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September Gardening Tips

General Gardening Tips

Stop using fast-acting fertilizers!  As days shorten and the weather cools, plants grow less quickly and use fewer nutrients.  Using chemical fertilizers, particularly high-nitrogen ones, can be useless or even harmful at this time of the season—they can stimulate lush new growth that will be vulnerable to fungus or frosts. 

Some plants, including lawn grass and all new plantings (including bulbs), benefit from fall applications of organic, slow-release fertilizers to support root growth through the winter.  (Bone meal is a traditional example of this fertilizer type, used alone or included as an ingredient.)  For many other plants, including most houseplants, it’s a good idea to simply stopping feeding after the equinox (9/22).  In the tips below, if we don’t tell you to fertilize a certain plant, don’t! 

Watering—you may or may not need to.  Fall rains should be starting soon, but go by the weather and soil conditions rather than the calendar.  

Normally we recommend NOT watering from overhead (using sprinkler or sprays).  Under most circumstances we advocate that you use soaker hoses, drip irrigation, watering bags, or other slow-release methods that deliver water directly to your plants’ root zone.  However, the recent smoke from wildfires may have left foliage filmed with ash.  If that’s the case, using sprinklers or sprays once or twice so you are simultaneously watering your plants and gently cleaning them may be beneficial.  If you’re using a spray nozzle, keep it on gentle–your plants won’t appreciate being pressure-washed.  And do any overhead watering early in the morning. Midday watering can cause leaf burn on sunny days from water droplets acting as prisms, while damp foliage in the evening can encourage the development of fungal diseases.


Vegetables and Herbs

Harvest!  Regular picking encourages continued production.  If hot-weather veggies such as squash, beans, peppers and tomatoes are still flowering, you might start picking off the flowers in order to encourage the plants to divert their energy to ripening their existing immature fruit.  (Remember that squash and scarlet runner bean flowers are edible!)

Plant your fall and winter vegetables!  Sky has vegetable starts for fall/winter harvest, plus it’s not too late for seeds of the fastest-growing varieties such as arugula and radish.  Not to mention overwintered peas for the earliest spring harvest.  Click for our winter vegetable information sheet and for Five Vegetables You Can Harvest All Winter Long.

Fertilize Fall Crops:  Radishes, spinach, lettuce, and other vegetables for fall harvest are an exception to that don’t-fertilize rule.  They need to grow quickly to get to harvestable size, so go ahead and feed them. A liquid organic fertilizer is ideal.  But hold off on fertilizing your spring-harvest crops until late winter/early spring.

Tomatoes:   Cool, moist nights can encourage the development of late blight, a fungal disease characterized by rapid browning or blackening of your plant.  Infected plants are best written off for the rest of the year; pick unaffected tomatoes to ripen on a windowsill and pull the plant before it can spread spores.  Do not try to compost blighted plants at home.  Put them in your yard waste bin instead, so that they can be heat-treated to kill any spores.  (You can email us a photo of your plant to confirm the diagnosis first.)   

Potatoes:  To maximize your harvest, wait to dig until after the tops die back (which may be October for the late varieties).  Ideally, leave the potatoes in the ground for two weeks after the tops die to allow the skins to thicken.  Then dig, and let the potatoes dry, unwashed, for 2-3 days to cure for better storage.

Pumpkins:  If you want big pumpkins, try removing all the smaller fruits on a plant to encourage the plant to send all its energy to the largest pumpkins. 

Pumpkins, squash, cucumbers, and melons:  As with tomatoes, the cool moist nights can encourage a fungus, in this case powdery mildew.  Click here for information on managing powdery mildew.

