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Wintering Herbs Indoors

Here are the things you should consider if you want fresh herbs all winter:

  • Soil: Potting soil needs to be rich, with good drainage. Garden soil or potting soil alone are way to heavy for any indoor plant. I like to mix 2 parts compost to 1 part vermiculite and 1 part perlite. If you are bothered with plant diseases, don't use soil in your mix. Purchase a soil-less potting mix from the nursery. Pots should be at least 4", but a 6" diameter pot is better.
  • Light: Even in the warmest and brightest of your windows, the light may not be enough during winter. Most herbs will need 4 to 5 hours of light a day. The ones that like partial or full shade will need less-mint, parsley, rosemary, thyme, and lemon balm. If you do not have a sunny window, set up 4' workshop lights with two cool, white fluorescent bulbs 6" to 8" above the herbs. If you are growing on a sill, remember to turn your plants regularly to make sure all sides get enough light.
  • Temperature: Most herbs like it on the cool side-daytime temperature of 65F and night time temperatures between 55 and 65 F. Be careful to keep plants from touching freezing windowpanes. If you are just keeping herbs alive and not trying to use them, they will survive in a greenhouse kept at 40 F, except for basil and scented geraniums. These can't take temperatures below 50 F.
  • Air-circulation: Herbs don't like stagnant air. Stale, dry air promotes fungus and insect infestations. Don't crowd pots. I have a fan that I turn on once a day for about an hour.
  • Water: Never let plants sit in water. Over-watering will cause root rot and other fungal diseases. Proper watering starts with a nice loose, well-drained potting mix. Water herbs thoroughly when the surface soil dries out. Never let rosemary dry out completely. Mints and lemon balm like it wetter than other herbs.
  • Fertilization: Feed indoor plants only once a month during winter. Over-fertilizing will cause your herbs to be leggy and lose flavor. I use fish emulsion mixed with seaweed at half strength. Yes, it smells, but my cats don't mind.
  • Pests are always a problem. Check the herbs you are bringing in very carefully for pest infestations. Close, dry, indoor conditions can bring out pests galore. Watch your plants like a hawk. You will be able to control whiteflies, aphids, and spider mites, if you do not let them get out of hand. Look for these pests on the UNDERSIDES of the leaves. That's where those pests hang out. Aphids like to hide in leaf creases, so look hard. They are usually green, but they can be black and brown. The black ones are the hardest to dislodge. Whiteflies are pretty easy to see because they're white! Mites are the peskiest because they're almost invisible. In good light, look on the undersides of the leaves for red specks, but sometimes they are black. If you see any pests, spray them with insecticidal soap. Watch for scale especially on bay. Dip a Q-Tip in alcohol and rub off the scale. If you can't control the infestation, get rid of the plant, or you may have a houseful of infested plants. In all cases, check out the Natural Remedies section of this booklet for my recipes for taking action against the little buggers.
  • Diseases may be a problem, especially if you aren't careful about hygiene of your pots. I always reuse pots from year to year, but first I soak them in a bleach solution of cup to 3 gallons of water for 5 to 10 minutes. Root rot is the most common problem. It's a fungus found in the soil, and you know you have it if the lower stems wither and die or the whole plant just lays down. Look at the stem where it comes out of the soil and you'll see that the outer layers of the stem and root sloughs right off, leaving just the center of the stem. The plants that have the worst problem with root rot are lavender, oregano, rosemary, sage, tarragon, thyme, and marjoram. Bleach your pots, give the plants really good drainage, and don't crowd them. Use a fan, if you need to, for better air circulation. If you do get root rot, destroy the plants (don't put them in the compost pile) and bag the soil and get rid of it. Powdery mildew may also show up, especially on rosemary. It is a white coating on the leaves and another soil-borne fungus. .