The September To-Do List
Fall is one of the best planting seasons, so get ready. It’s especially good for planting shrubs and trees, though if it’s still hot, delay for a week or two until weather is cooler. The ideal time is when the leaves begin to change color. For expert, detailed advice on planting trees, consult “Planting Trees,” Va.Coop.Ext., Va.Coop.Ext. Pub. No.426-702 and Charlottesville Tree Stewards/How to Select, Plant, and Care for your Tree. And don’t miss this new video, How to Plant a Tree/Va.Coop.Ext. Video.
Water newly-planted trees and shrubs regularly throughout the fall. Don’t rely on rainfall alone to maintain adequate moisture levels. To help hold in moisture as well as moderate the soil temperature, spread a three-inch layer of mulch over the rootball area after it has been well watered, but do not pile mulch against the plant’s stem or trunk. Check the soil near the roots once a week. If the soil near the trunk is dry, your new tree or shrub needs water, and plenty of it, especially if there’s been no rain. If the soil near the trunk is moist, don’t over-water as that can kill a tree as surely as under-watering.
September is an excellent time to start dividing those perennials that need it (daylilies, garden phlox, irises, and oriental poppies should be divided every three to four years), and the ideal time to do this — adjusted somewhat for climate change — appears to be around the 2nd or 3rd week of September. Your new divisions need at least six weeks to get established before winter. As a general rule, spring-blooming plants are divided in fall, and summer-blooming plants are divided in spring. Many plants, however, can be divided in either spring or fall. Some perennials, including many with long tap roots such as Baptisia australis, do not respond well to division. Check guidelines for specific plants before dividing. For detailed how-to instructions on division, see the Clemson Cooperative Extension publication on “Dividing Perennials”, which is very helpful.
September or early October is the ideal time to divide or move peonies so that they become well established before winter. Peony tubers are very fragile, so be particularly careful to keep the root mass as intact as possible. Space the plants at least three feet apart. This is important: Plant the roots so that the buds are only about one to two inches below the soil surface. If you plant them deeper, they will not bloom.
Spring-blooming bulbs can be planted throughout the fall until the ground is frozen. They do best if planted about a month before the ground freezes. A good rule of thumb is to plant bulbs when the average nighttime temperatures are in the 40° to 50° range. If bulbs are bought before planting time, store them in a cool, dry, dark place at a temperature of 60˚ to 65˚F. See the September 2015 Tasks and Tips article in The Garden Shed for recommendations of deer-resistant spring-blooming bulbs in addition to daffodils. Fall is also the time to divide bulbs.
Fertilize bulbs: Any time after your bulbs have finished blooming is a good time to fertilize them, but they may not need fertilizer if you regularly add organic matter, like compost, to your beds. But if your narcissus and other spring-blooming bulbs look like they’re not getting enough nutrients from the soil, you might consider fertilizing them lightly with a 5-10-10 fertilizer. Avoid high-nitrogen fertilizer (the N element, the first of the three numbers listed on the label).
Clean up your beds to prevent disease and pests next year. In addition to the all-important tasks of watering and weeding, it’s important to keep garden beds clear of plant foliage that might carry fungus or other diseases from this season to the next. Bag up diseased plant material and discard; don’t add it to the compost pile. As perennials die back in the fall, be sure to cut off the dead foliage and discard since the foliage could encourage insects and disease. Don’t cut off the brown fronds of your ferns in fall since the old fronds help protect the center through the winter. Once you see new green fronds in spring, you can carefully remove the brown fronds. Or, just let them fade off naturally.
Save seeds For suggestions and instructions on seed saving, see The Ornamental Garden in September/Garden Shed Sept. 2018. Don’t forget to save the seeds from the pods on your Hyacinth Bean vine (Dolichos lablab).
Avoid Fall Pruning. Resist the urge to grab your pruning shears. You can remove dead branches, but then put away the clippers. Shrubs that bloom on new wood can be pruned in late winter. Shrubs that bloom on old wood can be pruned right after they bloom in spring or summer. If you prune in fall, you might mistakenly prune off buds that have already set for spring bloom. Or, you could encourage a late season flush of growth that could be injured by cold weather.
Attack Invasives If you’ve got oriental bittersweet climbing in your trees, or autumn olives filling in every available space on your property, fall is the ideal time to go on the attack. The “cut stump” method is quite effective for both these invaders. It does not involve spraying herbicide. Instead you “paint” the herbicide onto the cut stem or stump. If you’re not familiar with the cut-stump method, you might start by participating in an autumn-specific Zoom training session offered by Blue Ridge Prism, Fall Invasive Plant Workshops/Blue Ridge Prism. Now is also the best time to eliminate Ailanthus; read all about it at Ailanthus Alert/Blue Ridge Prism.
Fall Webworms Are you seeing more than usual? I’m wondering if it’s related to climate change, which is supposed to lead to an increase in insect pests. Have you noticed any new pests? For more about fall webworms, see last month’s article, In the Ornamental Garden/Garden Shed Aug. 2020.
For more tips, see Monthly Gardening Tips, September .