Our temperatures have been all over the place this winter, and we had especially warm weather just a few weeks ago. Most plants use temperature as an important cue in deciding when to break dormancy, so you have probably seen bulbs coming up much earlier than usual.
But as we saw last week and over the weekend, the risk of frost or a hard freeze is far from over. Here in central Indiana, a good rule of thumb is that Mother's Day is the last frost date. So what happens to your plants when we have a warm spell followed by a cold snap? Today we'll go through a couple different categories of plants and how they could be affected.
Late Winter and Early Spring Bloomers
First off we have the earliest bloomers, the bulbs, perennials, and shrubs that start showing their colors in February or early March in a normal winter. These plants will not be harmed much, if at all, if temperatures drop again. They are adapted to tolerate the unpredictable freezing and light thawing we normally see at this time of year. See the list below for some examples.
Mid Spring Bloomers
This next category of plants includes the perennials, bulbs, shrubs, and annuals that are ready for action in mid March to mid April in a normal year - a lot of the plants that we mentally associate with spring. These species have already emerged more or less, and they can be harmed with another hard freeze. Flower buds can be killed, and the leaves, stems, and flowers of bulbs can suffer freeze damage (see what that looks like here). While this will spoil their looks for the year, no long-term damage will be done. A light frost won't affect them much, since they are acclimated to blooming when light frost is a risk. Here are some plants for this category.
Bulbs, Perennials, & Annuals
Early Tulip varieties
Late Spring Bloomers
This group includes the plants that spring to life from mid April to mid May. Seeing them coming up this early is really unusual, and they won't be prepared if it gets cold again. They are going to be a little bit more sensitive than the mid spring plants, and quicker to lose all flowers. If you love magnolias, this won't be an unusual experience for you - their flowers are sensitive to the cold, but the early blooming varieties often get spoiled here by late frosts.
Bulbs, Perennials, & Annuals
Late Tulip varieties
Sometimes we take risks and plant species that aren't quite cold hardy in our area. Some varieties of Japanese maple or magnolia can make it here in mild or moderate winters, but die off in bitterly cold years. While these plants are probably very happy right now, if we have a sudden cold snap - even without polar vortex temperatures - these plants could suffer significant twig damage or even death because they haven't had time to gradually acclimate to colder temperatures. If you have any of these plants in your yard, you need to be on the lookout for sudden drops below about 20 degrees F, and be prepared to insulate them for protection. Check out this article for information about how to do that.
In addition to plants that might die, and all the spring blooms that could be affected, all the rest of our trees and perennials will be tempted to green up early. What happens to them? The most likely scenario is that a quick freeze will damage tips of shoots and leaves, but the plants will be able to recover just fine this year. Plants that can will replace damaged leaves, others will just look a little weird this year.