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Gardening in a Changing Climate, Part III: A World Without Water

By: Sierra Laverty, Assistant Horticulture Director | 01/14/2019

Boise, Idaho—home to Idaho Botanical Garden—is known as the City of Trees. There is an elusive fact that our city’s shade trees and lush lawns conspire to hide. We live in the desert.

Southwest Idaho lies in the heart of the sagebrush steppe, a xeric grasslands ecosystem that covers 120 million acres across the North American West. The annual precipitation of the Treasure Valley is 12 inches. A foot of rain and snow is meager compared to precipitation in Seattle, Washington (37.5 inches), but three times that of Las Vegas, Nevada. Water has always been precious in the West. How will that resource fluctuate as our climate continues to change?

In Idaho, the amount of annual precipitation may not be as important as the timing and form that water comes in. As we discussed in Parts I and II, warming winters have rippling effects on our wild and urban landscapes. Our “snow days” may turn into rain days.

Water combined with warmer temperatures can cause gardens to “wake up” and break their dormancy too early. Leaf and floral bud creation are dangerous in the winter due to the likelihood of frost damage. New foliage will turn black and die when this occurs. Snow can act as both a water tower and cozy blanket; gradually releasing water while insulating the soil. Excessive rain can cause soil saturation and erosion, especially in urban clayey soils. Plant roots need air to survive, and too much water can asphyxiate them.

Southwest Idaho summers are predicted to be hotter, drier and longer. Just like people can experience heat illness, many plants suffer stress and can even perish in the hot sun. Take stock of your plants’ reactions to heat stress. Do some plants appear fine, while another’s leaf margins are brown? Are the leaves “flagging” or wilting downwards? If so that species or cultivar may not be suited to our climate.

Plants have fascinating ways of dealing with heat. Some have fleshy, water-storing roots (like four o’clock, Mirabilis spp.), and some go dormant during the summer (like spring bulbs). Some mine deep into the soil with a tap root (e.g. netleaf hackberry, Celtis reticulata), or spread their roots thin to collect every drop of rain (like Cacti). Others, like Sedum, have even adapted a novel and more efficient way of photosynthesizing (CAM photosynthesis).

As our seasons offer new challenges, we have the opportunity to evolve with them!
● Install a roof rainwater collection system
● Plant xeric or water-wise plants (visit our many demonstration gardens for inspiration!)
● Choose shade trees that are drought tolerant and adapted to our high pH soils
● Convert unutilized turf and lawn areas to drip irrigation and perennial plantings
● Consider replacing annual bedding plants like Petunia with perennials, which may consume less water from year to year
● Use bark or other organic mulches to conserve soil moisture, especially around the base of your trees