Drought and Perennials
by Martha Smith
July 27, 2008

"All plants need water. When natural rainfall does not provide enough, plants suffer," said Martha Smith. "The term 'drought' refers to a period of time when precipitation is well below normal or completely lacking. During this time, the water content of the soil is reduced to such an extent that plant material can no longer extract sufficient water for normal life processes.

"Large plants such as trees have extensive root systems capable of reaching other possible water sources nearby or deep in the ground. Perennials and annuals don't have as extensive a root system and therefore are subjected to the conditions immediately around them."

Some annuals and perennials can tolerate dry conditions for a long time, while others succumb very quickly, she added.

Wilting is often the first visible effect of drought noticed by the home gardener. Perennials and annuals are herbaceous ornamentals. They do not have a woody structure to support them.

"What holds a herbaceous stem up has a lot to do with the water pressure within," she explained. "Water is pulled into roots because of evaporation at the leaf surface. A water column forms because water sticks together. As it evaporates into the atmosphere through the foliage, it is replaced through the roots.

"If foliage evaporation exceeds the roots' ability to pull in water due to drought, this column breaks and wilting results. If additional water is supplied within a reasonable amount of time, most perennials and annuals will respond and continue normal growth. If drought conditions continue, plants wilt beyond their ability to recover."

Plants that have suffered through one drought may not be able to store enough food to help them through another drought year. They may be smaller in overall size or produce fewer flowers. If dry conditions and high temperatures continue over several seasons, these plants may be lost.

What can be done to minimize drought injury to herbaceous ornamentals? Smith suggested seven tips.

First, prepare the soil. Increase the moisture retentiveness of your garden soil before you plant. Dry, sandy soils benefit when compost and aged manure products are incorporated.

"These materials give soil water something to adhere to," Smith explained. "Tight clay soils also benefit from the addition of these materials. During drought, clay soils become very hard and are difficult to re-wet. Water runs off before it can penetrate. Compost and aged manure open up clay soils and allow easier water penetration."

The second tip is mulch--adding a two-to-three inch layer of mulch helps soil retain moisture.

"Third, group plants according to their watering needs," she said. "Most plant reference books have plant lists for various growing conditions. Artemisia species, Coreopsis species, and Sedum species are all tolerant of sunny, dry locations. These should be considered for a potentially dry area.

"Avoid moisture lovers such as Astilbe species, Trollius species, and ferns."

Fourth, consider planting time. Perennials planted in late spring or early summer may not have enough time to establish a sufficient root system needed to survive a drought. Plant perennials in early spring or fall to avoid this problem.

Fifth, remove moisture competitors.

"Weeds and turf are also seeking water to survive," she said. "Get rid of these and you will have more available soil moisture for your ornamental plants."

Cutting back on fertilizing is the sixth tip. These additional nutrients promote growth, which increases water demand. Plants should be kept lean and mean during a drought.

"Finally, supply additional water," said Smith. "Herbaceous ornamentals benefit from one inch of water every seven to 10 days during the growing season. If this is not supplied by rainfall, drag out the hoses."

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