Knowing when to start vegetable seeds indoors is an important step in growing a healthy and productive food garden. This is because proper timing helps ensure your seedlings have reached the ideal growth stage for transplanting outdoors. Seedlings that are too immature or overmature can struggle, so getting a handle on when to sow seeds indoors is crucial. In this article you’ll learn which types of vegetable seeds are started indoors and how to calculate the best timing for seed sowing.

Why you need to know when to start vegetable seeds indoors

The first time I started vegetable seeds inside I sowed them all at the same time. Within weeks the squash and cucumber plants were taking over my windowsill, but it was still too cold to move them outdoors. The tomato and pepper seedlings, on the other hand, were still tiny and nowhere near ready for the garden. I didn’t realize that certain crops are faster to grow than others and that seeds have varied germination times. Oops!

It may seem like a good idea to give vegetable plants an extra-early start so that you can transplant larger and more mature seedlings outside. This, however, isn’t the case as the plants quickly outgrow their indoor spaces as well as their cell packs or pots. It’s a hassle to repot seedlings into larger containers every few weeks, but there’s another issue to keep in mind: overmature seedlings don’t transplant well and never live up to their yield potential. This is especially true if the plants, in the case of cucumbers and tomatoes, have begun to flower and fruit. 

Waiting too long to start your vegetable seeds indoors can also cause problems. Trying to rush very immature seedlings into the garden leaves the tiny plants vulnerable to pests and weather fluctuations. You can also expect a delayed harvest, and if you live in a short season climate this may impact overall production. Following proper indoor sowing dates for vegetable seeds produces strong, stocky seedlings that are ready for the garden at the right time. 

For more information on when to start vegetable seeds indoors, watch this video:

Should all types of vegetable seeds be started indoors?

Nope! Many vegetables, in particular root vegetables and fast-growing crops like beans, peas, and arugula, don’t transplant well and are direct seeded outdoors. This is especially true of carrots, radish types, and parsnips. The crops most often started indoors are the long season vegetables, like celery, that take many weeks to grow large enough for transplanting. Or warm season crops, like tomatoes, that need an indoor start time when grown in short season climates. In most regions the growing season isn’t long enough for tomato plants to grow, flower, and fruit from seeds planted outdoors.

Finally, there are vegetables, like beets, cabbage, cucumbers, and squash that you can direct seed outside or start indoors. In many cases there are benefits to starting these seeds indoors. I find direct seeding broccoli and cabbage in my garden is risky as slugs often devour the just-sprouted seedlings. Giving them a head start inside allows me to set out strong seedlings that settle in nicely. 

Learn more about which vegetables are started indoors, outdoors, or both in this helpful chart below.

How to calculate when to start vegetable seeds indoors

There are a few ways to figure out when to start vegetable seeds indoors. Before you get started, you need to know the number of weeks of indoor growth needed for the vegetables you want to grow and your last expected spring frost date. Once you have these pieces of information, you can use them to calculate seed sowing. Use any of these tools below to find out the number of indoor grow weeks:

1. Seed packet – Most seed companies print helpful tips on their seed packets (or in their seed catalogs) including ‘days to maturity’ and the recommended number of weeks of indoor growth needed for a specific crop.
2. Planting calendar – A planting calendar may be a chart, like my handy chart below, or it can be a seed-starting calendar where you enter the last frost date for your region and it calculates when you should start various vegetable seeds indoors.
3. Good book – In my best-selling books The Year Round Vegetable Gardener and Growing Under Cover, I teach gardeners how to figure out timing for starting vegetable seeds indoors, when to transplant those seedlings outside, and which crops are best direct sown outdoors.  

More tips on timing

As you can see from the above chart, most of the vegetable seedlings you start indoors need to be transplanted outside a little before or after the last frost date. This is because some are cool season vegetables and others are warm season crops. Cool season crops like spinach and kale not only withstand cool temperatures and light frost but grow best before the heat of summer arrives. Transplant warm season vegetables, like tomatoes and peppers, into the garden after the risk of frost has passed. Why? Because cold temperatures and frost damages or kills the plants.

