When to Start Sowing Seeds: A List of the First Seeds to Start Indoors

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A guide for sowing seeds undercover for a head start on spring. Includes information on when to start sowing seeds based on last frost dates, hardiness zones, and getting earlier crops. Follows with a list of the first seeds to plant for this year’s vegetable garden.

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In spring, when days are warm, and plants are growing, it’s fairly easy to scatter some seeds in the soil and watch them grow. A lot of vegetable seeds are started this way, and if nights are warm enough, they’ll produce lush crops right there in the ground before you know it. Especially if you’ve sown fast-maturing veggies like radishes, lettuce, and spinach. Some people even have climates that are kind enough to let them grow this way through much of the year.

Many of us have to work around long winters and cold springs, though. So if we want to grow crops that have long maturing periods, like peppers, eggplant, and tomatoes, then we start these seeds early and undercover. We also start seeds early to get a headstart on faster-growing crops so that as soon as it’s warm enough, we can plant them outside and have fresh food on the table weeks earlier than with direct sowing. This guide is here to help you to understand the first seeds you can sow in the gardening year and why. It includes an explanation of last frost dates and how that relates to the first seeds that you can confidently sow.

When to Start Sowing Seeds

One major challenge to early seed sowing, for beginners and experienced alike, is the feeling of FOMO (fear of missing out). Early on in the year, I tend to spot people sharing photos of trays of their beautiful seedlings. This even happens in January, and the sight of lush green can fill you with such excitement. Young green plants stretching upwards, seeking the sun. It’s such an optimistic sight to behold! I know that I just want to shoot over to my seed storage and see if there’s anything worth starting.

Tomato seedlings growing inside a heated propagator in early spring

Unfortunately, most seedlings sown during winter won’t make it. Unless, of course, the soil and outside temperatures (day and night) are warm enough to plant them in their final positions within a couple of weeks. If not, then they need to stay inside, growing ever larger, needing more light, more nutrients, and more space for their roots. That’s how sowing seeds too early can cause you more stress, money, time, and effort than it’s worth in indoor garden therapy.

Seedlings that spend too much time indoors or in smaller containers can become sickly and weak. However, giving the first seeds of the year a head start by growing undercover can also give you much earlier harvests. It can also help you grow crops that would never have a chance of fully maturing if you started them outdoors in spring.

Tomatoes growing in my polytunnel started from seedlings raised undercover

Sowing seeds before spring is a balancing act. You want to give young plants a good head start without having to keep them growing undercover any longer than they need. That’s why getting your seed sowing timings right is crucial.

Some Crops Benefit from Early Sowing

The main reason for starting seeds undercover is to get a head start on the growing season. That could mean early harvests of spring vegetables such as lettuce and spinach. It could also mean nurturing plants that need a long time to grow and produce a crop.

Young eggplant and pepper plants growing under a grow light

For example, I love growing tomatoes! I live in Britain, though, and in a part that seldom gets hotter than about 75F (24C) in the summer. Springs can be cold, and autumn can start early. That means that if I don’t start my tomato plants undercover, then the best I will see is a harvest of green tomatoes by the end of summer. That’s fine if all I want to eat is green tomato chutney, but that’s not the case. That’s why I start my tomato seeds in late winter each year. If I want eggplant (aubergines) or peppers, I need to start those even earlier than tomatoes.

Starting seeds a little early is also a great way to squeeze in an early crop. Though I can sow a lot of seeds direct in the soil after my last frost date, I can also plant small plants then, too. Providing they’re healthy and hardened off, the small plants will produce much quicker than the seeds. Seeds started indoors often give us the first harvests of spinach, radishes, lettuce, and other spring vegetables.

The information on seed packets isn’t that accurate when it comes to early sowings.

Seed Packet Sowing Times

Before we continue, let’s address the easiest reference for determining the earliest time to sow seeds: seed packets. However, the small amount of information that seed companies are able to squeeze onto them is generalized. Accurate early sowing times are generally not provided, and for good reason.

Onion seeds benefit from a very early start

The best seed-sowing times for seeds are based on preparing plants to be planted in a warm and favorable environment to grow. Seed packets are mass-produced and sold in areas with wildly varying climates, especially in large markets like the United States. March temperatures in California are quite a bit different from those in Illinois. Even in smaller countries, like the United Kingdom, early sowing times in London can be several weeks before sowing times in Scotland. That’s why it’s best to understand how to work out sowing times specific to your own special region of the world. You and your plants will be far happier and healthier for it!

