If the cooler weather and turning leaves haven’t alerted you, the calendar can’t lie. Tomorrow is the first day of autumn. Can our first frost be far behind? It’s tempting to let the change of seasons put a stop to gardening for the year, but there’s still much to do. (See my previous post on “Putting Your Garden to Bed” for ideas.) Of course we know that many spring-blooming bulbs go in the ground now. But how about perennials, shrubs, and even trees? Can we plant (or transplant) them now? Even for those of us who live in places with cold winters, fall is a terrific time to plant.
Although subject to occasional cold snaps or mild heat waves, fall in Colorado probably offers the most consistent weather of the year, with clear, sunshine-filled days and increasingly chilly nights. The ground is still summer-warm, which encourages root growth; adding several inches of mulch will help maintain that soil temperature and delay freezing. Early fall’s cooler air temperatures mean reduced transpiration from the leaves and also keep moisture levels more consistent so plants don’t dry out.
In addition, many pests have completed their life cycles by now, and are less of a problem. (Be sure to protect tender bark from gnawing rodents and rabbits. They will have fewer options as winter sets in, and will eye your new offerings with gusto). At this time of year, garden centers discount their merchandise in order to reduce the stock they have to overwinter, and bargains abound. Check the roots on potted plants and avoid those that are root-bound after growing all summer.
Finally, there’s my favorite reason to plant now—fall is a delightful time to be outside working in the garden!
Which plants are good candidates for fall planting? One general recommendation is to avoid species that are slow to establish, such as lilacs, viburnum, hawthorns, and oaks.
Even more important is to choose plants that are well within the hardiness designation for your zone. Borderline plants do best if you wait for spring, giving them an entire growing season to establish vigorous roots before they face the cold. For example, in my zone 5a garden, I plant Sunset Hyssop (Agastache rupestris) and Hummingbird Trumpet (Epilobium [Zauschneria] garrettii) in the spring, since they’re only hardy to zone 5. But hardy geraniums (aka cranesbill), Sedum ‘Autumn Joy’ (right), and other zone 4 plants can go in now.
Don’t wait until later in the season; it’s important to give the plants plenty of time to get established. Those roots need to spread out and anchor themselves before the soil freezes to prevent frost heave. Once the ground is frozen, add several inches of mulch over the roots to keep it that way until spring. (Just make sure to avoid creating a mulch “volcano”!)
When you buy your new plant, you may be encouraged to also pick up some soil amendments, “transplant fertilizer,” or mycorrhizae to “help your new plant thrive.” Save your money; none of these are necessary. Your woody plants’ roots will soon extend far past the planting hole and any soil you amend. Changing the soil around the rootball will just discourage those roots from venturing any further, and could eventually stunt your plant. Adding fertilizer while the plant is under stress (and anything you just planted is definitely under stress) won’t make the roots grow faster, and you don’t want to promote top growth until the roots are happy. Plus, multiple studies show that adding vitamin B to new plants does nothing to reduce transplant shock or stimulate root growth.
Mycorrhizae are suddenly very popular, with a host of new products available. But decent soil already has plenty of mycorrhizae, and studies show that adding more makes no discernable difference. And if the soil is lacking these fungi, it’s because there isn’t sufficient organic matter to sustain them. Adding them in this case just dooms them to starvation. Either way, there’s no reason to buy them.
Work you do now in your garden will pay off big time come spring. Adding new plants is a great excuse to get outside and enjoy our spectacular fall weather!