How to Grow Basil

Warmth decides everything

Basil comes from tropical Africa and Asia, and it behaves like it. It does not just tolerate cold poorly; it stops growing in it. University of Illinois Extension notes that cold soil and air stunt basil and can blacken its leaves at about 50°F, and UF/IFAS says the leaves of many varieties turn black and drop below 40°F. A plant set out into cold soil can stay stunted, so waiting a week or two for warm weather costs little.

So the rule is not “plant on the last frost date.” It is to plant after it, once three conditions line up:

• Nights stay above about 50°F (Clemson) or 55°F (UF/IFAS).
• The soil is at least 60°F (Illinois).
• Days are reliably warm. Clemson waits until daytime highs are consistently in the 70s.

In most of the country that puts outdoor planting 1-2 weeks after the last frost; the dates for your zone above already account for that. If a late cold snap arrives after planting, Clemson suggests covering plants overnight with pine straw, an inverted pot or a bucket and uncovering them the next day.

Choosing a basil

Sweet basil in the Genovese style is the standard for pesto and Italian cooking, but basils range from 1-foot globes to plants 3 feet tall, with green or purple leaves and lemon, lime, cinnamon, licorice or Thai flavors. Choosing well matters more than with most herbs, because two diseases decide whether basil lasts all summer.

Downy mildew resistance is the most useful trait wherever summers are warm and humid, and increasingly elsewhere: the disease was first reported in the U.S. in 2007 and has spread wherever basil is grown (UW-Madison). Clemson lists ‘Prospera’, ‘Rutgers Devotion’ and ‘Rutgers Obsession’ as strongly resistant. Maryland recommends ‘Obsession’, ‘Devotion’, ‘Prospera’ and ‘Amazel’. In a North Carolina Extension Master Gardener trial, ‘Prospera’ was the overall favorite (though tall enough to need staking in some gardens), ‘Rutgers Passion’ scored well for vigor and branching, and ‘Rutgers Obsession’ had strong resistance, compact growth and the best taste scores. UW-Madison suggests ‘Eleonora’ for Wisconsin gardeners who want a sweet basil with some resistance. If you would rather avoid sweet basil, UF/IFAS and UW-Madison both note that purple-leaved, Thai, lemon, lime and other spice basils are less susceptible; all sweet green varieties are very susceptible.

Fusarium wilt resistance matters if basil has ever wilted suddenly in your garden, because the fungus survives in soil for years. Clemson names ‘Nufar’, ‘Aroma 1’ and ‘Aroma 2’, and newer, more strongly resistant ‘Newfar’, ‘Newton’, ‘Elidia’ and ‘Prospera’.

For pots, UF/IFAS points to compact basils such as ‘Spicy Globe’ and dwarf Greek basil, and calls the foot-tall ‘Marseillaise Dwarf’ a fit for almost any space. Utah State reports that most varieties perform well in Utah, where ‘Genoa Green’ is a top leaf producer and ‘Purple Ruffles’ a low one. If you want big harvests with less picking, Clemson suggests large-leaved types.

Starting seed indoors

Transplants give you a head start, but basil needs only a short time indoors. Clemson and Utah State allow 4-6 weeks; Illinois says 3-4 weeks is enough. Seed started too early simply sits indoors waiting for the weather, so count back from the date nights will reliably stay above 50°F, not from the frost date itself.

Sow seed about ¼ inch deep (Clemson and Hawaii’s CTAHR; Utah State says ⅛ inch). Warmth drives germination: Illinois wants the mix at 70°F or warmer, CTAHR gives 70°F as the optimum with good germination from 65-85°F in about 7 days, and Utah State calls 75-85°F ideal. A heat mat speeds things up in a cool house. Expect sprouts in 5-14 days, faster in warmer mix. When seedlings have two sets of true leaves, Clemson recommends moving each into its own small pot for faster growth.

Before planting out, get seedlings used to sun and wind over several days. Penn State notes that gradually acclimating transplants to outdoor conditions improves their survival.

