Fresh herbs can become a little more difficult to grow over winter, particularly if you are in a cooler climate, but many of our most popular herbs can still grow remarkably well in a pot on the kitchen bench. Pots of herbs can be one of the most useful and decorative indoor plants for your kitchen, all year round - but particularly in winter when it gives us the opportunity to use fresh herbs that we may be dormant outdoors at that time.
Let’s not kid ourselves - all plants would prefer to be outdoors (where they belong), except perhaps on the frostiest of days or if it is snowing; but many plants exposed to these conditions have adapted to survive them. Although hardier herbs, like twiggy stemmed rosemary and lavender, can withstand winter temperatures, tender herbs like basil are no match for the cold but will survive if protected indoors or in a glasshouse.
Many herbs are pretty hardy plants that will grow all year round so long as the temperature is kept above 10°C or 15°C, and they are provided with adequate light, water and nutrition.
In order to get the most out of indoor winter herbs you’ll need to place them near a source of natural light, ideally on a window sill in the kitchen, where you will remember to water and use them. If the sill is too narrow, you may have to choose a table or bench top or a plant stand on casters that you can move from window to window in your house.
Some herbs (e.g. mint) may grow with less light than others, and certain herbs may struggle no matter what you do in places where winter day length becomes much shorter (e.g. Tasmania). Tarragon is a good example - it normally dies down in winter in cold areas with lower winter light. No matter how bright winter light (through your windows) may seem, it is never as bright as summer light - so even herbs grown in front of a bright window may not be getting enough light.
There are many other herbs that react to day-length and unless you can extend the light (e.g. using artificial light) the plants will not thrive, no matter how ideal all the other conditions are. Cornell University research found that two 75watt incandescent lights hung above tarragon plants (on for 16 hours a day) is enough light to keep the plant growing all winter indoors. If you live nearer the equator and have naturally long days then of course you won’t need to use lights.
How to Treat Herbs Inside
Think about watering - watering is affected by potting soil type, amount of drainage, size of pot (smaller pots will dry out faster), what pot is made of (unglazed ceramics and wood absorb water) and air conditioning. Most people go wrong by watering every “x” number of days. You can’t set watering by the clock. You must feel the soil, and water when it is drying out, not too early not too late. Most herbs (other than annuals such as parsley and mint), prefer the soil to dry out before watering. Remember too that plants transpire less in cool winter conditions (just like we perspire less) so they need less water as a result. Use a good quality, free draining, sandy potting mix that provides excellent drainage with a pH of 6-7 - this will produce great herbs.
Temperature - most like 15 to 20°C and can do OK a little higher, which falls within the scope of most room temperatures over winter.
Light - most do better if they receive light, so either place pots near a window or consider using artificial lights – discussed in more detail below.
Humidity and oxygen - there’s a danger of low humidity and low oxygen becoming a problem if your house is heated 24 hours a day, 7 days a week. Solution: give plants some fresh air occasionally by placing them outside or opening the doors and windows.
Fertilize with a weak fertiliser (seaweed or fish emulsion is great e.g. Seasol) every 2 weeks, provided new growth is coming on – even grown indoors in ideal conditions growth is slower than it would be outdoors in ideal conditions so a gentle fertiliser as suggested is better than trying to force plants with a general pelletised fertiliser.