Frost

by Ida Livingston of Davis City, IA

How many times has the gardener, even the most seasoned of gardeners, been coaxed and tempted by the warm spring sun and breezes to chance the frosts and plant too early? Slightly crazed by cabin fever and beautiful seed catalogs, these Sirens have waylaid many a gardener who knows better into a garden of frosted tomato and pepper plants.

Spring

Any area will have an average last frost date. Here in southern Iowa our technical last frost date is May 10th. This means it can frost that late and still be a “typical” year. However, I, like any normal impatient gardener, plant my garden just as early as I think I can get away with. I read the weather forecast and then try to slip transplants in the ground while Jack Frost isn’t looking. When he catches me though I sure pay for it in spades, or at least row cover.

It is not uncommon for an area to have lovely spring weather, a nice long string of frost-free days, broken by a cool dip not too far out from that last frost date. Note, there is a difference between a frost and a freeze. A frost is a breath of ice crystals brushed over the leaves. A freeze is well self-described. The grass can crunch as you walk over it, plants are frozen through and brittle.

What is a chance of frost? When the forecast calls for 32 degrees Fahrenheit? Honestly, I have seen patchy frost when the forecast called for 39 degrees. When it calls for anything 40 degrees or less, I am out there covering anything I care about. If I don’t know what the forecast says, and my thermometer reads anything near 40 degrees when I go to bed, I expect a frost. It is just better to play it safe.

There are conditions that affect whether it can frost or not, even if it is cold enough to. First of all, there needs to be a dew. Warm air can hold more moisture than cold so when it cools off at night the air releases the moisture as dew. If it is breezy and wicking the moisture off of the plants, this leaves no surface moisture to crystalize and burn the sensitive leaves. If there is no dew the temperatures have to actually get down far enough to freeze the plant to damage it. If it is overcast, the cloud cover can increase the greenhouse effect, stabilizing the temperatures just enough to keep them from dipping as dramatically.

Covering plants does a couple of things to protect them from frost. First it keeps that breath of frost from having direct contact with the tender leaves it covers. Second, it catches just a little of the heat coming out of the ground and slows its release into the night air. This can raise the temperature around the plants just enough to help keep off that film of ice crystals.

Plants can be covered with buckets, sheets, shade cloth, just about anything can be used in a pinch. When using metal or plastic containers, try to keep the leaves from touching the sides. Sometimes the leaves can be damaged by touching the sides when the outside temperature is conducted by contact instead of buffered by air.

If it is going to be particularly cold, cover the plants earlier in the day. This allows the air under the cover to be warmed a little with some solar heat before bedding down for the night. Wait until the sun is out and the temperatures are above frost before you begin uncovering. Be cautious about leaving plants covered through the day. Do not underestimate solar power, it can get almighty hot under that shade cloth by noon.

If you were caught unawares and wake up to lightly frosted plants, before the sun hits them, spray them with water, rinsing that frost off. This slows the shock that they get from the sun and can lessen the extent of the damage. This is akin to the principle of not putting frostbit fingers straight into warm water, start with cool. Plants need this too. This will not save the plants every time but does enough to mention.

Even cold tolerant vegetables, such as cabbages, lettuce and other greens, can be susceptible to frost if not hardened off. So, when you buy your tray of cabbages in March or April, introduce them to cool but not cold temperatures and then bring them indoors when the air has a bite. Expose them to cooler and cooler temperatures before putting them in the ground for best results.

Corn can be frosted off up to 4 inches tall and still come back. Potatoes can be frosted off and regrow, but they will dock the harvest for it.

Last spring, I started a bunch of tomatoes and other vegetable seed in a cold frame. Well winter wanted a last hurrah and wasn’t just going to frost but wanted to freeze hard. My tomatoes and peppers were 8 inches tall in this glass covered frame but had no heat at night besides what it collected solar during the day. The solar heat it caught and then covered with a blanket was not going to save them from this freeze. So, I took several votive candles, buried them in the dirt a little bit and lit them. They put out just enough heat in the air to preserve my plants without injury.

Fruit Bloom

Strawberry blooms are particularly susceptible to frost. Any little frost can bite them. In strawberries, frosted bloom means no berries. How do you tell if they were damaged? The little yellow/green center of the bloom, when frosted, turns black. Now this bloom is dead and will produce no fruit. These blooms will show a black center within a day of the damaging frost.

