Thaw spawns summer’s blooms Signs of spring put gardeners on notice

Tiny bubbling pools of water erupt like miniature volcanoes throughout the landscape this time of year. Not only do they provide the first visual clue that spring is on its way, but also they offer their perennial soundtrack, along with robins and other spring birds. The pools and streams are our first sign that the planting season is just around the corner.

The thawing earth is retreating to its autumn form. When the ground froze in fall and winter, the water contained within the soil expanded and caused the soil mass to change shape. Thus, the ground is awkwardly formed with what we call “frost.” Dips and mounds can be seen here and there. Patches of snow prevent the soil beneath from warming in the spring air, while other areas are well on their way to greening the slender blades of grass. Frost-heaved fence posts ease back into the ground where they belong, righting gate latches so they again are flush with their posts.

In the spring months, as we await the thawing of the earth, we keep a keen eye on temperature readings. We are in tune with highs and lows and observe that the thermometer may hover near freezing, both day and night. As the daytime temperatures slowly and consistently gain an edge over the nighttime readings, plant activity will commence and hope for the new season will likewise begin.

Many environmental factors influence plant development here in northern New England and across the globe. The soil, human and animal activity and even other plants may affect plant growth. Above all, however, the ambient climate in which a plant is propagated and grown has a dramatic effect on the production and quality of the plant. Climatic factors influencing plants include light, temperature, precipitation, relative humidity, carbon dioxide concentration and wind.

These factors occur simultaneously and may individually or collectively determine the health of any given plant. In spring, we are especially aware of the interplay between temperature and moisture. A second kind of frost of utmost interest to horticulturists may strike on a cool morning in spring or autumn. A “late frost” can be the bane of any gardener. A late incidence of reduced temperatures in spring can easily damage or kill tender seedlings we may have sown or transplanted. A hard frost in fall can vanquish hopes for extending the gardening season into autumn.

Frost may be caused in one of two ways. “Air-mass frost” results with the inflow of a cold air mass that contains temperatures below 32 degrees F. Such frost is apt to happen during the winter months when cold air is prominent in the atmosphere. A second type, “radiation frost,” is most common in spring or autumn. This frost results from rapid radiational cooling of plant tissue to 32 degrees F or cooler.

Radiational cooling typically occurs in the wee hours, often between 4 and 6 a.m. In the darkness of night, a clear, black sky acts as an immense absorbing body that captures radiant heat from plants’ leaves. Plants may experience a crucial temperature decline of several degrees below the ambient air temperature.

A “white frost” occurs when moisture or “dew,” caused by the cooling plant tissue, collects on the leaves of plants and freezes into ice crystals. A “black frost” occurs when no evidence of moisture collected on the leaves is present, but the plant tissue is blackened from freezing injuries.

Radiational frost damages tender plant tissues in spring and can seriously damage plants in autumn if they have not yet been harvested or haven’t been stimulated to enter dormancy. However, nature can protect plants from the dangers of frost. A hefty cloud cover can act as a protective layer to plant material, effectively slowing radiational heat loss and shielding plants from the absorption of heat by the open sky.

Gardeners, of course, can employ several techniques to protect precious plants from frost. Gallon jugs or pails placed over tender tomato plants shield them from the dangers of heat loss. Row covers protect eggplant seedlings from frost. Rising early in the morning and monitoring and repeatedly spraying potential frost victims with water may shield them from heat loss.

Whatever the method, our efforts to temper nature’s fierce deliverances will soon begin!

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, 512 North Ridge Road, Montville 04941 or e-mail dianagc@midcoast.com. Selected questions will be answered in future columns. Include name, address and telephone number.