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Mulch is for Winter

Norm's mulchAll over the world, gardeners know how important mulch is for a successful garden. It’s a multi-purpose landscape essential; in addition to its practical purposes, it has aesthetic value. Covering bare soil with a consistent layer creates an attractive unified theme. It’s an effective way to connect the varied plantings in a landscape. In any climate, and certainly in a desert, mulch is an ecologically sound way to conserve our limited soil moisture and to control weeds.  Almost any mulch material – gravel for desert landscapes, or straw, wood chips, bark, even shredded newspaper for other types of gardens – will work.  Each kind has its most appropriate use. Newspaper’s probably not the best choice for a formal garden, but all mulches serve comparable roles: shielding the soil and the plants growing in it.

Mulch is a very simple way to protect the plant roots and lower stem.  During the Mojave summer, it provides enough shade to protect the soil from the heat of our blazing sun.  With this cooling action, even plants that evolved in environments that are more temperate can grow successfully here in the Mojave.  

Recently I suggested guarding landscape plants by applying mulch in winter. That might be a foreign concept for some people. We know it’s critical for keeping down unwelcome plants and modulating soil temperatures in the summer, but why winter?

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This might be less obvious. After all, weeds are generally a smaller problem in the dead of winter; and cooler, shorter days mean that our landscape plants are growing more slowly, if at all, so they tend to need less water. Plant roots experience lower stress when they’re sheltered from temperature and moisture extremes: hot/ cold, wet/ dry. So mulch isn’t only a summer lifesaver, it can benefit plants during the wintertime as well.  Why would mulch be an important consideration for perennial plants like trees and shrubs, not to mention our cool-season vegetables, around this time?

It’s not news to people who live in southern Nevada that desert winters, particularly winter nights, can be surprisingly chilly, even freezing at times. The stress is extreme, especially during those freaky seasons when nighttime temperatures can drop significantly below freezing.

Plants can be astonishingly smart; they have chemicals dissolved in their cells that have much the same function as the antifreeze in a car, or putting salt on a sidewalk to melt ice.  This is the reason why a number of cultivated plants won’t freeze until the temperature reaches about 28°.

Unless they’re perennials with the capacity to go dormant, very few plants survive, much less thrive, when it gets any lower than that, though.

Having said all this, many of our prized horticultural specimens originated in hot, sometimes moist, climates. They haven’t evolved the tools to withstand the challenges of their new desert homes. They scarcely have the capacity to tolerate our relatively mild winters, much less when the temperature drops down lower than the mid 20’s that we can experience.  Despite that, adventurous desert gardeners frequently want to try introducing them.  To keep them alive, we need to adjust their environment.

Let’s view mulch as a kind of protective insulation, something like a thermal blanket, or insulation in the ceiling. For any marginal landscape plants, winter mulch is essential. This is certainly no less important than it would be during hot weather. When we shelter their roots with an additional layer of protection, cold-intolerant plants are less vulnerable to extremely chilly temperatures.  Plants generally benefit when they experience a narrower range of heat and cold.

Over the course of a year, mulch moderates soil temperatures – keeping them warmer in the winter and cooler in summer, playing a similar role as the insulation in our homes. As an element of the landscape, there are choices of materials that can be an attractive addition throughout the garden.

Don’t forget to call the Master Gardener Help line to get details of my expanded “Growing in Small Places” class. In 2014, we’ll cover topics from proper pruning to creating an organic garden. Hope to see you there.

For KNPR’s Desert Bloom, this is Dr. Angela O’Callaghan of the University of Nevada Cooperative Extension.

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