceedingly obvious but don’t garden without water, sunglasses and sunscreen. Put on a wide-brimmed wet hat. Tie a cooling wrap around your neck – find one ready-made at a golf store or make one with a frozen wet towel or fabric tube filled with wet polymer crystals. Try wearing a wet long-sleeved shirt to cool you as the water evaporates. Work morning and evening, before and after heat of the day. Like plants, we need to cool ourselves during high temperatures.