How to Start Seeds Indoors, Step by Step

When to start, what equipment you actually need, and how to grow strong transplants — covering light, watering, and hardening off before your last frost.

Starting seeds indoors gives you a head start on the season and access to far more varieties than any garden center carries. It is not hard, but timing and light make all the difference.

Get the timing right

Each crop has a window before your last frost when it should be started. Tomatoes usually want about six weeks, peppers a bit more, and fast crops need only two or three. Start too early and seedlings get leggy and rootbound. Start too late and you lose the head start. Every crop page lists its start date by zone, and the state frost date tables show the last frost you count back from.

What you need

A sterile seed-starting mix, not garden soil.
Cells or small pots with drainage.
Something to make the holes with. A dibber or a dibble board sets every seed at the same depth, which is what makes a tray come up evenly.
A light source. An LED shop light a few inches above the seedlings beats a windowsill every time.
Gentle, consistent moisture and a little air movement.

Grow strong, then harden off

Keep the light close so plants stay stocky. Water from below when you can, and let the surface dry slightly between waterings. About a week before transplanting, harden the seedlings off: set them outside in shade for an hour, then add time and sun over seven to ten days. By the end they are ready for the open garden, which for tender crops means after your last frost. Hardy seeds can also be started outdoors in winter, in covered containers: see winter sowing.

Common questions

When do I start seeds indoors?

Count back from your last spring frost by the number of weeks the crop needs. Tomatoes are usually about six weeks before, peppers a little more. Each crop page lists its timing by zone.

Do I need a grow light?

Almost always, yes. A sunny window rarely gives enough light, and seedlings stretch and weaken without it. An inexpensive LED shop light a few inches above the plants works well.

What is hardening off?

It is the week of gradually exposing indoor seedlings to outdoor sun and wind before transplanting, so they do not get shocked. Start with an hour of shade and build up over seven to ten days.