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Wintering Herbs Indoors

Some herbs are too tender to leave outdoors for winter and should be potted up and brought in. Others you want to enjoy all winter on your windowsill so you can snip for your favorite recipes. I've found there are very different requirements depending on whether you're just keeping them alive during dormancy or you want actively growing plants to use. And growing indoors when the light is as low as it is during winter is quite different from growing outdoors.

Plants to winter indoors for use are:  rosemary, catnip, lemon verbena, scented geraniums, lemongrass, thyme, bay, chives, lemon balm, winter savory, sweet marjoram, oregano, parsley, mints, and sage. Beware the mints-the runners will take over the house just like they do outside (remove them or you'll have no leaves), and don't bother with St. John's wort or French tarragon-they don't like it inside or in pots at all, for that matter.

I plant herbs with the same light, soil, and water requirements together-thyme, marjoram, rosemary and sage, for instance.

The tender herbs like rosemary should be brought in as a whole plant or you'll forfeit what's left outdoors. Cut back tender perennials like lemon verbena, scented geraniums, rosemary, lavenders (if they are not hardy in your area) and lemongrass in midsummer and again before bringing them in. A special note on chives: Let the tops die back and the roots freeze before you dig them up and bring in.

Annuals, such as basil, dill, cilantro, and sweet marjoram (a tender perennial that should be treated as an annual) really cannot be dug up to bring indoors. Don't fret! You can still grow them indoors, just start them from seed, or buy them as plants in the fall specifically to grow indoors.

Some herbs like lemon verbena are actually deciduous shrubs and will lose their leaves when they go into dormancy. Don't panic. Keep the plants in a cool place, 40ºF is best, and don't water. In spring, they'll literally burst out into leaf. Scented geraniums don't lose their leaves, but they stop growing if you keep them this way. They should be kept on the dry side and watch them diligently for aphids. Aphids love them indoors!

Just pick the herbs according to what you like to use for cooking, and choose attractive pots. You will be living with these herbs as houseplants all winter.

Dig up the entire clump of perennial herbs, and divide the amount you want to bring in from the rest. Be careful not to disturb the roots. Pop the outdoor crop back into the ground and pot up the indoor herbs. These will winter inside and be transplanted out again once the spring comes. Let them sit in a shady spot for about 2 weeks before bringing in to help them adjust to the transition.

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 65°F 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 the hygiene of your pots. I always reuse pots from year to year, but first I soak them in a bleach solution of ¼ 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. .