Gardeners cringe when a forecast for frosty weather comes over the airwaves just after they've planted all their spring vegetables. A frenzy to find something to protect those tender seedlings usually follows, and much sleep may be lost wondering what the garden will look like the morning after an unsuspected freeze warning. A few tricks and techniques can prevent freeze damage and allay unnecessary concern.
Not all vegetables are equal in their ability to tolerate frosts and freezing weather. Cool-season vegetables have no problem with a mild freeze in spring. These include peas (Pisum sativum), carrots (Daucus carota), broccoli (Brassica oleracea), as well as lettuces (Lactuca sativa) and most other greens. It's the warm-season veggies that need to be protected if freezing weather threatens. Tomatoes (Solanum lycopersicum), peppers (Capsicum spp.), squash (Cucurbita spp.) and cucumbers (Cucumis sativus) will appear burned where frost settles on their leaves, and a hard freeze will kill them outright.
Average Date of Last Frost
Knowing the average date of last frost in spring is important to avoid frost damage to warm-season vegetables. Local cooperative extension offices provide this information in every region of the country, and seed packets usually indicate a general window in which the variety can be safely planted. Of course, the average date of last frost implies that sometimes freezing weather comes after that date, so it is a good idea to wait an additional three or four weeks to plant.
If the need to protect vegetables from freezing weather arises, You have numerous options, from quick and simple row covers to ornate glass enclosures. Often, throwing an old sheet or tarp over the vulnerable plants is effective and feasible for those with busy lives. It is important to place rocks or stakes along the edges to prevent the cover from blowing off in the wind. However, there is a risk that the weight of the covering will damage the tender stems of young vegetables. A frame can be constructed to support the covering just over the vegetables, and kits are available in garden centers specifically for this purpose. It is usually sufficient to just cover the plants -- providing an external heat source can be impractical and is rarely necessary.
On cold spring nights, the actual temperature and distribution of frost varies over the landscape. Cold air drains down slopes almost like water and collects where it runs into an impediment of some sort -- a house, fence or dense hedgerow, for example. Locating the vegetable garden on slightly higher ground and avoiding "frost pockets" can make a big difference in avoiding frost damage when there is a freeze warning. Likewise, planting the garden on the south side of a house or wall means it's a lot warmer during the day and is likely to stay a couple of degrees warmer at night. This is especially true if the structure is made of brick, concrete or stone, which all hold heat and release it into the surrounding air deep into a frosty night.