Most herbs suffer in silence indoors over the winter.
Can you really grow herbs indoors over the winter and thus always have fresh herbs to add to your menu. Yes, but… and it’s a big but.
And that’s in spite of the fact that lifestyle magazines and television shows keep on pushing the idea that growing herbs indoors is as easy as pie. I can pretty much guarantee that none of the journalists who promote the technique have ever tried it. They only repeat, like parrots, misinformation gleaned from others just as misinformed. So let’s dot a few i’s here.
Why Things Go Wrong
The problem is that most herbs just aren’t adapted to the conditions we can offer them in our homes during the winter, especially low light and dry air. Even in front of a large south-facing window, the light received by windowsill plants December through February is equivalent to no more than deep shade outdoors in June and July, notably because days are very short and often cloudy to boot. Fluorescent lights help, of course (set the timer for 16 to 18 hours a day to ensure maximum light), but give moderate light at best, enough to keep the plants alive, but not enough to promote the dense healthy growth of herbs grown outdoors.
Also, the atmospheric humidity in most homes is closer to that of the Sahara Deser than what herbs would like, that is at least 50%.
Barely 3 weeks after being brought indoors, this basil is already dead.
Plus plants weakened by a lack of light and dry air become prone to diseases and bugs, including whiteflies and spider mites. And eventually start to die back.
But don’t give up hope just yet: there are some herbs that do fairly well in the house and you can keep most at least alive if you know how to handle them.
Indoor Herb Scorecard
Bay laurel is one of the few herbs that actually does well indoors.
Bay Laurel or Bay Leaf (Laurus nobilis): This leathery-leaved plant is the easiest herb to grow indoors. You can place it in sun or partial shade or under a fluorescent lamp. Its growth is very slow, but it does grow, and it seems indifferent to the entire indoor temperature range, from 34˚ to 95˚F (1 to 35˚C). By bringing your bay laurel indoors every fall and putting it back outdoors every summer (it can stay outdoors all year in zones 8 to 11), it will gradually become a fairly sizeable shrub or even a small tree, but that will take decades.
Watch out for scale insects, though! Most bay laurels sold these days are already infested (read Another Bay Laurel Bites the Dust for more details), so make sure you get yours from a reliable source.
Parsley does fairly well indoors… if you give it your brightest light.
Parsley (Petroselinum crispum): If you pot yours up and place it in a brightly lit spot at normal room temperatures, it will grow slowly but steadily throughout the winter. The leaves may be a bit floppy, but they’ll still be edible. Watch out for spider mites though. It’s not worth saving parsley for a second year. It’s a biennial and begins to bloom in the spring, then becomes bitter. It’s done its job: just toss it in the compost!
Stevia is one of the happier campers indoors.
Stevia or Sweet Leaf (Stevia rebaudiana): This ultra sugary herb is one of the few that is of tropical origin (most classical herbs come from climates where winters are cold or even freezing) and, therefore, very tolerant of the warm temperatures we maintain in our homes. If you set the thermostat to please yourself and your family, it will be happy too. Give it as much sun as you can and if it starts to etiolate (stretch for the light), just chop it back. Give it normal houseplant care, including watering when the soil is dry to the touch.
There are other herbs of tropical origin that may not be traditional cooking herbs in European cuisine, but that have the advantage of doing fairly well in our homes. This group includes the various scented geraniums (Pelargonium spp.) and rau ram or Vietnamese coriander (Persicaria odorata). Lemongrass (Cymbopogon spp.) is a tropical grass (zones 9 to 12) that can also be grown indoors. Given it’s origins, you’d think it would do well there, but in fact, keeping it alive can be difficult unless you can offer it a very sunny windowsill.
“Think Twice” Herbs
These are herbs that will do all right indoors, but only if you meet their special needs. These aren’t necessarily going to be the stars of your windowsill, but they ought to be able to produce a bit of a crop indoors.
Young basil plants do all right indoors, but mature plants tend to keel over.
Basil or sweet basil (Ocimum basilicum): More people fail with basil than any other herb, but it will grow quite well indoors if you know what to do.
It’s important to understand that sweet basil is a naturally short-lived plant, little more than an annual. So when you try to bring in a basil plant from your summer garden, you’re dragging in a plant that is just about to croak: it rarely survives the transition between outdoors and in. However, if you sow basil seeds (ideally under lights), you’ll be starting afresh with a new young plant and should get excellent results… at first. It does go downhill more quickly indoors than out, so when it shows any sign of decline, harvest it and make a huge batch of pesto.
By resowing occasionally and always harvesting basil when it is still young and vigorous, you should get great results. Just give the plants the best light you can.
Note that basil loves heat: this is not a plant to put in a cool room. Keep in a spot that remains relatively warm even at night (preferably above 60˚F/15˚C) and it should to fine.