A hard frost can completely kill the foliage on crops, like these New Zealand yams

Cold and Frost Kills Young Plants

When you’re starting seeds indoors and undercover, you might not be as concerned about what’s happening outdoors. But if you’re growing seeds that you hope to plant outside in the garden, then you should. My general rule of thumb with most seed sowing is to sow seeds about a month before I plan to plant them outside. Some plants, like peppers and eggplant, take longer to grow, but many vegetables are of a size that needs planting out within 4-6 weeks of starting them from seed.

The thing is that most young annual vegetable plants are not cold or frost-tolerant. If you plant them outside before it’s warm enough, especially at night, then they will die or turn sickly. Frost is the biggest killer of annual plants at both ends of the growing year, and just the slightest dip in the temperature can wipe out all your plants. Or turn them into weak plants that refuse to pick up and grow.

Vegetable seedlings started off in trays

That’s why understanding when your garden’s last frost date occurs is key to knowing when it’s safe to plant out your seedlings. You can count back from that day to know when a good time to start seeds will be.

Know Your Last Frost Dates

The best place to check for your region’s average last frost dates is by looking at your USDA zone‘s information. It can even work for areas outside the United States, such as Canada, the United Kingdom, and Europe. The last frost dates listed for your growing zone are the result of records taken over many years and are fairly reliable. I’ve listed the dates further below, and you can use them to generally know when it’s safe to begin planting outside.

Hardiness zones for North America and Britain

Though the general dates are fairly dependable, a specific last frost date for any given year can be different. It can even occur weeks after the average last frost date if you’re experiencing a particularly cold spring. It’s part of the early spring gardening game, taking a chance and sowing seeds, planting potatoes, or planting out young plants. So keep your eyes on the weather report and be prepared with fleece, tunnels, and other tactics to protect crops from the cold.

ZoneAverage Last Frost Dates
1May 22 to June 4
2May 15-22
3May 1-16
4April 24 to May 12
5April 7-30
6April 1-21
7March 22 to April 3
8March 13-28
9February 6-28
10-13No freeze
Dates are based on the USDA Hardiness Zone Method
Simple seedling set-up by a bright window

Seed Sowing Based on Last Frost Date

The chart above gives you a range of dates for the average last frost date for a particular zone. However, there are more specific last frost dates given for your region here for the USA and here for the UK. If you are starting seeds to plant outdoors, use the latest date given in the chart to be your last frost date. Using this date, along with the chart below, can help you to work out the best time to sow seeds undercover for plants you want to grow outdoors.

As for me, living in zone 8, my average last frost date is March 28th. That means if I want to sow lettuce seeds early in the year, I can start them undercover up to six weeks before that date. So about February 14th. Tomatoes I’ll sow two weeks before my last frost date, around March 15th, but with bottom warmth to help with germination.

Tomato plants can get too large if you start them off too early. These had to wait it out in the greenhouse while my Polycrub was being built last year.

If I were planning on growing some of these crops inside my polytunnel or greenhouse, though, I could start them a couple of weeks earlier than the times listed. Sometimes, such as with chili plants, I’ll even sow in January. The hotter the chili, the longer it will take for it to produce a crop so if you have a quality undercover growing space to grow them in, feel free to start them pretty early. By that, I mean a sturdy greenhouse, polytunnel, or sunroom with plenty of light and warmth. Especially early in the growing year.

Seed Sowing Guide

Sowing Time before the last frost dateVegetables for Starting Undercover
10 weeks Eggplant (aubergines) – with warmth and grow lights.
Chilis (spicy) – with warmth and grow lights.
Onion and shallot seeds.
8 weeks Peppers (mild flavor) – with warmth and grow lights.
6 weeks Fava beans (broad beans), Early brassica varieties: broccoli, cabbage, cauliflower, kale, kohlrabi.
Leafy herbs: basil, cilantro (coriander), dill, parsley, lettuce, bulb fennel, peas for shoots, radish, spinach, spring onions, turnips, watercress.
4 weeks Same as above, plus beets and garden peas.
2 weeks Same as above, plus celeriac and celery.
Melons and tomatoes with warmth
I sow pumpkin, squash, and cucumber seeds in mid-April

Don’t Start These Seeds Yet

The above list of the first seeds to sow does not include many garden favorites. That’s because it’s just not time to sow them yet. Corn, cucumbers, pumpkins, melons, and squash are crops that do not tolerate cold temperatures very well. That makes them poor candidates for early seed sowings.

If you start them off too early, then the plants can swamp your house or greenhouse since it will be too cold to plant them outside. Large plants are not ideal for transplanting in the garden, and my experience is that they sulk and fail to thrive. It’s a fact that smaller plants establish better than larger ones once you transplant them.

Also, wait on sowing your carrots and leek seeds. Carrots have a tendency to fork if their little roots touch the sides of pots or modules. Direct sowing in the soil is best for them. Leeks sown too early can bolt earlier than leeks sown once it warms up. Brussels sprouts can also wait to be sown until around mid-April.

