Some herbs simply won’t grow in the UK over the winter: basil is an example, and chives will survive and return exuberantly in the spring, but won’t actually grow, even in a cold frame. However, quite a few herbs can be kept going, especially if you can grow them under glass.
Rosemary is cold-hardy, and will do fine even out of a cold frame. You should be able to crop a little from it if it’s a well-established plant, but be careful not to overdo it. It’s unlikely to put much new growth on unless the winter is quite mild, but taking some of the old growth is fine. Good for roast potatoes!
Mint and parsley are both cold-hardy, and parsley in particular will keep growing even through a little snow. It’s slow growth, though, and you won’t be able to use that much of it unless you have a lot of plants. If you’re keen to keep it cropping, it’s probably worth moving it into a cold frame to encourage more growth, especially if you’re a bit further north. Happily, parsley self-seeds with abandon, so after a season or two you may well have plenty of small plants in the corners of pots, and perhaps not even be all that bothered if you accidentally kill off a couple by over-harvesting.
Bay trees are cold-hardy, but like rosemary, there’s a limit to how much you can crop over the winter as the plant won’t be growing very much if at all.
Sage and thyme will survive without any protection over the winter, but you probably won’t be able to crop any of them unless the winter is unusually mild. They’re unlikely to put on much new growth and you don’t want to chop off all of the old growth for cooking or you’ll take away all the plant’s reserves. These will do well in a cold frame and should grow at least a handful of fresh leaves, in which case you should be able to keep getting (limited!) quantities of fresh herbs from them over the winter.
Oregano will die back altogether over winter, and even in a cold frame may not put on any new growth (though it’s worth a go). However, it will return in the spring unless the winter was unusually cold.
In general, though, bear in mind that the plants won’t be doing much in the way of new growth, so you should be careful when cropping. Don’t take so much that the plant can’t keep itself alive!
Keeping the more delicate herbs (thyme, oregano, even mint and parsley) in a cold frame will mean that they will pick up again much earlier in the spring than they would if just left outdoors, so it’s worth it even if you can’t use them very much over the winter. Parsley is biennial: it sets seed in its second year. So a parsley plant will keep going happily for a full year, before flowering and setting seed in the second year, after which it dies. Either save the seeds, or let it sow itself in the surrounding pots.
Remember that you can freeze or dry fresh herbs throughout the year to give yourself a supply of your own herbs through the winter months.
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