So what about plants that have been in your yard all winter? They naturally harden-off and are Michigan weather tough. They naturally harden-off to the elements and the process is completely natural. Frost becomes a problem in May however when the plants leave dormancy, begin to flower or produce tender foliage and a hard frost (temperatures in the 20’s) comes along at just the wrong time. These plants are well rooted and established in your landscape. They may suffer a setback for the season. The cold won’t kill them but a year’s “show” or fruiting could be affected. Plants that are not established yet, like tender annuals, fortunately we have options and can wait to plant them, or cover them or bring them inside until things improve. Tender plants like tomatoes that start life in a greenhouse or under lights in your home, have spent the first few weeks of their life in pampered comfort. The temperature is ideal, the air is calm, water is gently and readily applied. Protected from storms or scorching sun it is the life of Riley where everyday is perfect. Pampered plants are like humans, they can get “soft” unless tested and hardened off by environmental challenges. Some tender plants however are tough guys and can stand up to a frost, pansies and cole crops (broccoli or brussels sprouts anyone?) come to mind as tolerant to some frost.
A hard freeze can kill blooms on trees waiting to be pollinated. The hold your breath moment occurs when fruit trees and orchards face this seminal moment. If temperatures dip to or below 29 F, damage and death occurs to the flowers before the temperatures drop to 25. The end result is a lost or diminished crop for the year. The amount of time the freeze occurs overnight is one important factor. Freezing temperatures for an hour before sunrise is a lot different than freezing temperatures that last 3,4 or 6 hours.
The type of freeze matters. Windy conditions called “advective” or the rate of change of temperature caused by the horizontal movement of air are almost impossible to protect vulnerable plants. Classic freezes, where conditions are calm and the sky is clear, are at least a little easier to establish a battle plan of action. I know old seasoned horticulturists who have been through many spring battles with frost. They tend to pay attention to the dewpoint. The dewpoint is when the air becomes saturated with water and water needs to be condensed out of the air for the temperature to drop further. In dry air, often the clear sky calm air scenario, the temperature will drop quickly to the dewpoint and then slowly after the dew point is reached. If the dewpoint is lower than the critical temperature that will damage the fruit, injury is likely. Radiation freezes are characterized by clear skies and dry air. Often the day is warm and pleasant when the sun is out. During the day, the ground absorbs heat from the sun and gets warmer. At night, the ground radiates this heat back to the sky. A layer of clouds can help trap the heat by absorbing this radiation (and in this case clouds are our friends). A clear night sky allows that heat energy to escape to the open sky. As the ground cools, it chills the layer of air above it. A lack of wind prevents mixing in the atmosphere so temperatures near the ground can get very cold. Radiation freezes usually follow a cold front, preceding a mass of cool, dry air. There is a stormy period as the cold front moves through, followed by clearing and light winds. During the night, the ground cools by radiation to the sky. The cold ground chills the air close to the ground. This layer of cold air becomes thicker and thicker and a temperature gradient occurs between the cold air close to the ground and warmer air above it. Normally, warmer air is located near the ground and the air temperature falls as you rise upward in the atmosphere. In a radiation freeze, this is reversed and cold air is located close to the ground with a warm layer above it. This is called an inversion. Because most homeowners don’t own or live in close proximity to giant wind turbine fans, look for the high ground for safety. Low areas during frost events get the coldest and experience the most freeze damage. Colder air is denser and heavier than warm air.
Let’s look at some of the methods used by Fruit Growers to mitigate frost damage to flowering or budding fruit trees.
- They use air moving fans to increase air movement to decrease freeze injury. I have seen helicopters used for this purpose. Not a cheap proposition but if your livelihood hangs in the balance a possibly wise insurance investment.
- Weed-free soil retains more heat than freshly cultivated or unmoved sites and a few degrees may make a difference in a freeze.
- Frost fans mix warm air into the orchard when inversions are present (perhaps 2 to 5 F). Inversion towers measure the temperature above and if inversion is possible just a few degrees can make the difference.
- Under-tree sprinklers will provide heat from groundwater into the orchard.
