Strategies for Helping Plants through the Drought


Pine needles and hostas

With the continued onslaught of extreme heat and lack of rain, all vegetation is under a great deal of stress. Nothing we can do about the heat, except wait it out. Many communities and water utilities are starting to implement water use restrictions to help conserve this precious resource.

Mulching.  One way to help the soil hold water longer in shrub beds after a rain or irrigation is to apply a layer of mulch over the surface of the soil. I wrote about this a couple of months ago, but it bears repeating.

Mulch is a protective ground covering that reduces evaporation, moderates soil temperature, prevents erosion, and controls weeds. Probably the most effective mulches are organic, which include leaves, bark, pine needles, compost, wood chips, straw and similar materials. Inorganic mulches include rocks, rock chips, synthetic fabrics, and other non-plant materials.

A major benefit of mulching is it greatly reduces soil moisture loss through evaporation. Both plants and our water bill are helped by reducing the need to water plants so frequently by applying mulch around flowers, shrubs, young trees and in the vegetable garden.

The insulating quality of mulch helps keep the soil cooler in the summer. By maintaining more even soil moisture and temperature, mulch promotes better root growth and plant health.

Mulch suppresses the growth of many weeds which compete with our plants for water. A 2- to 3-inch layer of organic mulch should be sufficient to prevent sunlight from reaching the soil, thereby reducing the chance of weed growth. Any weed seedlings that do manage to germinate and break through the layer of mulch are easily pulled. A mix of coarse and fine mulch particles helps reduce seed germination.

Organic mulches enrich the soil as they decay, forming a rich, dark organic material called humus that provides nutrients for the soil and improves its texture, which in turn enhances root growth.

Because organic mulches break down over time, they need to periodically be replenished. Therefore, it’s a good idea to check the garden a couple times during the year and renew areas where the mulch has gotten thin. There is no need to remove the old and replace with new mulch, since soil organisms will work the decomposing organic matter into the soil, increasing the health of the soil.

Just don’t pile it on too deep! One of the biggest mistakes folks make when applying mulch is to create little (or big) volcanoes of mulch piled up against the trunk and around the base of newly planted shrubs and young trees. If the mulch stays wet, this can cause decay problems on the bark of the trunk. And, if the mulch is too deep, the roots can be starved for oxygen, and/or grow up into the mulch. The International Society of Arboriculture recommends a mulching depth of 2 to 4 inches, pulled away from the trunk of the tree (kind of like a donut) to expose the natural “flare” of the roots.

Watering Trees.  A common question is how to water established trees. This is a little difficult to answer because trees have such an extensive root system, often extending 2 or more times beyond the ends of the branches. A tree’s root system is not a mirror image of the above ground portion. The majority of the roots involved in water and nutrient uptake are in the top 18 inches of soil.

The root system near the trunk of an older, established tree is mainly involved with support, with fewer feeder roots located in that area. For this reason, watering of an established tree should be concentrated more towards the outer spread of the branches, an area often referred to as the drip-line.

Water established trees every couple of weeks during a drought. Apply enough water so the soil is moistened 6 to 8 inches deep. You can check depth of penetration by pushing a probe or long screwdriver into the soil. It will meet resistance when you hit the dry zone. Concentrate your efforts in shaded portion of the tree.

For younger trees, water more often, and consider eliminating grass in a larger area near the tree to reduce competition for soil moisture. Apply a mulch, not too deep, to stop evaporation. If applied too deep, light irrigation or rain may not penetrate to the soil.

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