Hardiness refers to the ability of a plant to survive cold winter temperatures. Plant scientists and meteorologists have mapped (sectioned) North America into plant hardiness zones. The zones have been assigned numbers ranging from 1 to 11. The zones start with the coldest, zone 1, and move south to the warmest, zone 11. Each one represents a minimum temperature range. For example, roughly the southern half of Minnesota (including the Metro area) is in zone 4 with an average annual minimum temperature of -20° to -30° F. Most of the northern half of Minnesota is zone 3 (average minimum temperature of -30° to -40° F.). Minnesota has the dubious distinction of having one of the few zone 2 pockets in the United States.
Lest you think that we have it bad — being a paltry zone 4 and having to live with unbearable winters, take heart in knowing that those at the other end of the spectrum (i.e. zones 9 and 10) have equally difficult challenges. If you want to grow tulips in Georgia, bulbs must be dug annually and artificially subjected to several weeks of freezing temperatures.
Hardiness is most often understood to mean only the cold tolerance of a plant. Every plant has a genetically determined resistance to a specific degree of cold. Although this is an important factor, there are several other variables such as sun exposure, watering, soil type, and drainage, which significantly affect the survival of perennials. Several plants that are typically listed as zone 5 plants can be successfully grown in our area (zone 4) provided they are given winter protection (i.e. a good mulch). Conversely, some plants that are listed as zone 4 will often not make it through our winters if certain conditions are not met. For example, some members of the Chrysanthemum family will grow quite well during the growing season in a wet, heavy soil. However, this condition (often referred to as "wet feet") can often be fatal to the same plants over the winter.
Ideally, your plants should be planted as soon as possible — within a few days of bringing them home. Drying can occur rapidly in small containers during hot or dry weather. If you are unable to plant right away, keep the plants in a spot that is sheltered from intense sun and drying winds. The north side of a building is a good spot. Lightly shaded areas at the bases of bushes or evergreens can also work well. Check the plants at least daily. Water enough to keep the soil moist but not soggy.
Most perennials can be planted at just about any time of the growing season. Spring and early summer are excellent times as this allows an entire growing season for the plant to become established before entering winter. Late summer and early fall also provide good conditions for setting plants. You may want to have planting completed by late September so that plants have a month or so to settle in before the ground freezes.
Planting can be done during the heat of summer — doing so will simply require more attention. Plant on a cooler day if possible and be sure to supply ample moisture until the plants are established. A surface mulch will help a great deal with summer planting. You may even want to use shade caps or lay some branches loosely over the plants to protect newly set plants from intense sun and drying winds.
A good time for transplanting is an overcast day that is on the cool side or just before a rain. Although plants can actually be set at any time, try to avoid very hot or windy days. Water plants thoroughly before planting. This will help in removing from the container as well as improve the success of transplanting. Dig a hole several inches larger than the container. This is an excellent time to amend the soil by combining 3 parts soil with 1 part compost. Refill the hole with the mixture.
Remove plants from their containers one at a time as you set them. Plants should also be removed from fiber and peat containers as they can restrict the growth of roots into surrounding soil. Most plants have fibrous, spreading roots - these should be loosened and spread out before placing in the planting hole to encourage their growth into surrounding soil. Other plants have tap roots (Lupines, Baby's Breath, Butterfly Weed, and Baptisia are examples) or brittle roots (Poppies). When setting these, do not disturb the roots. Rather, try to remove the plant from the container and set it with the soil intact and with minimal disturbance to the roots.
Set the plant in the hole at the same depth as it was in its container. Press the soil (or amended soil mixture) firmly around the base of the plant and then water thoroughly. Check newly set plants frequently during the first several weeks and water as necessary. Always water the soil — not the foliage. When active new growth is apparent, watering can be reduced to the plant's normal requirement. Your plant is now fairly well established and should reward you for years to come.
Once your plants are in the ground, water them thoroughly. Keep an eye on them over the next few weeks and make sure they don't dehydrate. Once the plants are established the frequency and need for water varies considerably depending on species. Some plants, particularly those that are drought tolerant and/or enjoy very well-drained soil, can suffer from over watering. Refer to the descriptions of individual plants for the needs of the plants you have. It obviously makes sense to group plants with similar needs together and to locate them according to their needs. When you do need to water, it is better to water thoroughly and less often than it is to provide small amounts of water more often. A thorough watering means that the top six inches of soil is moistened. Shallow watering will discourage deep root growth and can result in weak, wilt-prone plants.
Water is best applied to the soil — not sprayed on the plants. Watering with a soaker hose or with drip irrigation is not only more efficient in terms of water use; it is also much less likely to result in mildew and other foliage diseases. If you do water overhead with sprinklers, do so early in the day so that the foliage will dry before evening.
