How to Start Seeds Indoors: A Schedule by Plant Type

The key is timing: count backward from each plant's intended outdoor transplant date, not simply from the last frost. Tomatoes need roughly 6 to 8 weeks indoors, while cucumbers and squash need only 2 to 3 weeks. Follow the seed packet when its instructions differ from a general schedule.

Starting earlier is not automatically better. The goal is a healthy, appropriately sized seedling ready when the garden is ready, not a tall, leggy plant that has outgrown its pot weeks before it can go outside.

Use the tool: Seed Starting Countdown Planner. Enter your transplant date and your crops, and it will count backward to give you sowing dates by plant type, with a hardening off window at the end.

In this article

• When do you start seeds indoors?
• What is the vegetable schedule?
• What is the flower schedule?
• How do you turn the schedule into calendar dates?
• How do you start seeds, step by step?
• Which seeds are better sown outdoors?
• How do you harden off seedlings?
• How does the Seed Starting Countdown Planner work?
• Frequently asked questions

Key takeaways

• Count back from the outdoor transplant date for each crop, not from one shared frost date for all crops.
• Use clean containers with drainage holes, and premoistened seed starting mix, not garden soil, according to UMass Extension.
• Give seedlings 12 to 16 hours of light daily, with lights no more than 4 inches above the seedlings, according to University of Minnesota Extension.
• Keep mix moist, not saturated, and drain excess water from trays.
• Feed only after several sets of true leaves, with a weak general purpose fertilizer at one quarter strength, once weekly, according to Minnesota Extension.
• Harden off over about two weeks before transplanting, starting with a few hours in shade, then increasing sun and time outdoors.

When do you start seeds indoors?

It varies by plant and by the date of the last frost in your area, and more precisely by the outdoor planting date for that crop. UMass Extension says: count back the number of weeks from when each vegetable is to be transplanted into the garden. That will tell you when to start your seeds indoors.

The important distinction is between frost tolerance and warmth requirements. Cool season vegetables and heat loving vegetables should not automatically share one outdoor planting date. Local conditions and packet instructions should determine the date you count back from.

In many areas of Massachusetts, UMass notes, the last frost date is usually around May 15. That is an example, not a date for your garden. Use your own local last frost date and your crop transplant date.

What is the vegetable schedule?

The windows below describe how far ahead of outdoor transplanting to sow. They are planning ranges, not fixed deadlines. Variety and growing conditions affect development.

Vegetable timing is drawn from UMass Extension, with Minnesota Extension supplying the longer celery, onion, leek, and pepper windows. The broad onion and leek range reflects differences between published recommendations, not a requirement to keep every variety indoors for 15 weeks.

Caption: Use the weeks before transplanting, not a single start date for all crops. The packet for your variety wins if it differs.

What is the flower schedule?

Flower seedlings vary especially widely in how long they take to reach transplant size. These are Minnesota Extension approximate indoor growing periods. Use the durations to plan backward from your own planting date, rather than copying Minnesota calendar dates.

If you are starting only a few flowers, a south facing window may be enough space, according to Minnesota Extension. For a larger indoor start, artificial light is more dependable than a windowsill alone, because windowsills can be cold at night and hot during the day, and windowsill seedlings tend to be tall, with thin, bent stems.

How do you turn the schedule into calendar dates?

1. Establish an appropriate outdoor planting date for the particular crop and your local conditions.
2. Subtract its recommended indoor growing period.
3. Reserve the final 1 to 2 weeks before planting for gradually acclimating seedlings outdoors, a process called hardening off.

For example, if you plan to transplant tomatoes on May 15, a 6 to 8 week indoor period puts sowing around March 20 to April 3. That is an illustrative calculation, not a recommendation that May 15 suits every garden.

Worked example for a long crop: if you plan to transplant lisianthus on May 15, a 16 to 17 week indoor period puts sowing around mid January. That is why flower planning often starts months before vegetable planning, and why a countdown planner helps: it does the subtraction for each crop, instead of using one start date for everything.

How do you start seeds, step by step?

1. Prepare containers and growing mix

Use clean pots or cell trays with drainage holes. Fill them with premoistened seed starting mix rather than garden soil. A lightweight, soilless mix provides a more suitable environment for germination and reduces problems associated with weeds, pests, and waterlogged soil. Label each container with the plant name and sowing date.

If trays or pots have been used, clean and sanitize them first. UMass suggests a 10 percent bleach solution, 1 part bleach to 9 parts water, then rinse. Minnesota suggests soaking clean used containers in a disinfectant solution for 30 minutes, then rinsing.

2. Sow at the correct depth

Follow the packet sowing depth and light requirements. A general fallback, from Minnesota Extension, is a depth about twice the seed width, but some seeds need light to germinate and should not be deeply covered. For cucumbers and squash, use individual containers so their roots suffer less disturbance later.

Sow fresh seeds individually into each container according to package directions. If you are unsure, plant two or more seeds per cell when using older seeds, then once seedlings have true leaves, cut all but the healthiest one off at ground level with scissors. Avoid pulling unwanted seedlings, because that can damage the roots of the ones you want to keep.

3. Keep seeds warm and evenly moist

Many seeds germinate well in a warm location around 70 to 75 degrees Fahrenheit, though individual crops have different requirements. A clear cover helps retain moisture. Check it daily and vent it if condensation becomes excessive. A seedling heat mat can help maintain warm growing mix, and bottom heat can help prevent damping off, according to Minnesota Extension.

