Gardening Tips-September

September brings the cooler weather with it which is a great time to start planning for next year’s garden, prepping plants for the coming winter, and reorganizing your garden for next Spring.


Divide and Plant

  • Now is the time to divide and transplant many spring and summer-flowering perennials. Some to divide now include iris, yarrow, oriental poppies, Rudbeckia, and peonies. Care should be taken to replant them as soon as possible to prevent them from drying out. Try to finish this by the end of the month so that plants can be established before winter. Daylilies are more flexible and can wait until spring. Phlox should wait until spring.  Here’s a good YouTube video on dividing perennials. 
  • Select spring flowering bulbs as soon as they arrive in the garden centers. If you select your bulbs early, you will get the best. When selecting bulbs from the open bins, look for firm, undamaged bulbs without any nicks, soft spots or mold.  Here is a good site on bulb selection and planting.
  • Now’s the time to divide Bearded irises (6 to 8 weeks after they have bloomed back in July or August).  Plant the rhizomes shallow so that some of the top of the rhizome is exposed to sunlight. Siberian or Japanese Iris can be divided in spring or fall when two months of below 90°F can be assured. Our friends at Presby Memorial Iris Gardens have a great video on how to divide bearded irises.
  • Move self-sown biennials if needed. Now is a good time to plant peonies, poppies and bearded iris.
  • September is also the best time to find the greatest selection of mums for instant color in your pots and flowerbeds. Tip: chose plants that are fully budded with just a few blooms open – they will last longer.
  • Remember to keep new autumn plantings moist.
  • Cut back short-lived perennials in early September, if it was not done in late August.

Pruning & General Care

  • Generally, no more fertilizer should be applied to plants. Similarly, do not fertilize shrubs. Late-season nitrogen can prompt new growth in woody plants with insufficient time to develop cold hardiness.
  • This is not a good time for general pruning of shrubs, especially spring-flowering ones such as azaleas, forsythia, and rhododendrons, because they have set their buds for next year’s bloom. Restrict trimming to removing a few stray shoots.
  • Spruce mites are cool weather mites that become active now. Shake a branch vigorously over white paper to check for them.  Here is a good site on Spruce Mite management.

Vegetables & Herbs

  • Start cleaning up the garden. As soon as plants are spent – tomatoes, cucumbers, beans, squash – remove the plants from the garden. However, do not add plant materials from tomatoes, eggplant, or peppers to a compost pile as they may contain diseases that would be transmitted to the compost and to next year’s crop.
  • Early in the month it may seem warm for planting fall vegetables; however, September is actually a bit later than optimal for planting most of these crops. Give yourself until mid-month to plant things like spinach, most cabbages, collards and other leafy greens. Radishes will also do well as they are ready to harvest in only about thirty days.
  • Plant a Salad – a packet of “Mesclun Mix” might contain mustards, arugula, endive, and other interesting varieties along with lettuce, kale and spinach. Most of these greens are fairly cold hardy and can tolerate some nights below freezing without protection – if you don’t plant them too late.


  • September is the best time to reseed bare spots in fescue and bluegrass lawns.
  • Fertilize fescue and bluegrass lawns using slow-release fertilizers to reduce risk of nutrient run-off. Grass roots continue to grow until soil temperature falls below 40°F (late November usually).
  • Weeds may have been plentiful; however you cannot use pre-emergent herbicides (weed preventer) in the same season you are planting seeds.

For more information see our pages on Lawn Care and Lawn Pests.


  • Pull and trash any annual weeds, such as ragweed, nut sedge, creeping charlie, foxtail, or crab grass that went to seed in some neglected corner. These seeds live for years if allowed to reach the soil and get spaded under. Do not put into a compost pile.
  • Fall is a good time to start a new compost pile. You can build a compost bin or simply pile all that yard waste in a corner.
  • This is a good time to collect dried plant material for winter use. Seed heads, grasses, pods and dried flower heads can be hung upside down to dry for later use.
  • You may want to stop dead-heading coneflowers, sun flowers, and black-eye Susans in order to leave the seed heads for the birds to enjoy.
  • Don’t take down hummingbird feeders down when the resident birds disappear. There may be millions of other humming birds north of your backyard that will be passing through and will stop to recharge. Leave them up until at least October 1st for the stragglers.
  • House plants – move house plants indoors when the outside night temperatures are similar to your indoor night temperatures so as to minimize the shock…certainly before temperatures drop below 50º F. Start checking houseplants for signs of insects so they can be treated a few times before moving back indoors. While you are at it, give the plants a good bath with the garden hose, remove dead leaves and cut back long stems.