Layer fallen, shredded tree leaves, compost and fertilizer in the garden bed and turn under the soil. You could also dig trenches in the garden, pile in the leaves and compost and cover with soil. These organic soil amendments will decompose over winter and leave the soil more fertile when spring rolls around.
I usually go through and lightly rake my flower beds at least once in the fall and that cuts down on the amount of clean up I have to do in the spring. If you have a problem with rotting perennials or rotting bulbs, it's especially important to get as much leaf debris off your flower beds.
Also, do not cut back hardy perennials like garden mums (Chrysanthemum spp.), anise hyssop (Agastache foeniculum), red-hot poker (Kniphofia uvaria), and Montauk daisy (Nipponanthemum nipponicum). Leave the foliage. It's important to protect the root crowns over winter.
Whether you're new to gardening, or a seasoned pro, building better soil is the single most important thing you can do to improve your gardening success. And fall is the best time to do it! To learn more, read Building Healthy Soil.
A fall garden can be beautifully productive at a much slower pace. Even so, the trick is remembering that, like the summer garden, the fall garden needs to be started months before the cool weather comes. For me in USDA zone 6, I prepare for the fall garden in July and August.
Tip. Mid- to late-August is often an excellent time to plant fall flowers, as long as the weather isn't still so hot that the plants will suffer heat stress.
Fall mulching saves time and energy in the spring because it cuts down on the need to weed. It also saves you the labor of spreading new soil and mulch after winter snows. You may also save money too. Mulched perennials are likely to last the winter, reducing your outlay on new blooms.
While many gardeners believe that fall cleanup is the only way to go, there are actually several benefits to leaving a majority of cleanup for the spring. Since many animals and insects are adapted to use plants for winter survival, it makes for a healthier garden to keep their needs in mind.
In Northern California:
Plant cool-season annuals such as African daisy, Iceland poppy, larkspur, lobelia, pansy, petunia, phlox, snapdragon, stock, sweet alyssum, sweet pea, and viola. Water them frequently, especially during hot spells. Fertilize and water your roses, but do not prune them yet.
Plants that are fertilized in the fall have an earlier green up, and regular fertilizing throughout the year makes for healthier plants in general. The same logic applies to your shrubs, trees and perennials: give them a nutritional boost before winter to help them bounce back vigorously in the spring.
Adding organic matter in the form of compost and aged manure, or using mulch or growing cover crops (green manures), is the best way to prepare soil for planting. Adding chemical fertilizers will replenish only certain nutrients and do nothing for maintaining good, friable soil.
When to Cut Back Hostas. As a general rule, hostas should be cut back in the late fall. Start with leaves that have wilted or turned brown. Healthy leaves can stay a bit longer to help the roots store needed energy.
Violas and Pansies
These hardy little flowers not only have gorgeous blooms but can survive almost anything winter has to dish out. Violas and pansies grow best in partial shade but need a minimum of four hours of sunlight a day.
When To Winterize Your Gardens. The best time to start winterizing gardens is after the first hard freeze in the fall. A hard freeze occurs when the temperature gets below freezing overnight, killing off tender annual plants and vegetables.
The best time to apply winter mulch is just after the first hard frost. By putting mulch down at this time, you will help stabilize the temperature of the soil right around freezing. Applying mulch too soon may delay freezing and encourage heaving and thawing.