This is the seed storage container I use. I have two now!

How to Start Seeds Undercover

I think you’re well prepared with information now on the first seeds to sow in the year. So, have a look through your seed collection, make a planting calendar, and get your supplies ready. An unheated propagator, seed trays, fresh seed-starting mix, and a bright window sill are good enough to get kale, cabbage, broccoli, and onion seeds started undercover. You can usually get basic seed-starting materials at your local garden center.

You can purchase organic seed potting mix for starting seeds, but making it yourself is easy. My favorite mix is about 75% fine compost with 25% vermiculite. You can also use perlite, leaf mold, and other materials to create a fine but free-draining medium for your seeds to grow in.

For an introduction to the best ways to start warmth-loving seeds undercover, including information on grow lights, heated propagators, and caring for seedlings, read How to Start Seeds Indoors During the Winter.

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20 Comments

  1. You refer to both “under cover” and “indoors”. Is there a difference or do you use those terms interchangeably?

    1. Undercover could be indoors, in a greenhouse, polytunnel or cold frame. Indoors specifically is in the house where the temperature doesn’t tend to dip below about 60F/15C. Because it’s warmer indoors than in most places undercover in late winter and early spring, it causes some issues, including fungus gnats and green growth shooting up too quickly. Most plants that are ‘leggy’ are plants started off from seed in the house. Both from the warmth perspective but also the less adequate lighting.

  2. Great article!! Very detailed and informative.

  3. Thank you for this informative article.
    I am getting my sowing fix right now by planting microgreens such as sunflower, mellow microgreen mix, wheat grass and broccoli. These can be harvested in 10-12 days! They are so highly nutritious as well.
    I challenge you to try!
    Check out how to grow them on You Tube.
    Brenda

  4. Here in Ramsey, Isle of Man, I find that wind can be just as damaging to seedlings and young plants, as frost. We get strong, cold, salt laden winds blowing off the sea for about nine months of the year which have a dehydrating effect on plants, and some can tolerate it better than others. It took me a couple of years to work out why some of my seedlings thrived while others didn’t, and also, why some plants placed close to the garden wall (and sheltered from the wind) did better than plants which were more exposed.

  5. Deborah Maldonado says:

    I have been trying to find seeds for the 9 Star broccoli for years. I live in Texas in the United States. Do you know of any place that has this seed variety that I can purchase?

    1. I’m sorry but I’m not as familiar with seed companies in the USA. I’d suggest speaking to a seed supplier that supplies Nine Star Broccoli seeds in the UK and see if they know of anyone in the US.

    2. Sylvana Zeenny Pappas says:

      Smallislandseeds company on Etsy. They sell it and it’s in stock free shipping
      Nine Star Perennial Cauliflower/Broccoli Seeds – Brassica Oleraceae – Prolific, Delicious, and Perennial 30 seeds for $3.29 to the US

    3. Hi Penard Plants in the UK have them but they are rare and go quickly.

  6. LaurieAnn Lepoff says:

    seed packets always use the last frost date to tell you when to start. I live in the SF bay area where there is no frost. Does this mean I can start flowers indoors at any time?

    1. Hi LaurieAnn, technically your last frost date in San Francisco is February 1st. It doesn’t mean that you’ll have frost until that date every year, but rather the last chance of frost ends on that day. Find out more about last frost dates throughout California and even specific parts of the bay area here.

  7. Rick Laughlin says:

    You CAN use ordinary 40 watt double fixture lights suspended from a chain…I
    did this for many years and raised many seedlings for my expansive garden…

  8. Trying hard not to be drawn into sowing any seeds for a few weeks. Too much temptation online though.

    Those frost dates look a bit optimistic to me. I’m in zone 7 or 8 {Yorkshire UK} We often get a frost in the first week of May, with a below zero degree centigrade temperature. It usually sets my early potatoes back. Maybe I’ve mis-read the tables.

      1. Diane Cox says:

        Hi Tanya. I think you should update the text to link to this map for UK readers as March is much too early for last frost in majority of areas. You risk disappointing new gardeners who follow the March dates!
        You are in a mild area of the UK.

    1. Suzanne Douville says:

      I agree with you. It states I am zone 5 but I always use zone 4. We are in a weird location. With colder winds funneling down from Canada so I use zone 4. Unless I have a full sun, protected area. Use your best judgment is my philosophy.

  9. This is awesome, Tanya! I can actually start growing something now to get my gardening “fix!” Lol!

    1. lovelygreens says:

      I’d love to see what miniature garden plants and herbs could be cultivated now too — you’re the one in the know Janit :)