Timing When to Start Seeds Indoors for Thriving Plants

After you’ve finished mapping out your garden, it's time to figure out when to start seeds indoors. This very important question is on the minds of gardeners spanning across climates because timing your seed starting right increases yields and greatly improves planting success.

Along with the useful Seed Starting Chart in this post, you’ll learn how to calculate when to sow flower, herb, and vegetable seeds in any zone.

The Luxury of Seed Starting Indoors

A word of advice from the experts: starting seeds indoors is a complete luxury and not necessary to see fruitful results from your garden! Your garden CAN be successful without starting seeds indoors, and some plant varieties will do better when winter sown or directly sown in the garden.

So, if you can’t start seeds indoors, don’t worry! If you have the space and time to start your seeds in advance, knowing when to start seeds indoors along with these helpful resources on cold stratification, DIY seed starting soil mix, how to make soil blocks, and how to transplant seedlings will help you grow your best garden yet.

Which Seeds Should NOT Be Started Indoors

Not all seeds will thrive when started indoors. Some plants, such as root vegetables, corn, and garlic sets, don’t transplant well.

Other large varieties, like beans, sunflowers, and some squashes, need more space than indoor seed starting allows. Direct sowing will be the best method for planting these.

Winter sowing is another smart and green way to grow hearty plants without starting them indoors. With very low costs and little labor required, this hands-off technique for starting seeds may become your new favorite.

The Benefit of Properly Timed Seed Starting Indoors

Starting seeds indoors allows gardeners to extend the growing season and get a head start on the garden. This can be especially beneficial in northern climates.

We want to make sure that these seedlings are up and growing before we move them outside so that, when we do move them outside, we are essentially getting bonus growing time!

Plants given enough time to mature can produce a more bountiful harvest, making all your hard work worth it. But, there is an optimal target window of time you want to take advantage of.

If you start seeds too early, plants will outgrow the starting containers. You can let the overgrown plants become root-bound, up-pot them repeatedly, or plant them in the garden when the temperature is too cold. All of these will cause a lot of stress on the plant, and it may not thrive.

Starting seeds too late defeats the purpose of starting seeds indoors. Seedlings won’t have the time they need to mature for transplanting, and you will get very little return for the extra time and money you have put into it.

Timing is everything when it comes to seed starting. So, let’s find the sweet spot of timing it just right.

Calculating Indoor Seed Starting Dates

Figuring out when to start seeds indoors depends on a few variables. Use the information below to calculate starting dates for each variety you plan to grow.

Properly time starting plants from seed based on the following:

1. Last Spring Frost - This is the average date of the last spring frost of the year in your area.

2. Seed Packet Planting Recommendation - Each seed has an optimal indoor sowing time. If you get seeds from a reputable seed company, the seed packet will state how many weeks in advance to start those seeds. (If you don't have a seed packet for your seeds, see the Seed Starting Chart below!)

Last Spring Frost Date

The first thing you want to know is when to expect the last estimated frost (LEF) for your area, which will vary greatly depending on the climate where you garden. There are a handful of ways to find your LEF.

The NOAA Interactive Map of Average Date of Last Spring Freeze Across the United States and the Farmers Almanac are reliable online resources for finding rough LEF date averages. Using the Farmers Almanac link provided, type in the city and state where you live or the zip code, and it will calculate your average last spring frost date, first fall frost date, and the length of your growing season.

Once you know your last frost date, Clyde’s Garden Planner is a helpful all-in-one tool for all climates. It will tell you when to start seeds indoors, whether to start plants from seeds indoors or direct sow seeds outdoors, when to transplant seedlings, and more.

Reading Your Seed Packet

Now that you know your last estimated frost date (LEF), look at the back of your seed packet. It should tell you how many weeks before the LEF to start those seeds indoors.

Using a calendar, count backward from the LEF for the designated number of weeks. That will give you the date range for starting those seeds indoors.

For example, let’s say I want to grow tomatoes. The seed packet states that they should be started indoors 4 to 5 weeks before my last frost date. My estimated last frost date is May 11th, so I subtract 4 to 5 weeks from May 11th. That tells me to start those tomato seeds indoors between April 6th and April 13th.

Even with this information, your results will not be perfect. There is always the chance of a freak frost coming later and killing plants. But starting here is the safest route, especially for beginners, and usually yields good results.

Cold Stratification

Some seeds require cold stratification before seed starting indoors, which adds an entirely new layer to the process. The seed stratification process mimics cold winter weather for your seeds and dramatically increases germination rates for some seeds.

You will need to allow for an additional 2 to 3 weeks of cold stratification BEFORE you start these seeds indoors. Here is a step-by-step guide to cold stratification as well as how to identify seeds that require cold stratification.

When to Start Seeds Indoors (Seed Starting Chart)

If you saved your seeds or got seeds from a friend, you may not have a seed packet to look at. This planting chart will tell you when to start different seeds.

* Most flower varieties do well directly sown once the weather gets warm, but some benefit from being started indoors earlier or from winter sowing. Check the seed packet for the best time and conditions.

** Classic heading varieties of broccoli do not usually do well if started indoors. Use sprouting varieties like Broccoli Rabe and Purple Sprouting Broccoli for the best results!

Pro-Tip: Remember, it’s always important to source excellent seeds for successful seed starting. You can purchase quality heirloom seeds from the MIgardener Seed Collection and learn how to save seeds for a never-ending supply of your favorite garden varieties!

How to Start Seeds Indoors

Here is a step-by-step guide on how to start seeds indoors from start to finish. It contains information about selecting seed starting containers, using a seed starting heat mat, choosing a quality seed starting mix, how many seeds per cell to sow, how to soil block, and more.

Pro-Tip: When your seedlings are ready to be transplanted into the garden, start the hardening off process 7-10 days before your plants go out into the garden.

Keep A Garden Journal

The information in this post is a great starting point for when to start seeds indoors. But each year, you can fine-tune your seed-starting schedule and tailor it specifically for the area and microclimate where you garden!

An important part of this process is keeping good records! With a garden planner, you can record temperatures, weather patterns, and frost dates from year to year, plus the dates when you started seeds, when you transplanted seedlings, how those plants thrived, and the harvest results.

Good records will help you determine the prime seed starting dates specific to your garden so you can grow bigger!