Harvest Green Tomatoes
Once nights are consistently below 50 degrees F, it's best to harvest any remaining mature green tomatoes (those that have reached at least 3/4 of their full size and turned light green to white) even if the vines haven't yet been hit by frost. These tomatoes will ripen better indoors once the weather is this cool. Clip tomatoes from the vine with a short piece of stem attached. Red tomatoes well on their way to ripening can tolerate cooler temperatures and can be left on the vine until frost threatens.
Pinch Brussels Sprouts
To get the sprouts to ripen together, pinch off the top couple of inches of your Brussels sprouts plants to direct their energy into the sprouts that are already developing along the stem. Unpruned plants will continue to produce new sprouts until the weather is quite cool, though they may be smaller. Clip off any lower leaves that have yellowed, and keep plants watered if fall weather is dry. Sprouts harvested after the first frost will be sweetest. Harvest from the bottom up when the sprouts are between 3/4- 1 1/2 inches in diameter.
Cut Back Asparagus Tops When Yellowed
Wait until the ferny top growth of asparagus plants has yellowed and dried before cutting it back. Burn or discard the fronds if asparagus beetles were a problem, as this is where these pests overwinter. Then add a topdressing of compost to the asparagus bed.
Practice Good Garden Sanitation
Clean up garden beds well at the end of the season, removing plant debris (including weeds) that might harbor pests and diseases that could re-emerge the next year. If this plant debris shows obvious signs of insects or disease, it's better to dispose of it rather than add it to your compost pile. Taking it to a municipal compost drop-off rather than putting it in the trash is fine. Unlike most home compost piles, materials in these large-scale composting operations usually reach temperatures high enough to kill off pest and disease organisms.
Clean Up the Potato Patch to Control Late Blight
Unlike some other disease-causing organisms, the pathogen that causes late blight on tomatoes and potatoes can overwinter only on live plant tissue. In cold winter areas, infected potatoes that are left unharvested in the soil or stored and used as seed potatoes the following season are common ″Typhoid Marys″ that reintroduce the disease. (Late blight also arrives by working its way north on the wind from warmer southern areas over the course of the summer or it may be introduced earlier on shipped-in, infected transplants.) If late blight was a problem in your garden this season, be sure to dig up and destroy all infected potatoes (those with brownish purple spots that become a wet or dry rot), including those that rot in storage, and start with certified disease-free seed potatoes next spring. Don't put infected plant parts in the compost pile as some plant tissue may survive the cold in the center of the pile.
Clean Your Cages
Some pathogens that cause diseases on tomatoes, including bacterial spot, canker, and speck and fungal diseases such as early blight and alternaria, can be carried over from one year to the next on tomato cages and stakes. Prevent problems by cleaning soil and plant residues off cages and stakes, then disinfecting them by soaking them a solution of one part chlorine bleach to nine parts water for five minutes. Rinse well, then air dry before storing for the winter.
Cover Bare Soil
Once you've cleared away this year's plant debris, it's time to give your garden a winter blanket. Soil that is left bare over the winter is vulnerable to erosion from wind and water. Rain falling on bare soil can compact it, and rain and snow melt can leach nutrients in the soil out of the reach of plant roots. One way to counter this scenario is to cover the soil in your garden beds with a thick layer of mulch for the winter. Fallen leaves are generally plentiful (and free) in autumn and work well, especially if they are chopped first to make them less likely to blow in the wind. (An easy way to chop leaves is to run them over with a lawn mower and collect them in the mower's bag.) But other materials will also work, such as dried grass clippings and clean straw.
Grow a Winter Cover Crop
An even better method of winter soil protection is planting a ″cover crop.″ A good cover crop grows fast, blanketing the soil and competing with weeds. Cover crops also grab on to nutrients in the soil, keeping them from leaching away over the winter; then return them when the cover crop is tilled or dug in in spring. The decomposing cover crop plants add new nutrient wealth and ″tilth″ to the soil. Growing cover crops also provides opportunities for lessons on root function, nutrient recycling, and decomposition.
The types of winter cover crops that are suitable for planting in your food garden in late summer and early fall will depend on your region of the country and your gardening schedule. In many areas annual ryegrass (Lolium multiflorum) or oats (Avena sativa) make good winter cover crops, since they grow quickly and are hardy enough to grow late into the fall. In colder zones (Zone 5 and lower) they are often killed by cold over the winter. The dead tops act as mulch over the winter and it's easy to turn under the killed tops in the spring or rake them and add them to your compost pile, leaving the roots in the ground to decompose.
Winter rye (Secale cereale) is another hardy cover crop that can be started as late as mid-fall, so it makes a good choice if your garden beds are ″occupied″ until late in the season with cool season crops. Winter rye will survive the cold in most climates to grow again in spring, so it adds lots of organic matter to the soil when the green, growing plants are tilled under in spring. Be sure to till growing cover crops in before they go to seed to prevent unwanted self-sowing. Allow 3 weeks between turning under a green cover crop and planting the garden to avoid the temporary nitrogen tie-up that occurs as the cover crop decomposes. This wait may delay the planting of cool-season crops, so in beds earmarked for spinach, lettuce, and other spring crops, choose a cover crop that winterkills reliably or cover the bare soil with mulch instead.
Southern gardeners who can grow year-round might choose to rotate a cover crop into one section of the garden to improve the soil and give it a rest. To learn about recommended cover crops and planting times for your area, contact your local Cooperative Extension Service or Master Gardeners program. For more general information, check out Fall Garden Cover Crops.
Make Leaf Mold
Even easier than making compost, but not for the impatient, leaf mold is made simply by piling fall leaves inside a corral of wire fencing. Shredding the leaves first and turning the pile every six months will speed the process some, but because fall leaves are low in nitrogen, decomposition will take place slowly over a period of six months to two years. Leaf mold is a great soil amendment, helping to increase water retention, improve soil structure and provide habitat for beneficial soil organisms. It also makes great mulch.