Beware of frost til mid-May

By Julie Artz, Special to the Rocky

Published April 17, 2008 at 7 p.m.

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The woman at the nursery told me to harden off the tomato plants I just bought. What does that mean?

Tender plants like basil, tomatoes, peppers and annual bedding plants frequently appear in the nursery well before they can be planted outside safely. These plants, raised in controlled conditions, cannot survive the winds, snow, hail and freezing temperatures often seen in Colorado's Front Range at this time of year.

Plants that have already been purchased need to be kept in a sheltered location (a sunny window or even the basement or garage) until they can be hardened off. To harden off, place plants outside on days where temperatures are above 50 degrees, gradually increasing their time outside each day until planting-out time.

Cut back water while hardening off the plants to help them adjust to the drier climate; seedlings should not wilt, but the soil should be allowed to dry slightly between waterings. This process should be started at least one week before planting outdoors.

For the most tender plants, plant outdoors in mid-May unless they have some protection from the elements. A cold frame or cloche would work. Even an old milk jug or empty 2-liter bottle, when inverted over the new seedling, can provide enough protection to withstand a light frost.

In more inclement weather, covering the plants with a tarp or sheet overnight may be necessary.

When should I prune my roses?

Most rose cultivars lose canes to cold, ice or wind during the winter in Colorado. Pruning these damaged canes helps prevent disease and improves the shape and flowering of the plant the following season.

However, pruning also releases hormones that stimulate the kind of lush growth that is particularly susceptible to frost burn. CSU Extension recommends waiting until two weeks before the average last frost date (which occurs in mid-May for most of the Front Range and later in mountain areas) to prune roses.

Before pruning, clean and sharpen your tools. Then prune the roses back to live wood, which can be recognized either by the appearance of swelling buds or by a somewhat-flexible cane that, when cut, has a green ring of live growth around a white center.

Use a 30-45 degree angle when pruning and leave no more than a quarter-inch of growth above the last swelling bud on the cane.

Although this type of winter damage removal should happen in the spring, restoration pruning and thinning of shrub roses, climbing hybrid tea roses and climbing grandiflora roses should wait until immediately after the June bloom.

Julie Artz is a master gardener at Colorado State University Extension Agency in Boulder County.

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