With summer starting to wind down and September on the horizon, the thoughts of many gardeners may turn to winding down their gardens as well. Not so fast. Late summer and early fall are not only times for harvest and garden cleanup; they are times to plant. And no, it’s not just about bulbs. Fall is a great time to plant perennial flowers, either in plant or seed form, to be ready to grow and flower the following year. There’s even still time to plant vegetables, and harvest them, before the year is over.
The first thing to do is find out your USDA hardiness zone, as when to plant will depend to some extent on your first frost date. Our own area is divided between two zones: 5 and 6. Visit planthardiness.ars.usda.gov/PHZMWeb to pinpoint which zone you live in. The first frost date for 5 is considered to be October 1 through 15, and for 6 is considered to be October 16-31; there’s also a great tool at https://garden.org/apps/frost-dates that will give you your average first frost date by zipcode.
You can fall plant perennials purchased in pots (on which you should be able to get great deals at this time of year); dig up and, for clumping plants, divide your own perennials when they have spread naturally, or plant seeds.
For perennial plants (as opposed to seeds), leave about six weeks between planting and your first frost date to allow them to settle in.
Don’t skimp on soil preparation. Soil should be dug the depth of at least a shovel’s head. Add a three-inch or so layer of compost and mix it in with the dirt.
For potted plants, loosen the potting soil and the roots of the plants, which, especially late in the season when you purchase them for fall planting, may have become pot bound.
Water the plants before they are planted, and also water the hole into which you will put them before the planting (also known as “puddling in.”
To divide established perennials, dig around entire clumps with a garden fork, lift up a clump and then divide it into smaller ones with a sharp implement. Be careful to preserve roots on each portion divided as much as possible.
The hole you dig for the plant should be about 1.5 times as deep and wide as its roots.
Place the plant in the hole, making sure the roots are spread out and downwards over the dirt.. Fill the hole with more dirt and firm.
Cover the planting area with mulch, which will provide good winter protection as well as keeping the soil moist for as long as the roots are still active. Water after planting for several weeks, but not after frost.
You can also plant perennial and biennial seeds in fall; indeed, there are some seeds that will not grow without a cold season, and therefore should not be planted in spring (e.g. false indigo, catmint, evening primrose). Plant the seeds as you would in spring, but wait until after first frost (though obviously on a day and time that you can still work the soil)—you do not want them to soften or germinate before winter sets in.
Late vegetable crops
There are a number of fast-growing vegetables that can be harvested before winter in our hardiness zones even if they are planted as late as early September. Some of them grow fast enough that they will be ready to harvest before the first frost date; others are actually improved by frost.
There are no special rules for how you plant vegetables for a late crop other than when. If the vegetable will be killed by frost, just find your first frost date and subtract the days to maturity listed on the seed package from that date; unless that plant is cold tolerant (see below), that will be the last date appropriate for planting. There are many lettuces and other tender greens, such as arugula, that mature in 40-55 days; in zone 5 those could still be planted into mid August and in zone 6 into early September. And of course with tender greens, if you want to get some of those “baby” ones that are so expensive in the supermarket, you can even plant them later in the season and harvest them well before maturity for an epicurian delight. In fact, somewhat counterintuitively, young lettuce is more resistant to frost than mature lettuce, so late plantings could yield you crops even after frost hits.
Tougher greens like kale and collards will not only survive frost but improve after a freeze, so they can be planted closer to the first frost date. And some of the root crops are also good for fall planting: radishes mature extremely rapidly—as little as three weeks, and will survive quite heavy frosts to boot; turnips mature almost as fast, though they are not as cold-hardy; and carrot roots will survive heavy frosts and sweeten even after their tops have died off.