Sowing directly in the garden

Direct sowing works well once the soil is truly warm, and Maryland says basil grows easily from seed sown after all danger of frost. Utah State sows ⅛ inch deep, keeps the surface moist, thins to 3-4 inches apart after emergence, and keeps thinning until plants stand 12 inches apart. The thinnings can be eaten or moved to fill gaps. Basil competes poorly with weeds as a seedling, so weed shallowly to avoid cutting its roots.

Site, soil and spacing

Give basil full sun, at least 6 hours a day (Maryland). Utah State notes that sun-grown basil has more essential oil and better flavor than basil grown in shade. The exception is very hot climates, covered below.

Basil wants rich, well-drained soil with steady moisture. Work compost into the bed before planting; Clemson recommends compost, manure or composted pine bark and a pH of 6.0-6.5, while CTAHR gives a wider 6.0-7.5. Raised beds and containers help where soil drains slowly, and UF/IFAS recommends them in Florida.

Most sources space plants 10-12 inches apart (UF/IFAS, Utah State, Maryland, Texas A&M), with Maryland using rows 18 inches apart. Big Genovese types that reach 3-4 feet need about 2 feet, per Clemson. Do not crowd basil to save space: air moving through the planting dries leaves faster after rain and dew, which is the main defense against downy mildew and leaf spots.

Water, mulch and fertilizer

Utah State recommends 1 to 1½ inches of water a week, more in sandy soil and heat. Wilting checks growth and stresses roots, and stress pushes basil toward flowering. Water at the base with drip, a soaker hose or a can, because wet leaves invite gray mold, leaf spots and downy mildew. Clemson suggests mulching with 3 inches of pine needles or shredded leaves to steady soil moisture, keep soil from splashing onto leaves, and hold down weeds.

Feed moderately. Clemson, Maryland and CTAHR all warn that heavy fertilizing produces lush growth with weaker flavor. Clemson’s approach, without a soil test, is 3 pounds (3 pints) of 5-10-10 per 100 square feet worked in before planting, then about two months later a light side-dressing of ¼ to ½ pound of calcium nitrate per 100 square feet only if growth slows and leaves turn pale. Utah State, which pushes for continuous leaf production, side-dresses with a liquid nitrogen source every 2-3 weeks once harvest starts and feeds potted basil weekly. The difference is mostly about how hard you pick: the more you cut, the more nitrogen the plant needs to regrow.

Pinching, flowering and harvest

Start picking once plants have at least 6-8 leaves (Utah State), about 6 weeks after planting according to Maryland. Cut each stem just above a pair of leaves, leaving 2-4 leaves below the cut. Two new shoots grow from that leaf pair, which is how a single stem becomes a bushy plant. Leave enough foliage that the plant keeps growing strongly, and rotate among plants if you have several.

Harvest when the leaves are dry. Clemson explains that gray mold usually starts on fresh stem cuts made in wet weather; the cuts are much less vulnerable after 24 hours and fully healed by 48 hours, so avoid both rainy-day harvests and overhead watering right after cutting.

By midsummer basil starts making flower spikes, and leaf flavor and production decline once it does. Clemson, Illinois and UF/IFAS say to pinch flower buds as soon as they appear. Utah State takes a different view: pinching flowers does not stimulate new leaves and can push flowers to form in the leaf joints instead. Its advice is to avoid heat and water stress, which trigger flowering, and to sow a new batch every other week so young plants replace old ones. In practice, keep cutting stem tips regularly (which removes most buds before they form) and start a second sowing in early summer. For drying, Maryland suggests cutting just before the flowers open, when flavor peaks.

Regional differences

Cold northern gardens. The season is short, so transplants are worth it. Utah State plants after frost in mid to late May. Do not rush: cold nights after planting stunt basil (Illinois). Before the first fall frost, harvest heavily, or dig and pot a plant for a sunny window, watering regularly but never leaving the pot standing in water (Clemson). Illinois sows several seeds in a small pot every 2 weeks for winter windowsill basil, ready to pick in about 4-5 weeks.