These temperature sensitivities affect when to start vegetable seeds indoors. For example, to figure out when I need to start jalapeño pepper seeds inside, I’ll look at above chart and see that peppers need 8 to 10 weeks of indoor growth. Also note that they are transplanted outdoors 1 to 2 weeks after the last frost date. That means the seeds should be started inside about 7 to 9 weeks before the last frost date. If May 1st if my frost-free date, than I’ll sow the seeds between February 27th to March 13th.

Once you’ve figured out the seed-starting dates for your region for each type of vegetable you can create a list or chart. I suggest printing it and keeping it tucked inside your seed storage box or binder. That way you don’t need to do the math every year.

A guide for when to start vegetable seeds indoors 

• Beets – Direct sow beet seeds outdoors or give them a head indoors by planting them about 7 to 8 weeks before the last frost date.
• Broccoli and cauliflower – These cool season crops need 4 to 6 weeks of indoor growth. Transplant them a couple of weeks before the last frost date.
• Cabbage – Cabbage plants are more cold hardy than broccoli and cauliflower so the seedlings can be set outside even earlier. They require 4 to 6 weeks of indoor growth, so start the seeds 8 weeks before your last frost date.
• Celery – Celery is slow to grow so plant the seeds inside 2 1/2 to 3 months before you wish to set them outside.
• Corn – I rarely start corn seeds indoors, but it does result in strong seedlings. Sow them just 2 weeks before the last frost date.
• Cucumber – My first cucumber crop of the season is started indoors a month before the last frost date. I then direct sow more seeds until mid-July for a non-stop crop.
• Eggplant – Eggplant seedlings are slow-growing so start them indoors 2 months before the last frost.
• Kale – Sow kale seeds inside around 10 weeks before the last frost date.
• Lettuce – Start heading lettuce varieties inside 8 weeks before the last frost date.
• Melons – Sow heat-loving melons inside 2 weeks before the last frost date and set out 2 weeks afterwards.
• Onions and Leeks – I start both under grow lights 3 months before the last spring frost.
• Peppers – Start pepper seeds indoors about 10 weeks before you intend to transplant.
• Pumpkins and Squash – These fast growing plants only need 3 to 4 weeks indoors.
• Swiss chard – Plant Swiss chard seeds 7 to 8 weeks before the last frost.
• Tomatoes – Sow tomato seeds inside 5 to 7 weeks before the last frost and set out a week after.

A quick guide to starting vegetable seeds indoors

• Step 1: Gather your materials. You’ll need potting mix, containers, a tray, labels, a waterproof marker, and seeds. Many gardeners use 1020 seeding trays and cell packs, but you can reduce your plastic use and make your own soil blocks. For heat-loving vegetables like peppers and tomatoes I also use a seedling heat mat to encourage good germination rates. It’s not essential, but it does come in handy. 
• Step 2: Plant the seeds at the ideal depth. A good rule of thumb is to plant vegetable seeds 2 to 3 times as deep as their diameter. Don’t plant them too deeply or they may not emerge. 
• Step 3: Cover with plastic wrap or humidity domes. These covers keep humidity high to prompt good germination. When you see sprouting, remove the covers. 
• Step 4: Provide a light source. I use grow lights because they allow me to control the amount of light I give my seedlings. You can also use a sunny window for seed-starting, but if there isn’t enough light for healthy plant growth, the tiny seedlings will stretch and grow leggy. I keep my grow light fixture turned on for 16 hours each day. 
• Step 5: Encourage good air circulation. I use a small fan in my seed-starting room to keep air moving. This reduces the risk of fungal diseases and helps toughen up stems and leaves. 
• Step 6: Provide moisture and fertilize. Aim to keep the growing mix lightly damp; don’t over or under water. Once the first true leaves have developed, use a diluted water-soluble fertilizer every 12 to 14 days. 

For more specific information on starting vegetable seeds, be sure to read these expert articles:

• Growing tomatoes from seeds
• Two techniques for planting pepper seeds
• Learn how to grow broccoli from seed
• A guide to planting basil from seed