Several years ago, Frank was traveling in Costa Rica. It was January, and as it happened, an unusually strong cold front had made its way across the Gulf of Mexico to Central America. Of course this front had hit Texas on its way through, but it was rare that a cold front would push that far south. The temperature in Costa Rica had dropped down into the low 70’s, and people were complaining! When folks in Costa Rica found out that Frank was from Texas, the response was universal–Texas! Hace muy frio in Texas! ( It’s very cold in Texas!) Apparently the average Costa Rican’s impression was that it was always cold in Texas! That’s where the cold fronts came from anyway.

It’s refreshing to have someone think of Texas as a cold place, given its reputation for blistering heat, but we do get our share of freezes. Texas lulls you into thinking you’re in the tropics, and then slams you with a Blue Norther that can drop the temperature 40, 50 or even 60 degrees in the same day. This year was no exception, and after six weeks of beautiful spring weather, we had a frost in mid April. This would be similar to New England getting a frost on June 1st. The peach trees had fruit the size of golf balls and we were picking lilies. We went into crisis mode! We had about $100,000 dollars worth of plant material out there–good grief!

There are many variables that come into play when trying to protect plants from frosts. There are frosts, and there are freezes. Generally, a freeze is when a cold air mass moves in and the overall temperature of the air is below freezing and stays there for a while. A frost occurs when heat is lost to the atmosphere on a cool, clear night. This is called radiational heat loss, or a radiant frost. Radiational frost can occur even when the air temperature stays above freezing, around 35 or 36 degrees, for example. Heat from the plant tissue is actually drawn out of the leaves into the surrounding air, and the tissue can fall below freezing and be killed. Radiational frosts are generally of short duration – a few hours or more before sunrise – but they can still be devastating.

Cold air is heavier than warm air, and on a still night, the cold air will sink below the warm air and actually flow downhill. This is what happens when people say you have a “frost pocket.” As cold air flows downhill, it gets trapped in valleys and low spots. Even a row of trees can hold cold air in a pocket. Unless your farm is perfectly flat, you probably have a frost pocket somewhere, and you can avoid trouble by planting that spot later in the spring, or with hardy plants.

As the cool air sinks, the warm air is pushed up, and settles in a layer just above the field as an “inversion layer.” This is a pretty neat phenomenon that occurs on perfectly still nights. The inversion layer can be quite warm, and often is not very far off the ground. Last year’s frost damaged a young sycamore in our field that had leafed out. All the leaves below 25 feet were killed by the frost, but above 25 feet the leaves were fine. This left the tree bare below with a “poodle-dog” tuft of leaves at the top.

In big orchard operations, you will often see giant fans on towers among the trees. These fans are there to take advantage of the trapped inversion zone. They mix the warm air above and prevent the cold air from settling among the trees. They are super-expensive, but so is losing a crop. Fans like this only work in a still, radiational frost.

There are several ways growers try to combat late frosts. One is direct heat. For many years, orchard growers have used “smudge pots”–heaters that burn oil and are spaced out in the orchard. It takes a lot of heat to offset a frost. Many years ago Frank worked for a big orchard in Michigan that collected old tires all year and threw them into a hollow. On cold nights, they would burn the tires. Nowadays, the EPA and the neighbors frown on this method! Lately we have seen an ad for a nifty, tractor mounted propane burner/blower that you drive through the crop, mixing up the air and adding heat at the same time.

Water can be an effective tool in frost control. Water holds a significant amount of heat, and we make sure our beds are well watered when we expect a frost. Not only does wet soil hold more heat, but the darkened wet soil absorbs more solar radiation on the sunny days that usually precede a frost.

Water can also be applied directly to the plants during a freeze. Water coming from a well can be quite warm, often 55 or 60 degrees. That’s a lot of heat. Also, when water cools down and freezes, it gives off heat (80 kilocalories per liter). Some growers use sprinklers to coat the plants with ice during a frost. This works because the freezing water maintains the ice at a temperature of 32 degrees. This is not cold enough to harm the plant tissue. We visited a large grower in Central Florida that has used this method on all stages of a snapdragon crop, from small plants right up to bloom.

Freezing water has its disadvantages though. Ice is heavy, and often damages the crop. And the water must be applied continuously until the ice melts in the morning. Otherwise the temperature of the ice would drop below freezing and the tissue would be damaged. Also, applying water takes a great amount of volume, and most farms aren’t set up to overhead irrigate the entire crop at one time, especially if you rely mostly on drip irrigation.

We use floating row cover for frost protection. Row cover can be very effective, but it too has limitations. Row cover comes in different thicknesses, based on weight. For example, .5 ounce row cover means that a square yard weighs 1/2 oz. In theory thicker row cover provides greater frost protection, but we have found that not always to be the case. We buy thicker (.9 oz) row cover mostly because it doesn’t tear as easily and we can get more seasons for our money from thicker row cover, despite the additional cost up front.