When I grew up, we had a semi-commercial strawberry patch in northwest Ohio. Every year it tried to frost our bloom. So, we’d be out there spreading huge sheets of shade cloth over a couple acres of strawberries. You had to be careful with those large expanses of shade cloth, just the slightest breeze could pick it up. We blocked it down carefully. It got away from Dad and Mom once while they were uncovering the patches in the morning after us kids were in school. The wind draped that huge sheet over a large oak in the fence line.

Tree fruit, particularly stone fruit such as peaches, tend to bloom during the season of frost danger. When my family moved to Tennessee and was planting the orchard, we were notified by anyone who noticed that it was a waste of time. “You can’t grow peaches in Tennessee.”

Undeterred, they were planted anyway. But we did find out why we were warned. Warm, beautiful early springs break trees’ dormancy causing them to bloom. Then the predictably fickle spring weather delivers what the locals affectionately call “blackberry winter,” a hard frost around the time when the blackberries bloom. This frost somehow never seemed to slow the blackberries down too much but sure was hard on peaches.

The peaches however, put up a good fight. When the bloom is wide open and at its most vulnerable, it can withstand 27 degrees at a 10% loss. By 24 degrees, they lose 90% of their fruit. Miraculously, not all of it. It also depends how long the temperatures were held if it reached down and just touched that low temperature or if it stayed down there several hours. When the bloom has not yet opened it is less vulnerable; the tighter the bud, the colder the temperatures it can withstand. Once the bloom has set fruit it is also less vulnerable.

Almost every year without fail we had a cold snap that threatened our peach bloom. To combat this, my father set up a sprinkler system. The hope was to encase the bloom in ice, locking the temperature in at 32 degrees, and thus no loss. He would wait as long as he dared to turn on the sprinklers. If he turned it on sooner than necessary the limbs could, and sometimes did, break from the weight of the ice. When it began to reach this point, he would turn off the sprinklers. The water spraying through the air warmed it slightly and the fresh coats of water freezing over the bloom kept them at 32 degrees. But when the water had to be turned off, the cold could seep through the ice to the bloom and take the temperature much lower than the safety of 32 degrees.

Our apples bloomed later and were in less danger of those late frosts, or rather, freezes. We did have a couple times where they were threatened though. The apples were not as conveniently planted in rows as the peaches were, so sprinklers were not as easy to set up. We covered the tree completely, all the way to the ground with shade cloth and then hung a lit kerosene lantern in its branches (but well away from the cloth). This small heat source enclosed under cover kept the temperature just high enough we suffered no damage. This really only works if there is little to no wind and the shade cloth touches the ground all the way around the tree, or it is bunched and tied to the trunk of the tree. Sometimes smudge pot fires are burned in orchards to ward off frost. We did this once or twice but did not have our trees planted close enough to make it as effective as it could have been. Smudge fires are just little bonfires burned throughout the orchard to offer radiating heat and warm wafting smoke as a protective barrier against the cold.

Fall

Every area has a first frost date. It does not guarantee anything but gives you an idea where to base some expectations. It can frost a few days on either side of this date.

In my experience, the plants that are so tender in the spring, are much less sensitive in the fall. I have seen peppers and tomatoes shrug off light to medium frosts and usually take a freeze to actually kill them. One thing in their favor is they have more mass and whether or not they actually generate any extra heat they certainly are able to retain it. If your temperatures are going to dip down for a night or two and then warm right back up, it may be worthwhile to cover the plants to coax a few more days out of the summer garden before it is done for the year.

The plants that need immediate attention and care at the slightest chance of a fall frost are sweet potatoes. If these have their leaves frosted and are not immediately dealt with it can pull the decay down into the roots and render the tubers inedible. If you see it is going to frost you do not have to dig them immediately, but you do need to disconnect the tubers from the vines. Pull out the vines from the plant base. This will not usually unearth the potatoes; it simply pulls the vines off. If you find them already frosted in the morning, pull the vines immediately. Once this is done you can dig them at your leisure within a few days.

Frost usually occurs just before dawn, the coldest part of the night. I saw it happen once. Having to leave very early and realizing it was going to frost on my sweet potatoes, I was out in the wee hours of the morning freezing my fingers in the cold wet dew pulling vines. At a certain point, it went from dew to frost within a couple minutes. It really did feel like a breath of frost moved over the landscape. Remarkable. But it didn’t make my fingers feel any better.

The fall frosts are followed shortly by freezes, effectively ending the regular growing season and bedding down the earth for its season of rest.