There is no need to apply any fertilizer to newly set plants until they start actively growing. If you have prepared the soil well and added organic matter such as compost it is probably not necessary to fertilize them at all for the first growing season. In following years you may want to supply fertilizer or work in additional compost. Keep in mind that there are some plants that do best at low fertility levels and too much fertilizer may encourage excessive foliage growth at the expense of flowering.
Winter mulching is done not so much to protect plants from the cold but rather to protect them from alternate freezing and warming. The vast majority of perennials that are hardy this far north actually require or benefit from a dormant, frozen period — it is a natural part of their cycle. They do best when they are gradually brought into dormancy and gradually brought out of dormancy. Abrupt, extremely cold weather in the fall or early winter can be somewhat of a shock to plants. This is especially true with wet, heavy ground; the alternate freezing and thawing means expansion and contraction of the soil around the roots. The pressure can be detrimental to the roots and can cause shallow rooted plants to "heave" out of the soil.
Thus, winter mulch should be applied after the ground has frozen hard and not removed until the potential for deep freezes has passed. Light spring frosts will generally not harm perennials from which mulch has been removed.
The ultimate mulch is that provided by nature itself — snow. It is both a great insulator and a great reflector. Unfortunately, even in Minnesota, snow can prove to be very unreliable, coming too late in the year or disappearing too early. Straw is a very popular material for winter mulching. In my opinion, its only drawback is that it often contains a lot of weed seeds. On the positive side, it is lightweight and easy to work with, both in the fall and spring. It is a good insulator and remains so throughout the winter because it does not absorb a lot of water and mat down. Pine boughs placed loosely over plants can provide a lot of protection. They are especially useful for silver- and gray-leafed plants that tend to get moldy under other types of mulches. I generally discourage the use of leaves because they absorb moisture and mat with time, though several people have told me that leaves work quite well for them. If you have straight oak leaves they should work well as they have a leathery texture and tend to resist moisture absorption.
Whether or not to mulch depends a lot on personal choice and how much you are willing to pamper plants to ensure their survival. I personally mulch only a very few plants. Lavender and tarragon are religiously covered every year. With other species, I mulch only if the plants have been set in the fall. Of course I have access to lots of plants so it's fairly easy for me to take a "laissez-faire" attitude. I also have a very well-drained soil and am not usually faced with the problem of "wet feet". Experience in your own garden will be your best teacher.
Several plants require periodic division in order to maintain vigor. Members of the chrysanthemum family (mums, shasta daisy, etc.) and irises are classic examples. Without division these plants will begin to die out in the center and lose vigor. They are more susceptible to disease and harsh weather. Eventually, they will die from overcrowding.
Some plants will do fine without division and still others resent being disturbed at all once they are established and attempts to divide them are most often unsuccessful. Examples of the latter include Baby's Breath and Baptisia. There are also a handful of perennials that are not exceptionally long lived, making division unnecessary or not worthwhile. Examples of these include Feverfew, Lychnis, and Dame's Rocket. Fortunately, many of the plants in this group are prolific self-sowers, so one can maintain a continual supply by simply allowing some seedlings to develop each year. The descriptions of specific plants include suggestions on division. The best times to divide plants are in early spring and fall. Plants with fleshy roots, rhizomes, tubers, bulbs, or corms are best divided in the fall. Fall division should be done early enough to allow roots to become established before the soil freezes. I recommend that fall division, as well as new plantings, be completed by mid to late September. I also suggest that new plants and divisions set in the fall be mulched for the first winter. As with planting, divisions are best made on a cool, cloudy day.
Thoroughly water plants to be divided before undertaking the task. Then, use a shovel or spading fork to dig all around the plant to be divided. Work in a circle as large as the plant's foliage to minimize damage. Carefully lift the plant out of the ground and remove excess soil from the roots. This can usually be done with a gentle shaking. Washing the roots, while not necessary, can help you see what you are doing. With some plants it will help to loosen matted roots. Pull the clump into sections. Some plants have extremely tough or woody root systems that will not separate by hand. It may be necessary to insert two spading forks back-to-back to pry apart roots. I sometimes use a large, strong knife to cut a plant apart. This can, however, be a risky business as the cut surfaces are more susceptible to rot and/or disease.
Each division should have a good mass of roots and a strong crown or group of crowns. (A crown is the point at which the roots connect to the upper leafy portion of the plant.) Discard old woody roots as well those that are mushy or appear to be infected with disease or insects. The new divisions should be planted as soon as possible in soil that has been improved with compost or other soil amendments. If you are dividing plants in the late summer or fall, cut back the foliage to minimize water loss and maximize root establishment. The newly set divisions should then be treated as new plantings — give them frequent watering and close attention until it is apparent they have become established.