Row cover provides only a few degrees of frost protection, but sometimes that’s all you need. We have found that it is most effective when it is not touching the plants directly. In a radiational frost, some materials lose heat faster that others. That is why your car frosts up before the lawn does. Row cover fabric loses heat like car metal does, and will damage the plant material it is resting on. This isn’t a problem when you lose a few of the top leaves of a tomato plant, but when it is resting on flower buds, the buds will be killed.
Row cover works best when it is used as a barrier between the air around the plant and the surrounding atmosphere that is sucking heat from the plant. In that respect, lighter row cover can be just as effective as heavier material. By holding the row cover off the plant you create, in effect, a small greenhouse. The air inside insulates the plants.

We have found that the best way to hold the row cover off the plant is to use Tenax crop support netting. We often have this netting on the crop already. In a crisis (and trust us, these late frosts are indeed crises!), we will lift the netting up off the plants to the top of the posts. It will sag, but it will hold up the row cover enough to work. The hard part is getting the netting back down on the crop, but at least there is a crop to worry about!

We also run the drip irrigation under these row cover “tents”. The water is warm, and it does add a small amount of heat.

When row cover works
In a truly serious freeze we can get a great deal of protection using two layers of row cover. This usually applies to high-dollar, early crops such as lilies that are in bud. In such a case, we will pull up the netting to the tops of the posts, and then put a layer of cover underneath it, right on the plants. Then we will put another layer of cover over the netting so there is a layer of air between the row covers. We have had crops survive sustained freezes as low as 20 degrees. Sure, it’s a lot of work, but desperate times call for desperate measures!

Northern growers often use a clear plastic, slitted “tunnel” for season extension. This also provides some measure of frost control based on the insulating air space. Opaque plastic loses less heat than clear. We don’t use these tunnels here because we can sometimes get so hot in late winter that we would cook our plants! One year we had a week of 99 degree weather in late February. Right after that the temperature dropped to 14 degrees, and we lost everything. Good ol’ Texas!

One interesting concept in frost control that we are looking at is spraying kelp extract on the plants. According to the Peaceful Valley Farm Supply catalog (888-784-1722) kelp raises the mineral content of the plants, thereby adding to frost resistance. Mark Itz at McCall Creek Farms here in Blanco told us that he uses Maxi-crop on his tomatoes for frost and that it works. You need to start spraying well ahead of any cold weather so the plants have time to build up resistance. Kelp is a good idea anyway, and will help the plants in many ways.

We have learned by trial and error that plants differ in their ability to take cold based on the season and their growth stage. We have two basic types of plants to worry about in a spring frost. The first are the obviously frost tender plants such as zinnias and celosia. These plants will never be frost tolerant, and always need to be covered. The other group is more tricky. These are plants that are generally hardy and have overwintered for us, but now are actively growing and tender. Plants like snapdragons, ammi majus and statice can be wiped out by a late frost. Row cover is very expensive, so we have to make crucial decisions about which plants get covered. Here are a few specific crops that we have learned the limits for! These are just guidelines – you’ll need to use your own experience too.

•Dutch Iris- Very hardy until the bud is up in the leaves. If you can feel the bud down low, it can take about 28 degrees. When the bud is visible, a light frost can damage the flower. Row cover is very effective here.

•Snapdragons- Snap plants are very hardy but if the stems are really soft and tender, they can freeze. Buds can take a frost, but buds at a certain stage can be damaged. This will give you stems later on that will have flowers below and above with a large skip in the middle. Greenhouse varieties are more tender that the garden varieties and shouldn’t be planted early.

•Ammi majus- Ammi can take a hard freeze when acclimated, but when it is soft or growing fast, it will die to the ground at 26 degrees. We always cover the ammi.

•Dianthus- Our dianthus has always taken freezes well, except the year when it was 99 in February. We don’t usually cover it unless things are dire. The buds are usually hardy too.

•Lilies – Lilies always grow fast for us in the spring, and we have to cover them. We have had buds freeze in the winter cold frames, and they do thaw out, but below about 28 degrees, we see some damage. In the field, the frozen buds can desiccate easily, and they suffer more. This is a high dollar crop that we always cover.

•Sunflowers- Sunflowers are amazingly hardy, especially if they have had time to harden off. We have had them survive to 25 degrees without cover. Very young plants and older plants do best in a freeze. Plants about 4 weeks old have buds down deep that are most susceptible to damage.

•Statice – Statice plants are hardy to 26 degrees, but the flower stems will be killed in a frost. Statice is hard to cover with row cover because the cover rests on the stems and they freeze. We don’t usually net our statice, so we are always scrambling for a way to hold the cover off the stems.

•Delphinium – We cover the plants during the hard winter freezes, but the stems and buds can take some frost. Same for larkspur.

•Summer annuals – Zinnias, celosias, marigolds, gomphrena and the like. These plants can’t take a breath of frost. We always cover them, but we don’t go to the trouble of netting the beds to make a tent. That’s a lot of work, and these crops can be quickly replanted if they freeze through the cover.

So..good luck this spring. We survived our late freeze, so now we can look forward to droughts and hurricanes.
Our new website is up so take a look: www.texascolor.com. We make it look like it’s all wine and roses. Ah–the good life!

Pamela and Frank Arnosky are the owners of Texas Specialty Cut Flowers in Blanco, Texas. A collection of their columns, We’re Gonna Be Rich! Growing Specialty Cut Flowers is available for $19.95 plus $4 shipping from GFM Books, PO Box 3747, Lawrence, KS 66046; 800-307-8949, Or order online at www.growingformarket.com