The basil looks perfect at dusk, glossy and green beside the back step, and then a clear, cold night rolls through. By morning the leaves are dark and limp, and the pesto plans are over for the year.
NC State Extension has a short list of herbs that meet that fate. Its leaflet on winterizing the herb garden labels six as tender annuals: basil, cardamom, coriander (cilantro), dill, geranium and lemon grass. The leaflet states that annual herbs are killed by the first hard frost in the fall.
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The Six Tender Annuals on the List
The publication, Winterizing the Herb Garden from NC State Extension, was originally published in 1999 and was reviewed and revised on Dec. 13, 2024. Its Table 1 lists the hardiness of dozens of herbs, and six carry the same label: tender annual.
Basil leads the list for most home cooks. Wikipedia notes that basil is sensitive to cold and is grown as an annual in temperate climates.
Cardamom and coriander, which the leaflet also calls cilantro, carry the same label, and so does dill, the feathery herb that shows up in pickling jars and summer salads.
The last two are geranium, listed under its genus name Pelargonium, and lemon grass. The leaflet adds a note to geranium: bring it indoors. Parsley is a separate case. The table calls it a biennial that is grown like an annual, so it appears on the page but not among the six.
Anyone keeping a garden bed through the holidays may recognize the pattern. A look at elevated garden beds for fall greens and herbs shows how some growers stretch the season with the right setup.
Basil and Geranium Can Come Indoors
NC State says some frost-sensitive herbs, such as basil and geranium, can be brought indoors for the winter. There are two routes. Cuttings can be taken and rooted, or the entire plant can be potted up and moved inside.
Penn State Extension offers guidance for the indoor stage. Its page on growing herbs indoors names chives, basil and cilantro among the good options for a winter windowsill, and notes that most herbs need about 6 hours of direct sunlight.
Penn State also gives a temperature warning: basil cannot survive temperatures lower than 50 degrees F and should not sit on a cold windowsill during the winter months. Rooms at 65 to 70 degrees F during the day suit most herbs.
Heated homes bring a second challenge, dry air. Penn State says homes with forced-air heating tend to have the driest winter conditions, and grouping pots together is one way to raise humidity around the plants.
What Counts as a Hard Frost
The Old Farmer’s Almanac draws a line between the terms. It says a light frost, 32 degrees F and colder, kills all tender plants, while hardier plants such as spinach and kale survive until a hard freeze at 28 degrees F and colder.
The Almanac also notes that frost can form on the ground and on plants even when the air temperature is as high as 38 degrees F. Frost itself is a layer of ice, and Wikipedia explains that frost forms when water vapor deposits onto a surface that has dropped below freezing.
That is why a forecast of 36 degrees F is not always safe for a basil plant. The Almanac adds that frost dates are averages and do not reflect smaller microclimates, such as a low spot in the yard or an area near water or pavement.
For a tender annual, the first frost mostly settles the question. Once leaves turn black, NC State advises removing the dead plants, which cuts down on overwintering insects and plant problems.
A guide to the worst garden pests and how to get rid of them covers the usual offenders.
Rosemary and other Mediterranean herbs favor dry, well-drained ground, and dry-soil gardening is the subject of a list of xeriscaping ideas that save money at home.
Mulch After the Freeze, Not Before
For perennial herbs, NC State’s timing advice is specific. Heavy mulching before cold weather should be avoided, since it keeps the soil warmer and may actually reduce winter hardiness. After the first hard freeze, the leaflet says to apply a 3 to 6 inch layer of straw, pine needles or chopped leaves.
That layer is enough on its own for mint, chives and fennel. NC State says mulch alone protects them to minus 20 degrees F. Most of it should come off in the spring as new growth begins.
Cold-season prep reaches beyond the herb bed, and a few things that should never be left outside through the winter are covered in a separate post. Wood stored for the winter fire is another seasonal chore, and a post on five things never to do with a winter firewood pile lays out the rules.
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Rosemary and Lemon Verbena Need Extra Help
Two herbs fall between hardy and tender. NC State says rosemary and lemon verbena are not reliably winter hardy. Its table lists rosemary as normally hardy in the Piedmont and Coastal Plain regions of North Carolina, and lemon verbena only in the Coastal Plain.
The leaflet’s method is to cut each plant back to within a couple of inches of the ground after the first hard frost, cover the remaining stub with soil, and then add a 4 to 5 inch layer of mulch.
For lemon verbena, a microfoam ground cover, the packing material used around fragile items, held down with soil works very well. NC State reports over 95% survival in most years. A cage of hardware cloth or chicken wire about 12 inches wider than the plant, filled with mulch, is another option.
Lemon verbena is deciduous, so it loses all of its leaves if brought indoors. Rosemary, a Mediterranean native according to Wikipedia’s entry on the herb, also dislikes wet soil, and NC State says excessively wet soil can reduce winter hardiness for rosemary, thyme, lavender and French tarragon.
For cooks, the takeaway is a short checklist. Harvest the six tender annuals before the first hard frost, pot up basil or take cuttings, mulch the hardy ones after the freeze, and give rosemary and lemon verbena a little extra cover. Anyone who has watched a basil plant go black overnight knows the first item matters most.
This article was produced with AI assistance and edited for accuracy against the sources linked above.
