BOULDER – Magnolias and apricots frolicked in the sunshine while Colorado weather lurked beyond the horizon, readying a wintry blast that would wipe those smiling flowers right off their branches. As the first tendrils of clouds slithered across the sky ahead of the storm, gardeners who’d been waiting for that frozen shoe to drop felt the oncoming doom in the pit of our stomachs, like watching a horror movie when you know the monster is about to appear and savage the innocent bystander.
Had it only been snow to blanket the landscape things would have been fine. But the wet, watery weather came with a blast of temperatures in the teens, which for buds isn’t so harmful. But for flowers, is a very bad thing.
Many plants awoke with the early warmth, stretching stems or leaves into the sunshine. Tree buds swelled and some even broke, covering branches with tender new leaves. In many cases, the early growth of perennials and bulbs isn’t harmed by a frosty nip; the tissues are fairly hardy. And tree buds are able to withstand cold temperatures.
But once buds begin to open, they lose much of their cold hardiness; the more open they become, the less tolerant they are of frosts. Magnolias, apricots, some cherries, and a few apples were in flower when this frozen blast hit them.
If the freeze hits the flower for a half hour or longer, damage to the fruit can occur. Because our ice stayed on the plants for two nights, chances are these trees won’t fruit this year. But you can check to see the extent of the damage to your tree if it was in flower.
At or near blooming, freezing temperatures of 28 degrees Fahrenheit will result in about a 10-percent loss of flowers and fruit; at 24-degrees F a 90-percent loss. Several hours after a freeze and once the tissues thaw, the undeveloped fruit become brownish-black if they were damaged or killed by the cold.
If your cherry or apricot was in flower, check it for damage. The flower contains a single pistil – the female part of the flower that becomes the cherry or apricot. It’s cupped by the sepals and petals of the flower. If the pistil is brown or black, it was killed by the freeze and won’t become fruit. If it looks healthy and green the fruit is alive.
Apples have clusters of flowers, the center-most of which is the most developed. Known as the King bloom, it is the first to open as a flower. The king Bloom developed the largest fruit and is considered the most desirable of the flowers to flourish. But it’s also the most likely to be killed in a frost since it opens first.
Checking for freeze damage to apples is slightly different than checking cherries or apricots. Apple pistils are located inside the base of the flower, making it necessary to tear the flower apart to see if the center of the flower is brown or black. Check both the king and side blooms separately; hopefully some of the side blooms weren’t harmed by the freeze.
Check a few of the flowers to see if your fruit crop was harmed by the ice, but don’t rip apart all of the flowers. Take a sample from various parts of the tree to give it an overall checkup.
Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development.