Humid South and Gulf Coast. Heat is fine; humidity is the problem. Clemson reports a severe downy mildew outbreak in South Carolina in 2014, and UF/IFAS calls the disease nearly impossible to prevent in Florida because the spores travel long distances on wind. Grow resistant varieties, give plants a very sunny, breezy spot, and use drip irrigation. UF/IFAS recommends planting in early spring or fall in Florida, with a bit of afternoon shade, and its Central Florida herb calendar marks February through April and September through October.

Arid West and desert Southwest. Low humidity keeps disease down, but sun and heat are brutal. University of Arizona Extension notes Tucson summers reach 105°F and suggests an east-facing spot with morning sun and afternoon shade, or 30-40% shade cloth. Pots should be at least 14 inches across; smaller ones are very hard to keep moist and may need water twice a day in summer. Arizona’s calendars differ by city: Tucson lists basil for May-June, Maricopa County (Phoenix) shows seed from mid-February and seed or transplants from March through May, and Yuma County lists basil year-round. Expect plants to struggle in the hottest months. Water deeply so it soaks at least 12 inches, rather than lightly every day.

Warm-winter zones 9-11. Where frost is rare, basil gets two seasons, spring and late summer into fall, with a slump in the hottest weather. In Hawaii, CTAHR says basil grows year-round below about 700 feet and from May to October at higher elevations.

Common problems

• Downy mildew. Yellowing that starts on lower leaves, limited by the veins and easily mistaken for nitrogen deficiency, followed by brown spots and gray-purple fuzz on leaf undersides. UF/IFAS suggests checking for spores in early morning, or sealing suspect leaves upside down on a wet paper towel in a plastic bag in the dark for a day. There is no cure. Harvest healthy leaves right away, bag and discard infected plants, and plant fresh seed elsewhere or in pots. Maryland notes that leaves from infected plants are safe to eat. Sources disagree on sprays: Clemson lists preventive Bacillus subtilis or phosphorous acid products every 7-10 days in warm, wet weather, while UW-Madison says home fungicides are unlikely to control it and does not recommend them. UW-Madison also suggests buying steam-treated seed when you can find it, since the pathogen can travel on seed.
• Fusarium wilt. Plants grow normally to 6-12 inches, then stunt, brown at the tips, and suddenly wilt (Clemson). Remove them, plant resistant varieties, and move basil to a new spot.
• Gray mold. Fuzzy gray rot that moves down from harvest cuts. Harvest only in dry weather and keep leaves dry.
• Root rot. Yellowing, stunting and dieback from the tips in poorly drained soil. Improve drainage with compost or composted pine bark, still watering often enough to prevent wilting, and rotate.
• Bacterial wilt. Clemson warns not to plant basil where tomatoes have died of bacterial wilt; basil is also susceptible and has no resistant varieties.
• Japanese beetles, slugs and aphids. Handpick beetles into soapy water, use iron phosphate slug bait, and spray aphids with insecticidal soap in the early evening, reaching leaf undersides.

Moving basil to a different bed each year helps with nearly all of the soil-borne diseases above.

Storing and preserving

Fresh basil does not keep long. Illinois recommends standing stems in a glass of water at room temperature; refrigerated leaves discolor because basil suffers chilling injury below about 40°F (CTAHR). Utah State offers a compromise for 7-10 days: recut the stems, stand them in water, cover with a plastic bag, and refrigerate.

To dry basil, hang small bundles in a warm, shaded, airy room or use a dehydrator below 125°F (Clemson). Store leaves whole and crumble them when you cook, and replace them after about a year. Frozen whole leaves turn mushy, so Utah State suggests freezing them in ice cubes or as pesto and using it within 3-6 months. Never can pesto or herbs in oil, because of the botulism risk (Clemson).

Saving seed

Basil varieties cross readily. To save seed that comes true, keep different basils about 150 feet apart (UF/IFAS, Clemson), let the flower spikes dry on the plant, and rub the seed out by hand. Stored sealed in a cool, dry place, it stays viable for several years. Do not save seed from plants that had Fusarium wilt or bacterial leaf spot, which can be carried in the seed.