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Over the Garden Fence: 06/15/2013

Summer heat, water, and mulch in the garden

Hot weather, followed by cool weather, followed by warm weather, followed by hot weather - sounds like summer in the Sonoma Valley. While it is easy for the gardener tlook at weather forecasts for the next week or tw- how dwe prepare garden plants for the ups and downs of summer temperatures?

Most native California plants have spent eons preparing for our summers. Some native California perennial flowers - many of our wildflowers - simply disappear in the summer after a spring of colorful display; some native shrubs and trees have waxy leaves that resist the loss of moisture when it gets hot - look at ceanothus and oaks; the California buckeye without waxy leaves will purposefully drop its leaves in summer tconserve moisture - a summer deciduous tree, exactly the opposite behavior of eastern woodland trees.

But many of our favorite garden plants are not California natives; many originated in Asia and Europe where summers are much wetter than ours (I suspect there is a camellia, an azalea, or a hydrangea in your garden - all hail from China). These plants need attention and water.

The water content of many common garden plants is more than 50 percent; the water content of most vegetables is nearly 90 percent. Providing the right amount of moisture tthe garden is as important as giving your plants the right amount of plant food.

Plants can become stressed in hot weather if not sufficiently watered. Insufficient or uneven water can cause plants tgrow in fits and starts and may result in wilting, blossom drop, leaf drop, malformed fruits in the case of vegetables and fruits, and sometimes death.

Keeping the soil evenly moist can slightly lower the soil temperature in your garden and increase humidity around garden plants. This will help alleviate stress in hot weather.

Summer watering tips for non-native California gardens

  • Group plants with similar water needs. Group plants int“hydrozones”-groups of plants with similar water, soil, and exposure needs. Trees and large shrubs often form the “bones” of a garden. These are commonly set around the perimeter of the garden - sthey are grouped and their watering needs can be similarly addressed. Small shrubs and perennials are likewise commonly grouped - they are more shallow rooted and will require more water.
  • Water according tneed. 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 twtfour inches of soil than from deeper soil. Plants that are flowering or setting fruit need deep, even watering. Hand water or use soaker hoses twater shallow-rooted plants.
  • Furrows and basins. Hand-water plants bordered by furrows or surrounded by basins. Basins 3 t6 inches deep can be built around widely spaced plants. Use a hose-end bubbler attachment tirrigate. Place a simple mechanical timer on the end of hose bibs - that way you can wander off and not worry about watering more than needed.
  • Soaker hoses. Use a soaker hose twater perennials or vegetables planted on flat beds. This is the most effective way twater intensively planted sections of the garden.
  • Drip irrigation. Use a low-volume irrigation system with emitter line for closely spaced plants. Use individual emitters for widely spaced plants. In sandy soil use 2-gph (gallons per hour) emitters; in loam soil use 1-gph emitters; in clay soil set several _-gph emitters; in containers using potting soil set one or more _- or 1-gph emitters in each container.
  • Mulching. Mulch reduces evaporation from the soil surface, moderates soil temperature, and insulates roots from summer heat (and winter cold). Mulch suppresses weeds, reduces soil compaction, prevents erosion, and adds organic matter tthe soil.
  • Mulch is any material that protects the soil surface and allows air and water through. Organic mulches - derived from plant materials - not only protect the soil but add nutrients over time while enriching overall soil composition. You can mulch with compost or shredded leaves - both will alsfeed the soil.
  • Summer mulch. Apply three tfour inches of dried leaves, compost, grass clippings or straw around plants at midsummer tprotect from hot weather and reduce soil temperature by 10°F or more. Soil temperatures of greater than 85°F can slow plant growth.
  • Keep an eye on your plants. Know the signs of water and heat stress: wilting foliage and sunburn. If plants are wilting at the end of a hot day there may be nconcern. But if plants are wilting at the start of the day, water immediately. If plants show signs of sunburn (leaves with yellow or white spots in the center), place shade cloth over the plants.
  • Know the soil in your garden. Examine the soil frequently tknow how much water it is retaining. Thrust a finger intthe 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 well-aged compost tyour garden tkeep soil moisture just right.
  • Water plants deeply. Irrigate sthat the root zone receives water, not just the top few inches. You can push a long wooden dowel intthe soil after watering tget a sense of how deeply you've watered.
  • Avoid runoff. Apply water until it begins tpuddle then stop until the water is absorbed; then repeat the cycle sthat the water penetrates the growing bed rather than runs off. A drip or low-volume irrigation system will aid in delivering water evenly. Furrows and basins can help stem runoff. Terraces on slopes will slow runoff.
  • Adjust the watering schedule as necessary. Water more frequently in hot or droughty 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.

    Steve Albert is the author of The Kitchen Garden Grower’s Guide available at He teaches in the landscape design program at the U.C. Berkeley Extension. He lives in Oakmont.


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