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Dodging the cold: Preparation key in safeguarding plants against springtime frost

Recently I was enjoying my favorite occasional special “creamed chipped beef” at the North Atherton Waffle Shop, and Candi, Mary, Jennifer, Tammy and I were chatting about the weather and how the arrival of spring is just around the corner on March 20.

I pointed out that they need to be aware that frost can still catch us and ruin our early gardening activities. This is certainly not uncommon for our part of the country, but we can take some precautions and minimize damage from frost.

Just like the ladies at the Waffle Shop, many of us are chomping at the bit to get out in the garden, but we have to remember that there could be killing frosts quite late in the spring.

Now is a good time to begin planning how to deal with low temperatures late in May when it’s time to move transplants outside. Don’t wait until forecasts call for a hard frost. Get ready now and decide what protective materials you will use and which plants will need your help.

It has been my rule of thumb to wait until mid- to late April before setting out hardy broccoli, cauliflower and cabbage transplants. Even these may need frost protection when temperatures are predicted to go well below freezing. Later in the season, daring gardeners, who are determined to have the first ripe tomato by setting out plants in mid-May, will definitely need to be prepared for frost protection.

Frost is not so mysterious as it may seem. During the night, when the earth receives no radiation from the sun, the ground loses heat to the atmosphere. If enough heat is lost, the surface temperatures can drop below freezing. Whenever two adjacent objects have unequal temperatures, (air and soil in this case), the colder always gains heat at the expense of the warmer.

When humidity is high and the night air is below the dew point (the temperature at which the relative humidity is equal to 100 percent), but above freezing, moisture condenses and forms dew on garden surfaces. If the air temperature drops below the dew point and also below freezing, then we have frost.

You can prevent frost from forming by using hot caps, cloches, rows of vegetables covered with plastic and other fabric materials placed over wire hoops, or floating row covers laid over the crop and finally a small high tunnel. These allow (or are removed to allow) short solar rays to penetrate the soil in the day. When set in place, at night, the protective coverings trap the long rays leaving the soil and create a warm soil environment around the plant.

Hot caps have been around for a long time. The nice thing is that they can be quickly placed over tender annuals when frosts or cold evenings are in the forecast. Unless they have holes in the top for venting, they must be removed in the morning, before sunlight raises temperatures inside the hot caps to levels dangerous to your young seedlings. While they are inexpensive and do provide protection against frost, they are not good for extended periods of freezing weather. I have found that plants often quickly outgrow the size of the hot caps. Commercial growers largely stopped using hot caps due to the labor involved in deploying them and went to row covers, etc.

Another technique is using water in a container. This group includes Frost Jackets and Wall O’Water, also called a “Garden Tepee.” A Frost Jacket is part hot cap, part miniature greenhouse. Set a Frost Jacket around a plant and fill it with water. It absorbs heat from the sun during the day and releases it at night. It can stay around the plant all season long, or be removed after the last spring frost. Because of its heating quality, it is better than a hot cap, and can give your plants an even earlier start. It is a great way to get a jump on the season. Something as simple as a large plastic drinking cup can be placed over small tomato or pepper seedling. If the plant is too big, try a gallon milk jug or a five-gallon bucket!

If you need to protect a long row of plants from a late frost or want to start a row of vegetables a couple weeks early, a cloche may be the answer. A cloche is a mini “Hoop House,” standing 1 to 3 feet high at the center of the row. A series of bent tubing, either metal tubing, PVC, poly tubing or No. 9 gauge wire, is hooped every couple of feet along the row. Solid plastic sheeting or row cover material is placed over the hoops. The edges can be buried. The ends are covered during cold weather, and uncovered on warm days. It offers inexpensive frost protection and allows a few weeks’ head start for your plants. It is removed as warm weather arrives. You can make your own or purchase already-manufactured kits.

When frost is in the forecast, just about anything will do for a quick cover-up. The object is to cover your plants so moisture in the air does not form on the plant’s leaves and freeze as the temperatures drop to 32 degrees or below. For covering large areas, a plastic sheet, a bed sheet or a blanket will do. These items need to be lightweight, as you don’t want to crush the plant’s leaves or break the stems. If you are concerned about this, place a bucket or two under the cover to keep it off the plants. Commercially, you can buy inexpensive row covers. These items are commonly used in the fall to extend the season beyond the first killing frost. A little effort can be very “fruitful” as vegetable prices rise at local stores as soon as the first frost hits. Lightweight row covers need to be anchored to keep the wind from blowing them off your plants.

A final method is the use of coldframes. They have been around for a really long time and are really popular for getting a much earlier start on the season. Keep tender, young seedlings warm and provide them with more sunlight than they get indoors. Coldframes can also used to “harden off” your tender indoor transplants. There are directions readily available for building your own coldframe, or you can purchase pre-made kits.

I hope now that you will be ready when Mother Nature’s frosty breath visits us this spring.

Bill Lamont is professor emeritus in the department of plant science at Penn State and can be reached by email at wlamont@psu.edu.
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