Keep the mix moist, not saturated. If watering from below, drain excess water from the tray rather than leaving pots standing in it.

4. Provide strong light

Give seedlings approximately 12 to 16 hours of light daily, followed by a dark period. Fluorescent or LED lighting provides more dependable conditions than relying on a windowsill alone. Insufficient light produces elongated, weak stems.

Hang lights from chains so you can raise them as plants grow. Keep lights no more than 4 inches above the tops of seedlings, as close as 2 inches is ideal, according to Minnesota Extension. Lack of light is the major cause of elongated, skinny stems. Use a timer so lights turn off and on automatically, and give plants a dark period each night.

5. Thin, feed, and repot as needed

• When multiple seedlings grow in one cell, snip off the weaker ones rather than pulling them and disturbing nearby roots.
• Once seedlings have several sets of true leaves, plants in soilless mix can benefit from a weak fertilizer solution. Minnesota Extension recommends a general purpose water soluble fertilizer at one quarter strength, once weekly.
• Move seedlings into slightly larger containers when they outgrow their cells. Transplant into containers 1 to 2 inches wider in diameter than the cell packs, handling the root ball gently rather than gripping the stem. Never hold a seedling by its stem, because you may crush it or harm the growing tip. If you need to steady it, hold it lightly by a leaf.

Which seeds are better sown outdoors?

For a straightforward spring garden, plan to direct sow carrots and other root crops, peas, beans, corn, and spinach. They generally do not need the same indoor head start as tomatoes or peppers. Lettuce can be either transplanted or direct sown.

Cucumbers, squash, and melons are optional indoor starts: their short indoor window can provide a head start, but they are not crops to sow months in advance.

Root crops in particular should be sown directly into the garden, according to UMass Extension, because transplanting can disturb the root.

How do you harden off seedlings?

Begin hardening off about two weeks before transplanting. Start with a few hours outside in a sheltered, shaded spot during the warmer part of the day. Gradually increase outdoor time and sunlight, bringing plants back inside before temperatures become unsuitable. This transition helps prevent sun and wind damage.

Minnesota Extension gives the same two week method: move seedlings outside, start in shade during the warmth of the afternoon, protected from wind, bring them back inside before temperatures drop at night, then each day leave them out a little longer and expose them to a little more direct sunshine. By the end of two weeks, unless freezing temperatures are forecast, seedlings can stay outside in a sunny area until you are ready to transplant.

Transplant on a cloudy day or in late afternoon, once conditions suit the crop. Hardened off plants may wilt when first exposed to full sun, but they generally recover within a day or so.

How does the Seed Starting Countdown Planner work?

The planner turns your transplant date into sowing dates, crop by crop, so you do not have to count backward for each plant by hand.

Enter your transplant date, or your last frost date and let the planner use crop transplant offsets, then pick your crops. The planner gives you a sowing date for each crop, in order, with the hardening off window marked at the end. For example, with a May 15 transplant date, tomatoes sow around March 20 to April 3, broccoli around late March to mid April, and cucumbers around late April, because their indoor windows are different.

That matters because a single start date for all seeds will make some seedlings too old and some too young on transplant day. Counting back by crop gives you seedlings ready at the same time as the garden.

Frequently asked questions

What if I start seeds too early?

You will get leggy, root bound plants that outgrow their pots and lights before the garden is ready. If that happens, repot into a larger container, give stronger light closer to the plants, and do not overfeed to force growth. Next year, count back from the transplant date, not from the first warm day in spring.

Can I use a windowsill to start seeds?

For a few plants, a south facing window may be enough, according to Minnesota Extension. For most indoor starts, artificial light is better, because windowsills can be cold at night and hot during the day, and windowsill seedlings tend to be tall and thin. If you use a window, rotate trays daily and move plants to stronger light if stems stretch.

How much light do seedlings need?

About 12 to 16 hours a day, with a dark period each night, according to Minnesota Extension. Keep lights 2 to 4 inches above seedlings, and raise them as plants grow. Do not leave lights on continuously.

When do I feed seedlings?

Not at germination. Seedlings draw energy from nutrients stored in the seed. Feed only after several sets of true leaves, with a weak general purpose fertilizer at one quarter strength, once weekly, according to Minnesota Extension. Water with plain water the rest of the week.

What if my seedlings are leggy?

Leggy seedlings usually mean not enough light, or lights too far away. Move lights closer, to 2 to 4 inches above the plants, increase light hours to 12 to 16 a day if you are below that, and avoid starting too early in a warm room with weak light. Snipping a leggy seedling will not fix the cause, so fix light first.

Do I need a heat mat?

Not for every crop, but bottom heat helps many seeds germinate sooner and produce healthier roots, according to Minnesota Extension. It is most useful for heat loving crops like tomatoes, peppers, and eggplant, and for a cool room where potting mix is 5 degrees Fahrenheit lower than air temperature. Remove heat once seedlings are up, unless the room is cold.

Next steps

Use the Seed Starting Countdown Planner for your own garden. Enter your transplant date, pick your crops, and let it give you sowing dates in order, with hardening off at the end.

For the broader vegetable calendar, see When to Plant Vegetables: A Planting Calendar for Every USDA Zone, and for tomato timing specifically, see When to Plant Tomatoes by USDA Zone (and When It's Too Late).