Herbs:  Most of your herbs won’t need any special attention to prepare for fall.  Stop fertilizing, reduce watering, and most will overwinter just fine.  Basil is a tender annual, and will be on its last legs soon; you should probably harvest it all and freeze it. (I like pureeing it with either olive oil or water and freezing it in ice cube trays—instant summer flavor in the winter!)  Lemon grass can be brought indoors as a houseplant.  Cilantro can still be replanted at the beginning of the month for a final fall harvest. Fall is an excellent time to plant hardy herbs, since it gives them the winter to get established.

Unused beds:  Plant cover crops now and next month to eliminate nutrient leaching and soil erosion and to discourage weeds.  Click for cover crop information.

Fruit Trees and Berries

Reduce irrigation if plants are growing vigorously; however, don’t eliminate irrigation entirely if the fall rains are slow to start.

Trim grape or kiwi vines to keep them under control.

Support or prune heavily fruited branches.

Net grapes if needed to protect from birds and other animals.

Monitor fruit trees and berries for insect and disease problems; consult a Sky Nursery associate for recommendations if you spot an issue. 

Harvest!  The early apples, pears, Asian pears, grapes, and blackberries should be joining your later peaches and plums.  It’s fruit cocktail time!

pale yellow roses with pink rose buds in fall


Stop deadheading; allowing hips to form will naturally tell your plants it’s time to start preparing for winter (and give birds a treat, too!). 

Stop fertilizing, but keep watering if we have dry spells.

light and dark purple asters and rudbeckia in fall

Ornamental Borders

Groom bedding plants, perennials and flowering shrubs, removing spent flowers.

Now is a good time to evaluate the “bones” of your garden—if you need to add evergreens or other structure plants, fall is the best time of year for planting and getting them established.  Sky gets fresh shipments of conifers and broadleaf evergreens in September specifically for fall planting.  Planting now gives them all winter to grow their roots and become established. 

Now is also the time to think about bulbs for spring color.  Early September sees the arrival of most of our spring bulbs:  crocus, tulips, daffodils, plus more exotic ones such as native camassia, chionodoxa, and the hardy white calla lily (the one that will naturalize for you).  Shop early for best selection.

If your garden needs an immediate pick-me-up–and most of them do–Sky has a great selection of the fall classics.  Asters, mums in every color, winter pansies and violas, ornamental kale, glowing Swiss chard, plus heathers, anemones, fall-blooming sedums, and more. 

We also bring in fresh stock of fall and winter color trees and shrubs-Japanese maples, burning bush, beautyberry, and fall-blooming camellias will start arriving in mid to late September.

Planters and Hanging Baskets

Water!  Remember that a container can still dry out in less than a day in warm or windy weather.  Deadhead regularly to encourage continued bloom.  However, you should stop fertilizing.

As the weather cools, it’s probably time to start swapping out your warm-weather bloomers for hardy and vivid fall color spots.  We have pre-planted containers, as well as everything you need to do it yourself. Or we can custom create container for you.

Lawn Care

Did you know that fall is the most important time of the year to ensure a robust, healthy lawn through the following summer? 

General tips to make sure next year’s lawn is lush and gorgeous:

  • Fertilize around Labor Day and then again around Thanksgiving with a good slow-release organic lawn food such as Dr. Earth.  The nutrients in these slow-release fertilizers will stimulate root growth through the winter and keep the grass strong. 
  • Spread dolomite lime to keep the rains from making your soil acidic. 
  • If necessary, thatch and overseed. The ideal time to overseed is when the fall rains have started but before the weather cools too much.
  • For a real treat for your lawn, topdress with a thin (1/4”) layer of screened compost. 
  • Watch for craneflies (they look like mosquitoes on steroids); if levels seem high, nematodes can be applied while the soil is still warm.

Fall is also the very best time to reseed or to install new lawns. Click here for pdf of our “Lawn Planting and Maintenance” instructions.

Water Gardening

Switch your fish to a spring/fall food; nutrient needs change as the water cools.

Stop fertilizing pond plants.

Tropical water plants such as lotus and papyrus will start to become unhappy as summer wanes; lift them to store for the winter, or compost them.