Push Back Your Garden's Frost-Free Date

By JEREMIAH McNICHOLS

For most gardeners, the year doesn't truly start on January 1, but on their local average frost-free date, and we prepare to make the most of it by starting seeds early indoors and in greenhouses, mixing early variety crops in with our best-tasting favorites, and anticipating the vagaries of the true frost-free date for the year by reading nature's cues and occasionally pressing our luck. In addition to these preparatory measures, an understanding of the effects of microclimate on the frosts and cool temperatures that keep us from our work can give gardeners an additional leg up on the growing season.

Visit potential or planned garden sites on a cold morning when there is frost on the ground, and examine the patterns of frost presence and intensity. Sketch it on a page in your garden journal for later reference. Then examine your sites for ideal locations based on the criteria below, and compare that with the frost map you created. Chances are you will see some corrolations, as well as get ideas for ways you can improve early spring heat-retention in your garden.

1. Location in relation to the sun.

South-facing slopes receive more intense sunlight than north-facing ones, and east-west running fences and walls reflect more sunlight to the south side and shade the north, while areas adjacent to east-west running ones are warmer on the west and shadier (and cooler) on the east.

During the summer, the earth itself acts as a giant "heat sink," radiating heat stored during the day through the cooler nighttime hours. Air temperature drops most rapidly during the winter months, when the sun, at its lowest point on the horizon and making its shortest appearances of the year, has the smallest effect on soil temperatures.

Small heat sinks in the garden can help maintain warmer winter nighttime temperatures in "small" climates. The most effective of these is water, which releases stored energy most slowly and thus provides heat as long as possible as the surrounding air gets colder and finally bottoms out in early morning. Small ponds can provide a source of evening warmth, as well as a habitat for pest-devouring amphibians and fish. On a larger scale, plastic drums of water (the 55-gallon variety used in the food industry, for example) can be used to build walls or spaced throughout the garden.

Concrete or brick walls or paths and rocks used for landscaping or pathways also serve as heat sinks. The larger the material's mass, the more heat it will store during the day and release gradually at night.

Trees, hedges, fences and walls can protect the garden space from night winds, which flush warm air out of the garden and from the immediate self-warmed space around individual plants.

Trees or arbors offering shade directly overhead can also slow cooling, as warm air will be discouraged from rising directly out of the garden. Such locations are only recommended for shade-loving plants, and are not likely to provide sufficient sunlight to satisfy garden crops. Keep in mind though that plants located along a wall that gets good sunlight but is sheltered by a shallow eave will have a similar advantage.

Cold air can be collected and retained by a dip or sheltered basin in the garden, and such areas are the first place for frost to collect. In contrast, cold air is easily flushed out in sloped gardens -- as long as the garden is not at the bottom of the slope.

Hedges, fences and other barriers that block wind can also stop frost on a hillside. A windbreak running along the slope of a hill can block frost from traveling down past it, as sinking cold air is stilled or slowed by the barrier.

A site that receives early-morning sunlight will offer plants some respite from serious frost damage when frost is unavoidable, and minimize the effects of a light frost on tender plants.

Coldframes and hothouses are mini-greenhouses that are easy to build and are excellent for starting or hardening off seedlings for the garden plot. The former relies on stored warmth (air warmed within the coldframe during the day, transpiration, and stored heat released into it from the soil during the night) to keep temperatures warmer, and the latter uses a subsoil warmer (a layer of manure, circulated hot water, or electricity) to generate heat.

Similarly, cloches can be used to maintain warmth for individual seedlings. Rich Victorian gardeners used glass bell-shaped cloches; today most of us are satisfied with liter plastic soda bottles. Since cloches work by retaining the warming effects of transpiration, not by keeping the day's warm air trapped inside (which cools too rapidly to be of much use) these should be removed during warm days, as the temperatures inside can get quite hot.

Visit Farming For Artists' GreenMachine garden planner, where you can adjust your average local frost-free date based on your garden's microclimate and your own best guess.