- Overhead sprinklers protect plants by using the heat given up by water when it turns from a liquid to a solid to warm the plants. Grower continuously add water, which turns to ice. This requires large amounts of water and the weight of the ice can sometimes break branches.
- Mowing the grass in the surrounding area close to the ground. Short grass will allow exposed soil to absorb heat all day from sunny conditions to release all night to the trees.
- Wet soils absorb more daytime heat because the water can load a lot of heat and radiates heat back out at night. Irrigate dry soils as much as possible before the frost event.
- Frost protection spray products might be useful, but results are not consistent.
- If you use sprays, do it several days before the freeze and not the day before as sprays before a freeze can increase freeze injury.
- Burning wood and hay has been used but it is a tough way to change the temperature of a large area effectively.
OK now let’s look at the homeowner and how practical it is to protect the landscape from frost (Unless you own a helicopter?)
When a frost freeze advisory is forecast I start getting messages like “Do I need to cover my plants?” One of those simple questions with a complicated answer like the question “who am I?” Well for the homeowner that all depends on what “plants” we are talking about. In some cases when plants are covered by the bed sheets from the bedroom or trash bags from the pantry more damage can be done through breakage and positioning as well as removal. If the protective material such as plastic is touching the plants it can do damage as it’s best they are “tented” allowing air space between the plant and the plastic. I’ve driven by some yards and wondered if anyone in the household that night had blankets for their beds or had made the sacrificial chilling donation for the cause of the hydrangeas in the side yard in lieu of a good night’s sleep. In many cases large landscape plants like a Lilac or a flowering tree are just not practical to cover and you take your chances. Again with these established landscape plants you might lose the blooms for a season but the plant will not die. I have been through many May frosts where Japanese Maples or Hosta leaves are severely damaged by a hard frost right after emergence. By the time we get to June the plant has begun producing new replacement foliage. It is often smaller than the original foliage potential, but foliage none the less. The plant lives for another day. As we head toward summer you can feed the plant and make sure it has proper water in hot dry months. Then begin hoping the following spring is more friendly. If you have some hydrangeas with thick buds on the stems and it’s practical to cover them go ahead. Again this frost isn’t going to kill these landscape plants, it just might set them back a little or sacrifice some bloom for this season.
I recommend you cover or bring in any tender annuals you have purchased in the past month. These will have to be protected until we have cleared our last frost date. Vegetable plants, flowering annuals can wait to be planted until the frost is past. Remember some annuals are much tougher than others. I find annuals like snapdragons, petunias, sweet alyssum, pansies, violas, cole crops very tough and cold tolerant. Other annuals like coleus, impatiens, begonias, cucumbers or potato vine are so frost sensitive you even say the frost word around them and they will surrender and wilt at a moments notice. When it comes to seasonal annuals and vegetables you are in control! So wait to plant, or cover or bring them in when in doubt. Perennials that have been in the ground for a few years are fine. They are hardened plants. They may show some scarring and if that bothers you go ahead and cover them if practical. You would be amazed how tough as examples daylilies, tulips or peonies are. If they are just recently planted, or are in bloom now and you want the blooms to last a little longer (bleeding hearts or creeping phlox) then go ahead and cover them to enjoy the blooms further into this season. Bring any tropical plants indoors or hanging baskets from the deck until better summer-like temperatures prevail. You can shuttle them in and out for a week or so until there is no more frost.
Finally some interesting facts on probability of blooms freezing in a frost event
- Flowers pointing downward will not radiate their heat as much as flowers pointing up toward the sky. These tend to survive cold frost events.
- Abundant bloom. Having numerous flowers helps with the survival of some bloom in a light to moderate frost event.
- Species. Some plants and flowers like people are just tougher than others.
- Flowers at a wide stage of development (pink to petal fall) will have different critical minimum temperatures and some may survive the freeze because of it.
- Is there abundant foliage? Leaves will provide protection to flowers hiding under the leaf and reduce radiation of heat. Of course some trees flower first then produce their foliage. It makes them more attractive to pollinators.
- Micro-climates make a difference. Is there strength in numbers or help from structural protection nearby?