When to start basil seeds

How many weeks before the last frost?

Sources differ more for basil than for most crops, so here is the range. University of New Hampshire Extension and University of Minnesota Extension both say 6 to 8 weeks from seed to transplant. Clemson Extension says 4 to 6 weeks, Iowa State 5 to 6 weeks, and University of Illinois Extension 3 to 4 weeks to produce plants ready for the garden. UNH sets basil out 2 weeks after the last frost. Using the UNH figures with an average last frost of May 10, you would sow indoors March 29 to April 12 and set plants out May 24.

In practice: if you have good lights and want bigger plants at planting time, use 6 to 8 weeks. If space is tight, a shorter start of 4 weeks or so still gives a usable transplant, since basil grows quickly once the weather is warm.

Germination

Basil likes warmth. Illinois Extension says seeds germinate and grow best when the mix is at least 70°F, sprouting in about 5 to 7 days. UMass Extension gives 65–80°F with an optimum of 70°F and 8 to 14 days. Iowa State lists 65–70°F and 10 to 14 days. Sow about a quarter inch deep (UMN; Clemson). The seedlings are easy to spot: two broad seed leaves shaped like a capital D, flat sides facing each other (UMN).

Growing on

Clemson suggests potting seedlings into small individual containers once they have two sets of true leaves, for faster growth. UMass notes transplants can be pruned to encourage branching. Give basil the same bright light as other seedlings, with grow lights a few inches above the plants.

Cold is the main risk

Basil is a tender annual. Illinois Extension notes that cold soil and air can stunt it and cause damaged, blackened leaves at 50°F. Clemson recommends waiting until days are consistently in the 70s and nights stay above 50°F before transplanting, and Illinois suggests waiting until the soil is at least 60°F. Harden plants off gradually over a week or so, and bring them in on cold nights.

Direct sowing

UMN says basil can be sown directly in the ground after the danger of spring frost has passed. Cover with a quarter inch of soil, keep it moist and weed-free, and thin to 6 to 12 inches apart when plants have two or three pairs of true leaves. Iowa State adds the condition that nights should be consistently above 50°F. A 2 to 3 inch mulch helps hold soil moisture (UMN).

Choosing a variety

Because of basil downy mildew, UMN recommends choosing a resistant variety. Beyond sweet basil, UMN describes types such as Spicy Globe (compact, good indoors), Lettuce Leaf, Lemon, Opal, Cinnamon and Thai basil.

Common mistakes

• Planting out with the cool-season crops. Basil waits until after the last frost, with warm nights.
• Cold germination. Below about 65°F, seed is slow.
• Letting it flower. Pinch flower spikes to keep leaves coming.
• Ignoring downy mildew. Choose resistant varieties where the disease is common.