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The Dry Garden: How to water and mulch in the heat of summer

August 5, 2011 |  9:16 am

Mulch

The end is in sight. On Sept. 30, California’s “water year” will end and meteorologists will begin measuring rain for another 12-month cycle. Gardeners will begin planting with the expectation of help from seasonal rains. Before then, however, we must get through August and September. Atop the to-do list: irrigate and mulch.

This finale to our dry season can be fierce. Apart from a freak thunderstorm, no rain will fall. As moisture from winter rains recedes in the soil, we need to recharge it. But using water to emulate spring and push growth only stresses plants. Rather, the object of watering in late summer is to maintain stasis and prevent damage. We need to make sure that sufficient moisture is in the soil for trees and shrubs to weather a day like last Sept. 27, when downtown L.A. reached 113 degrees.

Apart from water-hungry vegetable gardens, the landscape in summer is best irrigated infrequently and deeply. For those with established California native or Mediterranean-style gardens, follow Bart O’Brien’s advice. The senior horticulturist with the Rancho Santa Ana Botanic Garden in Claremont turns on a single, low-flow sprinkler overnight and allows slow, steady misting to recharge parched soil.

For hedges, a long run of soaker hoses works, provided that they aren’t clogged and the entire line is receiving water. If run overnight, the irrigation should have infiltrated the soil by morning. Do that once a month, and you should be fine.

SprinklerDon’t irrigate so heavily that you see runoff, and don't water during the heat of the day or early evening. You may lose 50% of the water to evaporation, and the rest will be promoting fungus. Some natives, such as the fremontodendron, can’t tolerate any summer water.

For those with mechanized watering systems, the protocols change with the type of technology. To help parse the differences, irrigation advisor Bob Galbreath has produced a series of water budgets for coastal cities and Pasadena. They err on the generous, so the coastal watering guidelines also should provide decent advice for the L.A. basin., and the Pasadena recommendations can be a starting point for anyone in hotter areas.

Along with Galbreath, the most trusted source on the subject in California is Cal Poly Pomona professor emeritus Robert Perry. Calculating how much water a garden will need is a through-line to his encyclopedic “Landscape Plants for California Gardens." Those planning a garden or learning to garden should use his book to choose the right plants and irrigation.

Perry was kind enough to pass on this most general, no-guarantees, advice for summer: Water every three weeks or four to five times from June to October. Run drip systems on two- to three-hour cycles with .9 gph emitters. This puts about three to four inches of water down on every foot of soil where emitters have been placed. Three to four inches of water will percolate 24 to 36 inches deep in sandy soils, 12 to 18 inches deep in loamy soils, and six to nine inches deep in clay soils. "Clay soils are such a challenge!” Perry said.

You can also set your sprinklers for multiple run times to also provide this amount of water if you know the precipitation rates of the nozzles, he said.

For more thirsty ornamentals, “I look at two-week intervals," he said, adding that you're essentially doubling their water budget.

There, in a nutshell, you have the reason why it is preferable to landscape with natives. Whatever type of plants you have, if they are young with poorly established root systems, you’ll need to water more frequently. It also merits giving your trees and shrubs a short, gentle shower with the hose once a month to rinse off urban grime.

To test how well water is sinking into the soil, Galbreath recommends a T-shaped soil probe that can produce a foot-long sample not much wider than a roll of quarters, so you’re not disturbing the earth. After checking the moisture content, you can simply plug the sample back in. Landscape instructors also encourage beginning gardeners to use a probe so they can learn their soil type. If your sample balls up in your hand when you add water, you have clay. The sweetest condition of soil will be lightly moist but also airy so roots can reach air and water.

The secret to making the most of your irrigation in summer while avoiding pitfalls of disease is mulch. This is the blanket of organic material -- wood chips, leaves, straw, coffee bean hulls if you can stand the smell -- that will keep the soil cool and reduce evaporation. By August, most yards can use a topping of it. You want a two- to three-inch layer in beds, less around tender vegetables where mulch can overwhelm them.

Bagged redwood chips sold in box stores work fine, but the city of Los Angeles gives free mulch made from green waste to anyone who can haul it away. Commercial suppliers, such as Cal Blend Soils, have mulch starting at $19 per cubic yard. According to Cal Blend, this should cover 150 square feet to a depth of roughly two inches.

A great source for mulch can be tree trimmers; however, if the wood and leaves are freshly ground, you should ask if the mulch came from healthy trees, not from a specimen felled because of disease or pestilence. There is also the issue of seeds. Be prepared for them to sprout into volunteer plants. A school where I once helped to mulch now features an unplanned eucalyptus among its courtyard willows.

The greenness of fresh mulch will have a deliciously woodsy smell that will intensify as the nitrogen in the leaves briefly heats and breaks down. Be careful about having a big pile of cooking leaves near existing plants because it might heat up the roots. At two to three inches deep, though, green mulch won’t heat up. Whatever nitrogen is leached as the leaves compost will be swiftly returned to the soil.

Don’t pile any mulch right up to the base of the plants, but leave three to six inches for small plants, at least a foot for trees.

Uncured stable straw can have manure and urea capable of burning the root zones of plants, so let it stand for a while. Then spread it on your vegetables and fruit trees first. Most natives and many Mediterranean plants are nitrogen fixing, meaning they take their fertilizer from the air, so they won’t need the manure. Some plants, such as lavender, even seem stunted by helpings of nitrogen. Buying new hay and straw bales, using them for garden seating, then un-baling them and spreading the straw is a fun option. Beware, straw can make the ground slippery for running kids, and there will be a certain amount of drift in wind.

The best way to build up mulch around trees is to ban leaf blowers and rakes, particularly around oaks and avocado trees. The needles of pines and cedars give a particularly healthful shot of acid to soils.

As for the other summer chores: Harvest tomatoes, thin grape leaves, clear dead wood in fire zones. Book an arborist for autumn tree trimming, then pore over seed catalogs for October planting season.

-- Emily Green

Photo credits, from top: Robert Gauthier / Los Angeles Times; Mladen Antonov / AFP / Getty Images

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