mountain gardener | Irrigation: Make every drop count – tap the banner

Put this under “Be careful what you want.” The days of summer haven’t appeared in the dogs yet, but the thermometer is approaching 100 degrees. What does your garden look like?

All plants need water—even those that tolerate drought conditions in the summer. Water makes up 90-98% of every plant we grow. It is essential for photosynthesis, as well as reproduction and defense against pests.

What happens to a plant when the thermometer reaches 100 degrees? Are there some plants that can survive tough times more easily?

Photosynthesis is one of the most remarkable biochemical processes on Earth and allows plants to use sunlight to produce food from water and carbon dioxide. However, at temperatures around 104 degrees, the enzymes that carry out photosynthesis lose their shape and function.

A lawn that provides optimal light and water but is extremely hot will be less vigorous. Tomatoes, for example, will drop flowers and not bear fruit if temperatures exceed 90 degrees. Plants that are tolerant of high temperatures may be stunted and less likely to attract pests and diseases even if water is available.

Plants have natural systems that respond to heat problems. Plants can cool themselves by pumping water through the leaves for a sort of swamp cooler effect. They can also produce “heat shock” proteins, which reduces problems with overheating. All of these strategies can take resources away from the plant’s other needs such as growth, flowering, and fruiting.

Not surprisingly, many Californians are adapted to high temperatures. Some California native plants that can handle heat with little water include salvia, mimulus, California fuchsia, eriogonum, manzanita, wormwood, California grass, ceanothus, mountain mahogany, shrub poppy, bush lupine, penstemon native, monardella, mahonia niveni, fremontodendron and mahony cherry.

Other well-adapted plants known to be more heat tolerant include leucadendron, butterfly bush, germander, rosemary, smoke tree, rudbeckia, coropsis, lantana, plumbago, gaillardia, lilac, sedum, oregano and verbena.

So how much water do different types of plants need during the summer heat?

Be sure to water trees and shrubs, checking soil moisture deeply first with a shovel. Small to medium shrubs should be watered when the top 3-6 inches is dry, and large shrubs and trees when the top 6-12 inches is dry.

As a general rule, large trees and shrubs need deep but infrequent watering. They should be on a separate valve from small shrubs and perennials. Watering ornamental trees 1-3 times a month, depending on the type and soil. The tree’s roots grow 12-36 inches deep and require 10 gallons of water per inch of trunk diameter.

Apply water with a soaker hose, drip system emitters or a handheld hose with shut-off and a fine mist attachment depending on the limitations of your water area. Don’t dig holes in the ground to try to water deeply. This dries out more of the roots.

Be sure to water the root zone to the indicated root depth each time you water. Watering deeper than the root zone only means you’re wasting water. You can test how deep you should water by driving a smooth rod 1/4 inch to 3/8 inch in diameter into the ground shortly after watering. The soil probe should glide easily through moist soil but be difficult to push when reaching dry soil.

The roots of the smaller shrubs are 12-24 inches deep in the soil. Established native shrubs may only need watering monthly to keep them looking their best, while other shrubs may need watering every seven to 10 days during the heat of summer. Perennial roots drop only 12 inches or so and may need watering once or twice a week depending on the variety.

With plenty of mulch and the above watering tips, you can keep all of your plants happy and healthy

California-based landscape designer and nursery expert Jan Nelson will answer questions about gardening in the Santa Cruz Mountains. Email her at, or visit

Leave a comment