Winterizing Your Perennials
Winterizing your perennial garden doesn't have to be a big job, especially if you take little steps all year to prepare your plants for whatever nature gives us. As everyone who has lived through a Minnesota winter can tell you, we all need a little help to make it through. Some plants are less hardy than others and need extra attention. And it's a great time of year to be out and enjoy the sights, sounds and smells of fall. Relax and enjoy the task, it's the one chance you have to do a cleaning job that stays done for 6 months.
How do I know what my plants need? While some varieties of perennials may need more protection than others, all of them will benefit from your attention.
How do they determine plant hardiness? The United States Department of Agriculture determines the hardiness zones based on average low temperatures during the winter. While low temperatures are the most important factor, other factors such as exposure, how much wind is received and how healthy the plant was going into winter will all influence the hardiness. Winters when we have little snow cover are very hard on plants.
What Zone are we in? The Twin City area is officially in the northern half of Zone 4. Zone 3 starts around St. Cloud and covers the majority of the northern half of the state. There are a few small pockets of Zone 2 in far north Minnesota. There are also sheltered areas that could be considered zone 5.
Is there anything I can do during the summer to make my plants hardier? There are several cultural practices you can follow to help insure your plants winter successfully. Grow your plants lean, giving them only the fertilizer they need. Avoid fertilizing any later than mid-August. Plants growing in soggy areas usually have a harder time in winter, so amend your soil with lots of organic matter to improve drainage, and water carefully.
When should I cut back my perennials? Most perennials are cut back after we have had a killing frost in the fall. This usually occurs in late September or early October. It is important to clean off all plant debris after the frost to help minimize soil-borne diseases.
What kind of protection should I provide? Most perennials simply need a good layer of mulch applied late in the fall. The purpose of mulching in this case is to protect the crowns of the plants from the alternate freezing and thawing that occurs very late in fall and in early spring. It is important that the ground be allowed to get cold before mulching, so wait until early to mid-November before covering the plants. Ideally an inch or two of frost in the ground is best.
Are some mulches better than other for perennials? There are several mulches that work well for winter protection of perennials. Straw, hay and leaves are the most common. Straw is the stems of wheat or oat plants and is usually a bright golden-yellow in color. Hay includes the leaves of plants, usually grasses such as Timothy or canary.
What are the pros and cons of mulching with straw? The advantages of straw are that it doesn't pack and it insulates well. The disadvantage of straw is that it takes so long to break down in the compost pile.
What are the pros and cons of using hay? Hay is a good insulator and will break down readily when composted. It does tend to pack down and may have a few weed seeds. Marsh hay is often recommended in gardening books, but it is not available locally.
What are the pros and cons of using leaves as mulch? Leaves from various trees and shrubs make excellent mulch when shredded. You can shred your leaves by running over them repeatedly with a lawn mower or using a leaf shredder. Avoid walnut leaves. Leaves are inexpensive and usually readily available. They offer good insulation and compost readily.
How much mulch should I use? A layer 4-6 inches deep is best for most perennials.
Are there any perennials I shouldn't mulch? Bearded iris should either go without mulch, or be mulched extremely late. The iris borer seems to be worse on mulched plants, especially those mulched early. If you have had any disease problems with your peonies, leave them unmulched as well.
Should I continue to water in the fall, even after a killing frost? Making sure your perennials stay well watered until the ground freezes is important to successful wintering. Quite often we go through several dry weeks late in October. If the soil is dry an inch or two below the surface, give the area a thorough soaking.
Can I divide or move my perennials in fall? Many perennials can be divided or moved in fall. Generally if a perennial blooms in spring or early summer, it can be divided or moved in fall. If it blooms in late summer or fall, it is best divided or moved in spring. There are a few exceptions, of course. Irises and daylilies prefer to be divided in August and a few plants with taproots don't ever want to be disturbed.
Should I do anything special for my Zone 5, marginal plants? With a little extra care, we can grow quite a few plants that aren't truly "hardy" in our area. The key to success seems to be getting a deep root system established (by watering deeply and preparing the soil with lots of organic matter) and mulching heavily the first winter or two. A small fence can be put up around the crown of the plant and the area filled with mulch.
Plants with tender crowns like Delphinium can be protected by filling an empty nursery pot with leaves and setting the pot over the crown of the plant. Place a rock or brick on the pot to keep it in place. The crown of the plant will stay dry and protected over the winter.
When can I remove the mulch in the spring? Wait until all the frost is out of the ground before removing the mulch. If it gets very warm early, you may want to pull back part of the mulch, but leave at least 2-3 inches.
Some gardeners leave mulches in the beds, just pulling them back away from the crown of the plants. This adds organic matter and helps suppress weeds. Mulches that have been removed can be composted.
Baled Straw, Canary Grass, Hay
Leaf and Lawn Rakes
Quality Hand Pruners
Shovels and spades
Additional Bachman's Information
Fertilizing Annuals and Perennials
All About Watering
Protecting Trees and Shrubs in Winter
Understanding Mulches and Mulching
© Bachman's 2007