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What to do in the garden in September & October

Hips on Rosa rugosa

Continue to deadhead some perennials and annuals to keep them blooming, others to avoid self-sowing. You may want to leave seedheads for the birds on plants like echinacea. 

Stop deadheading most roses. This will allow them to start transitioning to winter. Rosehips are an added bonus with some kinds of roses.

Keep container plants watered and fertilized.

Evergreens, including conifers, should be planted by mid-September to allow them plenty of time to root. The newly planted broadleaf evergreens will need winter protection from sun and wind. Continue to water all newly planted woody plants. Ten to fifteen gallons of water is needed weekly when rainfall is less than one inch. 

Protect tree trunks from buck rub as soon as possible.

Plan to protect woody plants from browsing by deer, rabbits and rodents. The bark and the buds on the branches are all susceptible.

Bearded irises should have been divided and/or planted last month but if you do so in September, place a stone or brick on top of the rhizome to prevent winter heaving (this tip courtesy of the Southern Tier Iris Society).

Keep water gardens full. Continue to prevent mosquito development. Use mosquito dunks if necessary—these contain a type of natural Bt that kills mosquito larvae.

September is the best time to renovate or install a lawn. Cooler weather and hopefully more moisture allow better germination and growth of the grass seedlings. Mowing the lawn as high as possible results in a healthier lawn with deeper roots more tolerant of drought and denser turf that will prevent germination of some weed seeds. 

Now is a good time to move spring-blooming bulbs if you can locate them. Many will already have roots so don’t let them dry out.

Photograph your garden and make notes of needed changes. I put notes on next year’s calendar so I don’t forget what I wanted to do next April or May.

Now is a good time to plant hardy perennials and woody plants. Keep them watered to encourage rooting.

Narcissus is best planted in September after the soil has cooled a little. Delicate bulbs such as fritillaria and trout lilies should be planted as soon as you get them. Winter aconite tubers and Anemone blanda tubers should be soaked in lukewarm water for several hours before planting. This is very effective for A. blanda, less so for winter aconites, which are best propagated by seed.

Nursery stock goes on sale and may be a good money saver if it has been well cared for. Score the rootball of pot-bound plants with vertical cuts to ensure root growth into the surrounding soil. If rain is insufficient, water weekly. Continue watering until the ground freezes.

Check viburnums for viburnum leaf beetle (VLB) adults, especially if the shrubs were defoliated by the larvae. Consider a pesticide treatment to save the shrubs. Do NOT cut back branches just because the leaves have been eaten or damaged. Scratch the bark with your fingernail, if it’s green underneath, the branch is alive. Snip off and destroy the twigs that contain the VLB eggs. Although the egg-laying sites are most obvious in the fall, one actually has until April to trim the affected twigs.  

It is too late to fertilize woody plants, as doing so may encourage tender late growth that may not harden off in time for winter.

It is also too late to prune woody plants, except for dead or diseased wood. Be especially mindful not to prune spring-blooming shrubs that have already formed next spring’s flower buds, such as forsythia.

Bring in poinsettias and Christmas cacti to get them adapted to indoor conditions. Start exposing them to long nights (short days) for flower buds to set. After checking for insects, bring in houseplants before nights cool off too much outside and heating systems start operating. 

Consider having windowsill herbs for winter use. You may pot up small ones or take cuttings—basil, sage, rosemary (especially susceptible to drying out in my experience) are some of the possibilities. Chives are a hardy perennial; pot them up and bring them inside in late fall.

If you live in a cold site, you may want to dig tender bulbs such as dahlias, tuberous begonias, and cannas before the frost hits. This winter I am planning to keep canna ‘Stuttgart’ growing on a windowsill. Cannas do not need a rest period.  Gladioli seem to be marginally hardy even in my cold site. One has persisted and bloomed for three years outdoors now, and others survived last winter, but may not bloom this year. I may leave them all in the ground and see what happens!

You should already have harvested garlic.

Keep up with weeding! If you can’t remove all the weeds right away, at least don’t let them go to seed.

Renew the mulch in your veggie garden or consider planting hardy cover crops to improve the soil.

Pick fall raspberries every day, especially if the weather is wet or humid. If raspberries or other soft fruits look moist or misshapen, check for the maggots of the spotted wing drosophila fruit fly. Destroy all the bad fruit. If a lot of fruit has been set, you can then use row cover to keep the fruit flies out, but this will also prevent further pollination. Also look out for the brown marmorated stink bug. 

Keep harvesting veggies and herbs and continue to water if it is dry.

If you garden in a cold site, start watching for frost after October 1 be prepared.  (The average first frost in zones 5 and 6 is in mid-October.) 

Now is a good time to do a soil test and make pH amendments as needed but wait until spring to apply fertilizer.

Continue to water newly planted woodies. You can continue to plant hardy perennials and woody plants such as tall phlox, hostas and lilacs. The shallow-rooted perennials such as Heuchera should have been planted earlier.

This is the best time to move peonies. Normally, they don’t need to be moved or divided unless they are growing in too much shade. It may take a couple of years for them to recover after dividing or moving. Do not plant them too deeply; doing so may cause them not to bloom.

Continue to plant spring-blooming bulbs. Tulips can be planted last. Many spring-blooming bulbs are deer-resistant, such as alliums, winter aconite, snowdrops, snowflake, Siberian squill, glory-of-the-snow, puschkinia, fritillaria, and Anemone blanda. Grape hyacinths send up fall foliage but even when it’s browsed it doesn’t seem to affect their vigor.

Cut off all the peony foliage down to the ground to remove botrytis spores.

Some perennials can be cut back now for the winter, if the foliage has senesced already.  Leave stalks of natives in place in case beneficial insects use them for overwintering.  Also, do not trim back the stalks of certain plants that overwinter better with the protection of the old stalks. This group includes mums, lavender, culinary sage, Kniphofia and butterfly bush.

Late in the month, look for spring bulbs on sale. Consider forcing some: daffodils, crocuses, hyacinths, and smaller bulbs like Siberian squill all force well. Tulips can be forced, too, but they require a longer rooting period and stronger light in the foliage-growing stage or they will be leggy and floppy.

Listen to the fall forecasts and be prepared to protect tender plants from an early frost with old sheets, towels, etc., as we frequently get a couple weeks of nice weather afterwards. Otherwise, when frost is predicted, do a quick harvest to get produce indoors.

Mid to late October is the best time to plant garlic. Be sure to rotate garlic; pick a new spot with lots of sun and good drainage. I mulch it with a couple inches of woodchips to give it plenty of time to root but preferably not to sprout.

Remove all the brown asparagus ferns to reduce the number of overwintering asparagus beetles.

Continue weeding, watering and mulching as needed.

— Pat Curran and the Tompkins County Master Gardeners

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