Assess Your Garden
A flower garden can tell you a lot at the end of the growing season. You'll want to assess the results of all your spring and summer work, and prepare the garden for next spring. First, take a walk around your garden and look at how all the plants did over the summer. Track successes and failures of individual plants. Identify which plants have outgrown their space and need to be divided.
Determine which bare areas could use soil amendment and new plants. Add mulch where necessary.
Check for Diseases
Check the overall health of plants — look for diseases and damage.
Replace Old with New
Replace summer annuals in window boxes and garden beds with cool-weather flowers. Dig up any bulb plants that aren't hardy in your zone.
You'll want to weed, deadhead faded blooms, divide overgrown plants, dig up non-hardy bulbs for winter storage, remove spent annuals, amend soil and add needed mulch. Replace ties with jute twine. Natural fibers make the best ties because they're more flexible. They'll break down over time, but at that point, it will be time to retie the plants anyway. Amend soil where there are bare spots or where you've removed annuals. Add compost and to replace nutrients lost during summer growth and to better prepare the soils for spring planting. Turn the amendments into the soil with a garden fork to distribute it evenly. Brush off any mulch that's sitting on branches of shrubs because it can cause leaves and needles to yellow.
Fall isn't the ideal time to prune roses. Pruning stimulates new growth that may not be able to survive the winter, especially in colder zones. Don't even cut off any dead wood.
Cut faded blossoms. If powdery mildew is present, remove most of the stem that has the worst of the problem. Discard any affected debris — do not compost.
It's important to get these out of the ground before the first killing frost; it doesn't harm the plants to do this while their foliage is still green. Dig out the bulbs and gently shake excess dirt from their roots. Cut off the stalks. Allow bulbs to "cure" (dry) for a couple of days. Shake any remaining soil from bulbs. Put bulbs in a cardboard box with some peat moss and store in a cool, dry place for the winter.
To divide, dig out the entire clump and then cut it into sections. Replace one section into the original hole and save the remaining sections for other bare areas in the garden.
Remove All Annuals from the Garden
Remove all annuals from the garden. You can save seeds from most annuals and plant them next year. Zinnias are an easy plant to collect seeds from and to grow from seed. For window boxes, simply remove summer annuals, add more potting mix and plant cool-weather bloomers like ornamental kale and pansies.
Ready Your Container Gardens
Believe it or not, the most overlooked group of plants this time of year is fall flower for container gardens, and there are plenty of things to consider with respect to their care.