Hot weather watering tips
Flowers - annuals and perennials, ornamental shrubs and trees, herbs and vegetables - even plants that are “drought tolerant” - are sure to suffer when the weather gets hot.
Sustained temperatures in the 80 to low- and mid-90 degree range can significantly slow plant growth and may even stunt or harm garden plants. Temperatures in the high 90s and greater than 100 degrees can even kill some garden plants. Even plants that are well-rooted and have been in the garden for years can suffer when temperatures get hot.
You've invested a lot of time and money in your garden. Here are a few tips to get your garden through hot weather:
Keep an eye on your plants. Wilting foliage and sunburnt leaves - yellow or white spots in the center - are signs plants need immediate attention. Water immediately if plants are wilting early in the day. Plants that show sign of sunburn should be protected by shade cloth - drape shade cloth over stakes driven into the ground near vulnerable plants. (If plants are wilting at the end of a hot day, check them again first thing in the morning then water if they are still wilted.)
Water plants deeply. Irrigate so that the root zone receives water, not just the top few inches. You can push a long wooden dowel or a metal rod into the soil after watering to get a sense of how deeply you've watered. The probe will move easily through wet soil and haltingly or not at all through dry soil. Annuals and perennials will be rooted to 6 to 10 inches; shrubs can be rooted to 18 inches or more. Snake a soaker hose through planting beds, turn the water on to a steady drip, and allow water to seep down into beds.
Water according to need. Shallow rooted plants, new transplants, and seeds require more frequent watering than established plants with deeper roots. Soil moisture evaporates more quickly from the top two to four inches of soil than from deeper soil. Annuals and perennials that are flowering or summer vegetables like tomatoes, cucumbers, peppers, and melons that are setting fruit need deep, even watering.
Avoid runoff. Use basins and furrows. Apply water until it begins to puddle then stop until the water is absorbed; then repeat the cycle so that the water penetrates the planting bed rather than runs off. Basins and furrows can help stem runoff. Terraces on slopes will slow runoff. Basins 3 to 6 inches deep can be built around widely spaced plants. Use a hose-end bubbler attachment to irrigate.
Drip irrigation. A drip or low-volume irrigation system will aid in delivering water evenly. Use a low-volume irrigation system with emitter line for closely spaced plants. Use individual emitters for widely spaced plants. For plants in sandy soil, set several 2 gallons per hour (gph) emitters about a foot apart in a row; in loam soil set several 1 gph emitters about 1 1/2 feet apart in a row; in clay soil set several 1/2 gph emitters about 1 1/2 feet apart in a row; in containers using potting soil set one or more 1/2 or 1 gph emitters in each container.
Adjust the watering schedule as necessary. Water more frequently in hot weather; water less frequently in cool or cloudy weather. Water in the morning or in the evening when evaporation is lower. Avoid watering in windy conditions.
Know the soil in your garden. Examine your soil frequently to know how much water it is retaining. Most plants grow best in soil that is evenly moist, meaning not too wet and not too dry. For shallow-rooted plants such as annual flowers and vegetables and herbs, you can test soil moisture by simply thrusting a finger into the soil; if it comes out dry, the soil needs water; if it comes out glistening wet, the soil is too wet; if it comes out just damp, the water is just right. Sandy soil requires more watering, clay soil less.
Add aged compost to your garden often. Well-aged compost is both moisture retentive and well draining. Add an inch or two of compost to your planting beds twice a year. Compost will help keep your garden soil evenly moist. Evenly moist soil can slightly lower the soil temperature and increase humidity around crops. This will help alleviate plant stress in hot weather.
Group plants with similar water needs. When you think about adding plants to your garden, group plants into “hydrozones” - place plants with similar water, soil, and exposure needs together. This will allow for the most efficient application of water. Deep rooted plants are best planted in one section of the garden or a section of a planting bed. As well, group medium-rooted plants together and shallow-rooted plants together.
The height and width of a plant at maturity is not a foolproof indication of root depth and watering needs, but it is a suitable guide for home gardeners. You can expect a mature 10-foot tall shrub to be rooted to at least 5 feet (when properly watered) and you can expect a 25-foot tall tree to be rooted to 10 feet or 15 feet (when properly watered). Roots follow the water, so early in a plant's life water deeply to insure the plant becomes well established and can survive hot weather.
Steve Albert is the author of The Kitchen Garden Grower’s Guide available at Amazon.com. He teaches in the landscape design program at the U.C. Berkeley Extension. He lives in Oakmont.