Early fall is your last chance to harvest most herbs before they go dormant for the winter. Think about the plant's natural growth patterns before you harvest. If the herb dies back completely, as do parsley, French sorrel, coriander/cilantro, and basil in cold climates, you should remove as many good leaves as you need before they are killed by winter cold.
But if you harvest hardy woody perennials such as thyme, savory, and tarragon heavily now, you can weaken the plants. (It won't hurt to take a couple of sprigs but discontinue heavy pruning of woody herbs and roses 45 days before you expect the first fall frost.) Avoid heavy fall pruning on roses, evergreens, and other trees and shrubs that haven't yet fallen dormant. As long as woody plants are still growing, they could resprout with soft new twigs that are certain to be lost to winter cold.
Bringing Herbs Indoors
Start new plants or bring in existing plants. About a month before the first fall frost, dig up the plants you want to keep indoors. Capture as many of the roots as you can. Mature plants, such as sweet marjoram, lavender, and scented geraniums, should be cut back by about one-third their full height to make them more manageable. Put each one into a pot that's slightly bigger than its roots. Fill in the vacant area with a soiless growing mix. Let the plants get settled in the pots in a lightly shaded outdoor location for a week or so. Then move them into deeper shade for another week to get them ready to come indoors. Before frost arrives, bring tender herbs indoors to the window or light garden you've prepared. But let chives and garlic chives stay out through a month or so of winter cold before you bring them indoors; they will grow much better indoors if they get a short winter to trick them into thinking it's spring.
Herbs for Fall Seeding
- Sweet cicely
- Winter onions
When your herbs are grouped together indoors, they may be more susceptible to pest problems. If you find whiteflies fluttering around the indoor herb garden, spray with to kill mature flies and repeat until you get rid of newly hatched generations. Red spider mites may attack because the humidity is low. If so, use a pebble tray and fight them with insecticidal soap. Also use a pebble tray or spray plants often with a mister to increase humidity. To discourage diseases, remove dark, dead, and sickly growth, and scrub your pruning shears or knife in a solution of 1 part bleach to 10 parts water between each cut.
Work fresh seed into well-prepared soil in early fall. The plants may germinate now and spend winter as small seedlings, or they may germinate first thing in spring. For many perennials, such as those listed here for fall seeding, exposure to winter's alternating periods of warm and cool temperatures, freezing and thawing, and wet and dry soil can coax reluctant seeds out of their shells faster than spring sowing.
In warm climates, fall and winter are ideal times to plant cool-season annual herbs such as mustard, cilantro, calendula, and arugula. Sow the seed while the temperatures hover around 70-degrees F or when cooler but mild weather can be expected to follow.