So much for sweet basil. There are perennial basil species that are naturally long-lived and that will do much better indoors, thriving over the entire winter in bright light. This group includes African blue basil (O. ‘African Blue’), holy basil (O. tenuiflorum) and lemon basil (O. x citriodorum), including the popular variegated form, O. x c. ‘Pesto Perpetuo’. Note though that these easier-to-grow basils don’t have the same taste as sweet basil (O. basilicum).
Chives needs a cold treatment before you bring it in.
Chives (Allium schoenoprasum), lemon balm (Melissa officinalis) and French tarragon (Artemisia dracunculus sativa): These three cold-climate plants tend to decline indoors… unless you give them a cold treatment first. Pot them up in September or October, but then leave them outdoors until they’ve gone through a good frost or two (or three). Now, when you do bring them in, they’ll think it’s spring and start to grow vigorously.
Oregano (Origanum vulgare), sage (Salvia officinalis) and thyme (Thymus vulgaris and others): These plants prefer to be outdoors in the winter and you may find mature plants don’t adapt well to being moved indoors. You’d do better to take stem cuttings in the autumn and grow the cuttings indoors instead. They’ll still be in their “young and vigorous” stage of life, allowing you a decent harvest.
Lemon thyme (T. x citriodorus) is particularly easy variety for indoor growing.
Rosemary: not the easiest herb indoors.
Rosemary (Rosmarinus officinalis): I’m always annoyed when gardeners gush on and on about how beautiful their rosemary is in the winter and how they have no problem growing it. You see, I just don’t have quite the indoor conditions it wants, that is a cool but intensely sunny room. Nor to most people. And a rosemary plant that doesn’t like its conditions is soon a dead rosemary plant.
Look for a spot when temperatures remain below 60˚C (15 ° C) at night (and in fact, during the day as well if possible), yet above freezing, and water it carefully, just enough so that the soil is barely moist and you too can have rosemary you can be proud of.
True lavender or English Lavender (Lavandula angustifolia): It prefers to spend its winters in the ground, but can sometimes remain alive indoors under the same conditions as rosemary – bright sun, low temperatures, and careful watering – although it won’t do much more than just sit there. My personal thought is that lavender really is an outdoor plant and there is no really good reason to want to grow it indoors.
Less hardy Spanish Lavender (L. stoechas and its hybrids, zones 7-9) can also spend the winter indoors under similar conditions. Bringing it indoors is a way for cold-climate gardeners to save it from the cold.
Mint (Mentha spp.): There are many species of mint and some do better indoors than others, but in general these are rock-hardy herbs that actually prefer a winter outdoors. When you try to grow them indoors, they tend to gradually deteriorate, unable to adapt to constant heat and dry air. But if you have a barely heated but brightly lit room or a cold greenhouse, they can do quite well. Unlike the two previous plants (rosemary and lavender), that prefer to their soil to remain barely moist, water mints well, although without exaggeration: they don’t like to dry out.
Herbs Not Worth Saving
Dill (Anethum graveolens), anise (Pimpinella anisum), coriander (Coriandrum sativum), borage (Borago officinalis) and chervil (Anthriscus cerefolium): These are annuals and will be nearing the end of their life cycle in fall, if in fact they are not already dead at that time. There is no use trying to save from the cold a plant that it is genetically programmed to die only a very short time later!
You can always sow seeds of these plants indoors (fluorescent light might well be necessary) if you want access to fresh leaves during the winter, but they probably won’t produce seed indoors and for several of these herbs, seeds are the part most often consumed.
Winter Care for Herbs
If you do decide to bring some herbs indoors for the winter, remember they’ll need as much light as you can give them as well as good atmospheric humidity. For the latter purpose, the use of a room humidifier may be necessary. Water them generously when the soil is dry to the touch.
If you grow your herbs in front of a window, stop fertilizing in late November. You don’t want to encourage them to grow under poor light, as that will lead to weak, etiolated growth. Start fertilizing again at the end of February or in early March, when the days are longer.
If you grow herbs under lights, that’s a different story: you’ll be able to maintain long days (16 to 18 hours) all year, so they’ll keeping growing all through the winter. Feed them monthly with an all purpose fertilizer.
Keep your eyes open for pests, especially spider mites and whiteflies, but also aphids. Treat them with insecticidal soap if you see any. And remove diseased leaves on sight, as they can spread like wildfire on stressed plants… and the average indoor herb is very stressed!
With a the information above, you should be able to cultivate at least a few herbs in your home this winter… and maybe even impress your guests with a bouquet of fresh herbs on the table in mid-January!
But be honest with yourself: growing herbs indoors may be trendy, but is not an ideal situation for most of them. You’re putting them through considerable stress and most would really prefer being left outdoors for the winter.
My thought is the following: for everything there is a season… and winter is just not the season for most herbs. A truly laidback gardener would learn to accept that herbs are summer plants and leave it at that.