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Washington State University Extension

Garden Tips

Helping Landscape and Garden Plants Cope with Drought

GARDEN TIPS – written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA

Written April 2, 2015

With the prospect of limited irrigation water in the coming months, we may have to make agonizing decisions regarding which plants in our yards and gardens to save and which plants to let go. To me, it is like making ASophie=s Choice.@  Before making these difficult decisions becomes a necessity, there are some things we can do to make the most of the water that will be available.

As much as 50 percent or more of the water that is applied to bare soil is lost to the atmosphere through evaporation from the soil. The rate of evaporation increases with increasing air temperatures, solar radiation, and wind. In addition, the lower the humidity, the faster the evaporation. By applying a mulch in our landscape and garden beds we can reduce the amount of soil moisture lost through evaporation by as much as 50 per cent, depending on the type of mulch.

For landscape plants and perennial flower beds, I recommend using shredded bark or wood chip mulches applied on top of bare soil and maintained at a depth of 3-4 inches. Bark and wood chip mulches should not be used in vegetable gardens and annual flower beds because they will become incorporated into the soil. This causes a problem because soil microbes will use the nitrogen in the soil for the decomposition process, thereby tying up the nitrogen and making it unavailable to garden plants.

Where annual crops are grown and the soil is regularly tilled or disturbed, organic mulches that break down more quickly are advisable. I recommend applying well-rotted compost, lawn clippings mixed with compost, or lawn clippings as mulches. Keep in mind that the general recommendation is not to collect lawn clippings, but if you do have them available they can be recycled as a mulch. However, you should never use clippings if they have been treated with an herbicide without waiting the amount of time specified on the product label.

Never apply more than a one-inch layer of fresh grass clippings at one time because they mat down and start to decompose anaerobically, making a gooey mess. Instead, wait until the clippings last applied have dried, and then apply another one-inch or less layer. The clippings can be tilled into the soil at the end of the season, adding organic matter to the soil.

To increase the effectiveness of a grass or compost mulches, place one to two moistened sheets of newspaper on top of the soil, overlapping the sheets as you place them in the garden, and then cover the paper with a layer of mulch. (Without a cover of mulch, the newspaper will easily be blown away by wind.) Do not use glossy color sections of newspaper, as they may contain heavy metals or other chemicals that will contaminate the soil. The newspaper will decay over the growing season and then can be tilled into the soil along with the layer of mulch on top.

Rock mulches are suitable for areas vulnerable to wildfires or non-plant areas, but they should generally be avoided around landscape plants because they are heat sinks. The rocks absorb heat during the day and then radiate that heat back at nighttime, increasing the heat stress and water needs of plants. Light-colored and white rock also reflects light back onto plants compounding

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