When to start seeds indoors, month by month

Why starting seeds indoors matters

In most climates, the outdoor growing season is too short for warm-season crops like tomatoes, peppers, and eggplant to go from seed to harvest if started directly in the ground. Starting them indoors gives you a head start — typically 6 to 12 weeks — so by the time the risk of frost is over, you're transplanting a plant that's already well established.

Indoor seed starting also gives you access to a much wider range of varieties than what's available as transplants at nurseries. And it costs a fraction of the price of buying plants.

How to calculate your seed starting dates

The process is straightforward. Every crop has a recommended number of weeks to start indoors before transplanting outside. You count backward from your last frost date by that many weeks to get your indoor start date.

For example: if your last frost date is April 30th, and tomatoes need to be started 6–8 weeks before transplanting (which happens after last frost), you'd start tomato seeds in early to mid-March.

Different crops also have different relationships with frost. Some (like lettuce and spinach) can go outside before the last frost date — they tolerate light frost. Others (like tomatoes and basil) must wait until all frost risk is gone and soil temperatures have warmed. The chart below accounts for all of this.

The master seed starting chart

All times are given in weeks before your last spring frost date (LFD). Negative numbers mean the plant can go outside before last frost — positive numbers mean after.

LFD = Last Frost Date. "Not recommended" for indoor starting means the crop prefers direct sowing — it either grows fast enough or dislikes root disturbance from transplanting.

Month-by-month schedule

This schedule assumes a last frost date of April 30th — the average for USDA Zones 5–6, which covers much of the US Midwest and Mid-Atlantic. Adjust earlier or later based on your own last frost date.

Crops that should NOT be started indoors

Some vegetables strongly prefer direct sowing. Starting them indoors and transplanting is either unnecessary or actively harmful because they dislike root disturbance.

Common seed starting mistakes

Starting tomatoes too early

This is the most common mistake. Tomatoes started 10–12 weeks before the last frost date will be root-bound, leggy, and stressed by the time it's warm enough to plant them out. 6–8 weeks is the right window for most varieties. For large-fruited heirloom varieties, 7–8 weeks is optimal.

Not enough light

A windowsill is rarely enough for healthy seedlings. Seedlings need 14–16 hours of direct light per day, which only a south-facing window in a sunny climate can provide reliably. For most gardeners, a simple LED grow light positioned 2–4 inches above the seedlings is essential. Leggy, stretched seedlings are almost always a sign of insufficient light, not poor seeds.

Overwatering

Seedling roots need oxygen as well as moisture. Consistently wet soil — with no chance to dry out slightly between waterings — leads to damping off, a fungal disease that collapses seedlings at the soil level. Water when the top inch of the growing medium feels dry. Use a mister for the first week after germination, then water normally.

Skipping hardening off

Seedlings grown indoors are not adapted to wind, direct sunlight, or temperature fluctuations. Moving them directly from the grow room to the garden on transplant day will stress or kill them. Harden off over 7–14 days: start with 1–2 hours outside in a sheltered, shaded spot, and gradually increase exposure to direct sun and wind each day until they're outside all day.

Using garden soil in seed trays

Garden soil and potting mix compact badly in small cells, preventing root development and good drainage. Use a dedicated seed starting mix — lightweight, fine-textured, and sterile. It doesn't need nutrients at the seed stage; start feeding only after the first true leaves appear.

How to track your seed starting schedule

The easiest system is a physical notebook or a calendar where you write each crop's start date, expected transplant date, and what variety you're growing. Make notes each year about which varieties performed well and whether your timing was right — after two or three seasons, you'll have a personalized guide that's more accurate than any generic chart.

Alternatively, a garden planning app that connects your crop list to a frost-date-based calendar does this automatically. Niwa does exactly this: add a crop to your garden, enter your location, and your personalized planting calendar is generated instantly — including when to start indoors, when to transplant, and when to expect harvest.

Succession planting for continuous harvest

One seed starting date gives you one harvest window. Succession planting — starting new trays of fast-maturing crops every 2–3 weeks — gives you a continuous supply throughout the season.

Good candidates for succession starting: lettuce, spinach, cilantro, bush beans, and radishes. Start a small tray every 2–3 weeks from your first possible start date through mid-summer. As one batch is harvested, the next is catching up.