YES, YOU CAN PLANT NOW
Dr. Leonard Perry, Extension Professor
University of Vermont
Often I find gardeners wonder if you can still plant in late summer, and if so, what.
Lawns, for instance, can be seeded in late summer. Sown then, they will have plenty of time to grow and become established during the cooler weather, prior to winter.
August is the ideal time for transplanting bearded iris and daylilies. By then, the fleshy rhizomes (underground stems) of iris, and the fleshy roots of daylilies, have stored enough food to encourage active root growth and re-establishment after moving.
Divide the clumps, and reset at distances of 18 to 24 inches apart. Iris rhizomes should be barely covered with soil. Plant them too deep and they will rot. Cut back the foliage, such as with grass shears, of both types of plants to about 6 to 10 inches high. This will help balance the loss of roots during dividing and transplanting.
Peonies can be divided from late summer through fall, preferably once the foliage starts to die. Cut the tops back, and divide the large fleshy roots with sharp pruners. Divide so each division has at least 3 to 5 new, plump reddish buds. Then when replanting, make sure these buds are no more than two inches below the soil surface. Deeper and the plants will likely not bloom.
Late summer is usually the time my flower gardens are finally under control, and I have a chance to visit specialty nurseries and garden centers. By doing so, and buying what is in bloom then, you'll be assured of that rare late season bloom in your own garden. Perennials or woody plants that are in containers, or balled and burlapped trees, can successfully be planted then.
Plants, especially in containers, purchased at that time of year may often be root bound. So when removing their pots, make sure and cut or loosen the roots. Otherwise, the roots may keep circling in their planting hole and never grow outwards.
Also make sure and mulch when finished planting. This will keep the ground warmer through the fall, leading to more root growth and better establishment. Make sure to water well at planting, and if insufficient rain, to keep plants well-watered.
Trees and shrubs may also be transplanted then, but special care must be taken, and this is not advisable if not essential. Transplanted in late summer, trees and shrubs wont have the full season to establish new roots. They often will need cutting back by half to two thirds to balance the loss of roots. Between the transplanting and cutting back, new growth may be stimulated which wont harden off by fall and be injured.
If you need or want to transplant such plants, but can wait until spring, that is the better time. You may want to root prune though this time of year. That simply means pushing a spade in the ground, in a circle around the plant, where you will be digging it next spring. This makes new roots form within the circle, which will help the plant establish better when moved. Allow enough soil to get the most roots possible when digging, yet make sure you'll be able to dig and move the heavy soil mass!
Although spring is also best to divide and transplant many perennial flowers, if you do so by late summer and no later than when leaves fall, plants should have a chance to establish some roots before winter. If you have heavy soils, such as clay, the earlier you divide the better. These soils are subject to frost heaving in winter and spring, so new plants in them need even longer to get extensive roots established to avoid such stress. As with daylilies and iris, the more roots you lose, the more the tops should be cut back.