According to climatologists at the U.S. National Weather Service, a heat wave is defined as three consecutive days with a minimum shade temperature of 90ºF (32ºC). For those of us in the Northern Hemisphere, that's a pretty good description of at least part of our summer. Global Warming has arrived to our gardens. Over the next few years we will continue to see increases in the average daytime and nighttime temperatures, higher humidity and more frequent heat waves. Here are ways you can help your plants beat the heat.
How Heat Harms Plants
Up to a certain temperature, heat is an advantage to growing plants because heat works to speed up a plant's physical and biological processes. As temperatures rise, certain metabolic and chemical processes speed up. This is to the plant's advantage, because these faster reactions allow it to absorb nutrients more efficiently and accelerate the plant's growth. Once the optimal temperature threshold is surpassed, (90ºF for many plants), these same metabolic and chemical processes start to become burdens rather than advantages. They start to metabolize their food for energy (respiration) faster than they can produce food (photosynthesis). As a result, the plants start to weaken. At the same time, other critical chemical changes start to occur due to the heat that causes the structure of the plant to break down, causes its internal temperature to rise and eventually renders the plant's roots incapable of taking up moisture at a fast enough rate to replace what's being lost.
Help Plants Beat the Heat Use the Right Plant for the Right Site:
Consider microclimates when planting your garden. The south side of the house is always going to be the hot side of the house in the northern hemisphere. Even when located away from the house, a bed with a southern exposure can be as much as 10 to 15 degrees warmer than other parts of the garden. These sites are best left for plants that are drought and heat resistant. Add plenty of organic nutrients to these sites and use a shallow mulch to help conserve soil moisture.
Enrich Your Soil: Add organic matter to your soil on a regular basis. It improves soil structure, improves soil drainage, increases aeration and increases your soil's ability to retain moisture. All of these elements work together to help keep you soil cool and your plants vigorous.
Cover Your Soil: When you dig down into bare soil on a sunny day, you'll notice immediately that although the top soil is hot, the soil is cool and moist just a few inches below the surface. When left unprotected, soil exposed to the sun can heat up to a whopping 130ºF near the surface. This can cause irreversible damage (even death) to the plant. Covering your soil with a shallow layer of mulch will help keep soil temperatures (and plant roots) cool.
Give Them Plenty to Drink
It's thought that as much as 90% of the water given to a plant is used for the purpose of cooling itself, and the other 10% for respiration and food production. Water your trees and plants thoroughly and deeply during heat waves. Extend your watering area out to just beyond the plant's foliage (drip line) to ensure it reaches the plant's feeder roots. Use a moisture meter on drought sensitive plants and keep a daily eye out for signs indicating heat stress (leaf curl and tip burns). This is even more critical for container plants. Avoid dark colored containers and containers made from plastic or metals. Even when given adequate water, many plants will naturally look wilted during the warmest part of the day.
Reduce Your Fertilizer Use
It may seem counterintuitive, but fertilizing during dry, hot weather will only serve to injure a plant's roots. As temperatures rise, plants start to shut down to conserve energy. Applying fertilizers several weeks before or during this time only results in a build up of salts in the soil, which will injure tender root tissues once growth resumes normally. Amend your soil with plenty of nutrient-rich organic matter and you'll all but eliminate any need for using fertilizers.