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Spring Frosts & Snows/ Perennials

April 19, 2016

Snow in the spring is welcome. The snow provides great insulation for plants and can protect hardy flowers from frosty temperatures. The moisture provided is a real boost to bulbs and perennials that are ready to emerge. If daffodils and hyacinths are blooming and snow is predicted, cover them with a bucket or similar container so the weight of the snow doesn’t break them.

Tolerance to freezing temperatures depends on the plant. Spring bulbs, such as tulips, crocus and daffodils, can tolerate a light frost or temperatures of 29 degrees Fahrenheit. Perennials that emerge early in the spring are rather cold hardy and can also tolerate a light frost.

Pansies, snapdragons, and dusty miller are examples of spring bedding plants that tolerate cold. Young petunias will tolerate some frost. Most new transplants don’t tolerate frost at all so wait to plant them. Plants like impatiens, tomatoes and basil should be planted in late May to June as they don’t tolerate cool weather.

If the forecast is for a hard frost, cover perennials and bulbs just to be safe. Plants located close to the house receive some protection and might not need additional protection. Plants on the south side of buildings and south-facing slopes come up earlier in the spring because the location is warmer and therefore are more at risk with a frost.

Colorado’s relatively warm days and cold nights, extreme temperature fluctuations and drying winds can wreak havoc with many of our commonly planted perennials.

Generally, after the first hard frost, the foliage of most perennials starts to die and wither. There are two philosophies on when to remove the dead foliage. One approach is to immediately remove the dead foliage. Many perennials will suffer no harm as a result, as long as you exercise great care when applying mulch. The other approach is waiting until spring to remove dead foliage. Always remove diseased foliage to discourage the spread of leaf-spot diseases and fungal problems.

Dehydration is a common problem when snowless winters occur. A layer of mulch several inches thick helps retain soil moisture. This mulch should be coarse and loose to permit air movement to roots. Root tissues continue to metabolize in the winter and requires oxygen for this process to take place. Reduced soil oxygen level increases the aggressiveness of many soil pathogens. Mulches which pack down should be avoided.

Watering at least monthly under dry winter conditions recharges the soil profile with moisture critical to plant survival.

Whether you decide to remove dead foliage in the fall or wait until spring, mulches provide the best protection for your perennials. Many types of mulches are available, and no matter which you choose, there are a few guidelines you should follow.

Mulches do a better job of insulating plants when space is allowed for air to circulate. Mulch that packs down to a dense mass during winter can cause mildews and molds to form. Shredded leaves from deciduous trees and pine boughs from discarded Christmas trees offer great winter protection. A good organic compost used as mulch is also effective and can be used as a soil amendment in the spring.

Established perennials and bulbs benefit from mulches that are applied after the ground freezes, because mulches don’t allow soil temperatures to fluctuate as much throughout the winter.

Mulch should not be removed too early in spring or plants will begin to grow too early. Plants located on the south side of a building or wall will emerge sooner than those in other areas, but may be subject to spring frost damage. Some perennials can be damaged by drying winter winds and western sun. Fencing can be used to effectively to protect plantings.

Denver, CO.


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