The cooler temperatures of fall have arrived and are quite a welcome change from our sweltering temperatures of the summer. Along with the cooler days, fall also provides a perfect opportunity to plant more perennials. Whether adding to an existing garden or starting from scratch, the soil and air temperatures during autumn offer the very best conditions for perennials to establish their roots. In addition, planting in the fall frees up your time for other spring activities. If you’re accustomed to waiting until spring to plant perennials, here are few important tips to keep in mind as you plan an autumn plant installation.
1. When to Plant. Don’t wait until the cold winds begin to howl to plant. The plant roots need time to get established, so try to install your plants at least six weeks before the ground freezes solid.
2. Soil Temperature. The soil temperature is warmer in the fall than it is in spring, and this encourages root growth. Although it is not forbidden to install a plant while in bloom, the best time to plant is when it is not. This way, the energy goes to the root and not the bloom, which will give a stronger plant and ensure better blooms in spring and summer.
3. What About Frost? Although frost will stop the growth of the plant above ground, it will not kill the plant. The roots will continue to grow until the soil freezes solid.
4. Plant Condition and Dormancy. At the end of the growing season many perennials have finished their growth cycles, and they may not be looking their best. Plants that bloomed earlier in the spring and summer may be thought of as past their prime and may a bit tired and ragged looking as they head towards winter dormancy. This is not a concern. Simply plant them with the same loving care that you would give a robust looking plant during spring planting.
5. Watering. Before planting, water the plants thoroughly and let the roots completely soak up the water. With the lower temperatures, the lower sun angle, and the shorter days, plants generally need less watering in the fall. Water them once per week until the soil freezes.
6. Mulching. Fall transplants do not need to be mulched immediately. The new growth will benefit from the sun warming the soil, even into early winter. The transplants should be mulched when evening temperatures are consistently 32 degree or lower and the soil is beginning to freeze. Mulch can be leaf litter or other spent plant material, straw, etc. Remove the winter mulch in the spring, after freeze-thaw cycles have passed.
7. Preventing Frost Heave. Once the plants are in the ground, a freezing and thawing of the soil can result in “frost heave” – wherein the thawing soil causes plants that aren’t fully anchored to raise up out of the soil, which puts the plant in danger of being killed by cold. After the soil has frozen hard, spread a few inches of straw mulch around the transplants. A good layer of straw will keep the ground from thawing around the plants. Once they are mulched you’re ready for winter. When spring arrives, pull the mulch away from the plants, to allow the soil warm up faster around the plants.
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