Last Updated on October 9, 2026

How to Start Basil Seeds Indoors

Start basil seeds indoors 5 to 4 weeks before your last spring frost, then plant them out 3 weeks after it, for 7 to 8 weeks total under lights. Sow them 1/4 inch deep in moist seed-starting mix on a heat mat, keep them warm and bright, and harden them off before they go outside.

That window is the one the Plan My Garden planner uses for basil. To turn it into dates, look up your last frost date by zip code or enter it on my seed starting calendar.

The basil seed starting timeline

Why 5 to 4 weeks and not 6 to 8

You will also see basil described as needing 6 to 8 weeks indoors before it goes outside; University of Minnesota Extension uses that figure, and it already covers the whole stretch under lights, not extra time added on top of the planner’s window. Sow 5 weeks before your frost and plant out 3 weeks after it, and that is 8 weeks indoors; sow 4 weeks before, and it is 7, the same range Minnesota’s number lands in.

How long basil seeds take to germinate

Basil is not on my seed germination temperature chart, so these numbers come from extension guides. Warm mix is what they agree on.

A seedling heat mat with a controller is the easy way to hold that range. Minnesota Extension’s seed starting guide notes that potting mix in indoor containers can run as much as 5°F cooler than the room air, so a 70°F room can leave the mix near 65°F, below the range above. When the seeds come up, Minnesota Extension says you will know basil by its two broad seed leaves, each shaped like a capital D.

Step by step

1. Moisten the mix. Wet the seed-starting mix in a bucket before you fill anything. I recommend a mix with compost in it, like Coast of Maine.
2. Pick cells or blocks. If you are just starting out, I recommend cell packs. Once you are comfortable, 2-inch soil blocks work too.
3. Sow shallow. Penn State and West Virginia University Extension say about 1/4 inch deep; Minnesota Extension gives the same figure for its direct-seeded (outdoor) basil. Utah State says 1/8 inch. West Virginia notes that basil seed is very small and somewhat hard to sow, and that pelleted (coated) seed is easier. I put two seeds in each cell.
4. Cover and warm. Put a clear dome or plastic wrap over the tray and set it on a heat mat with a controller at 75-85°F.
5. Uncover and light up. Take the cover off as soon as the seeds sprout, which helps prevent damping off. Hang the lights 2 to 4 inches above the seedlings and run them 12 to 16 hours a day. Minnesota Extension calls 2 inches ideal and says plants need some dark each night. Iowa State says bright light prevents lanky plants that do not transplant well.
6. Thin to one. Once the true leaves show, snip the weaker seedling at the soil line with scissors so the two stop competing.
7. Water from below. Pour water into the tray and let the cells drink it up through the holes. For soil blocks I add about an inch of water to the tray daily or every other day.
8. Pot up if needed. If a seedling outgrows its cell before plant-out, Minnesota Extension suggests a container 1 to 2 inches wider. Hold it by a leaf, never the stem.

The full method for every crop is in How to Start Seeds Indoors and my step-by-step seed starting guide.

Keep basil warm: where cold damage starts

Basil is a tender crop, readily damaged by frost. Iowa State and University of Maryland Extension both say it does not tolerate cold temperatures. Frost is not the only line to watch.

South Dakota State University Extension wrote its 50°F note about crops nearing maturity in fall, not about seedlings, and calls basil an extreme example. Penn State’s mark for moving seedlings out, nights above 60°F, keeps them well clear of it. Indoors, keep trays off windowsills, which Minnesota Extension says can be the coldest place in the house at night, and do not leave seedlings out on a cool night while you harden them off. My vegetable frost tolerance chart rates basil tender, and so does the planner, which is why it waits until 3 weeks after your frost.

Pinching basil seedlings

Pinch basil to make it branch and stay bushy instead of flowering early. Penn State, Iowa State and Maryland Extension all recommend it.

• Where to cut: Just above a pair of leaves. Minnesota Extension says new growth appears at the cut point within a week.
• Tips, not flowers: Utah State says pinching off flowers as they form does not make more leaves. It encourages flowers in the leaf axils instead. Pinch the leafy tip.
• Holy basil too: In my holy basil post I pinch off the top leaves and buds so the lower nodes grow.

Every pinch is a small harvest. How much to cut once the plant is in the bed is on How Many Basil Plants per Square Foot?

Downy mildew can start with the seed

Iowa State says basil downy mildew spreads through infected seed, transplants or fresh leaves, and calls Genovese one of the most susceptible varieties; its resistant list includes Amazel, Prospera, Rutgers Devotion and Rutgers Obsession. Thinning to one plant per cell, and later one per square, helps too: Iowa State says to avoid overcrowding and keep air moving.

Hardening off and planting out

Harden basil off over the 10 to 14 days before planting. Start with a few hours outdoors, out of direct sun at first, and add time and sun each day. Minnesota Extension says to start in the shade during the warmth of the afternoon, protected from wind, and to bring the plants back in before temperatures drop at night. A cold frame makes the job easier. The general steps are in How to Transplant Seedlings Into Your Garden.

Plant out 3 weeks after your last frost, one full-size plant per square foot. My raised bed herb garden layout gives basil three squares of a 4×4 bed.

Where to get basil seed

Wood Prairie’s organic herb seed includes three basils. Genovese is listed at 68 days, about 200 seeds to a packet, and yields several cuttings off the same planting, but it is not a DMR variety; if downy mildew has hit your basil before, treat Rosie or Lemon Basil as your Wood Prairie pick instead and look for a resistant variety elsewhere. Rosie is a red basil at 65 days with compact, upright plants, and Lemon Basil grows more compact than the common European basils. Code PMG5 takes 5 percent off.

Quick questions

When should I start basil seeds indoors?

5 to 4 weeks before your last spring frost, then plant out 3 weeks after it. That is the Plan My Garden planner’s window, and it gives the plants 7 to 8 weeks indoors.

How long do basil seeds take to germinate?

About 10 to 14 days in 75-85°F soil, according to Utah State University Extension. Penn State Extension says 1 to 2 weeks, and University of Minnesota Extension says within 5 to 7 days for seed sown outdoors after frost.

How deep do you plant basil seeds?

About 1/4 inch deep, which is what University of Minnesota, Penn State and West Virginia University Extension recommend. Utah State University Extension says 1/8 inch.

What temperature is too cold for basil seedlings?

Penn State Extension waits for nights above 60°F before moving seedlings outside. South Dakota State University Extension says warm-climate crops may begin to suffer below 50°F and basil leaves can turn black in the 40s. My frost tolerance chart says to cover basil at 36°F or colder.

Should you pinch basil seedlings?

Yes. Penn State Extension says to pinch out the growing tips to slow flowering and promote branching. Cut just above a pair of leaves, and University of Minnesota Extension says new growth appears at the cut within a week.

Get your basil start week

Basil goes out after your frost, and the planner dates it for you. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick basil and everything else you want to grow, and it places each plant at square-foot spacing and builds a planting schedule (start indoors, transplant and direct sow dates) from your frost dates.

Related: Salsa Garden Layout · How Many Parsley Plants per Square Foot? · How to Grow Peppers from Seed · Seed Starting Calendar