When Should I Start Seeds Indoors

Whether you’re just tipping your toe into starting seeds indoors or you’re an old pro, the goal each year is to have healthy, hardy sprouts ready to move outdoors at the correct time. Start seeds indoors too early and you might have plants that are root bound, perhaps with buds or flowers, which can impede reestablishment when they are transplanted. Start seeds indoors too late and you’ll end up with weak seedlings that may not survive the hardening off process.

So what is the perfect time to start seeds indoors? That golden moment depends upon you! The types of seeds you want to germinate and your plant hardiness zone will determine when to start seeds indoors.

When to start seeds indoors

Project Difficulty


What You Will Need


Mosser Lee SucSeed

Pots or Seed Trays


 First, create your garden plan. Click here to see our guide for a successful garden plan. 

Once you have created your garden plan, make a list of all the plants you intend to plant. This will help you determine how many plants of each variety you will need for a successful harvest or display. At Mosser Lee, we recommend planting at least 20% more seeds than you need. You may find additional spaces that need filling and of course, seedlings are always wonderful to share with neighbors or with plant swappers.

When to start seeds indoors

What do I need to Start Seeds Indoors?

Many people are very successful in starting their seeds with minimal equipment. When you have a bright sunny eastern-facing window, seed starting can be very cost effective. You can start seeds without expensive custom trays or containers. 

when to start seeds indoors - seedlings started in a toilet paper roll

Thoroughly wash yogurt or pudding containers and cut drainage holes. You can do the same with plastic milk or juice jugs, cutting them down to about 6” high and adding drainage holes. Collect toilet paper rolls and fold over about an inch, creating a bottom. Bonus – paper rolls can be moved directly into your garden as they will compost, providing nutrients to your soil. 

There is one thing you can’t skimp on. A high quality seed starting mix is vital for germination and growth success. Our SucSeed® Seed Starter medium is proven to help your seedlings develop strong and robust root systems, prevent overwatering and provide fast and high germination rates. It’s peat-free, lightweight and recommended by the National Home Garden Club. 

When the seed starting bug really takes hold, you may want to purchase items that allow you to better control the seedling’s environment.

Do I need a Grow Light? What kind of Grow Light should I get?

Grow lights help prevent leggy seedlings. When the seeds aren’t stretching for light, they grow more compactly, giving you fuller, more robust leaves instead of long, reaching stems.

In the past, grow lights were large, clunky, and cost-prohibitive. Today’s LED technology has now made grow lights adaptable, inexpensive and easy to manage. Shop lamps and fluorescents will work fine.  Photosynthesis happens best in a red/blue combination and doesn’t really require “daylight” spectrum bulbs. 

You can find grow lights with:

  • Bendable arms. Position your light close to the seeds, moving lights up as plants grow. 
  • Timer capability. Seedlings typically need 16-18 hours of light each day.
  • Multiple settings. These can help you accommodate plants with different needs. 

Should I use a heating pad to start seeds indoors?

Beware – a heating pad that you might use for an aching back is not appropriate for seed starting. These pads are not waterproof, do not maintain a steady temperature and can easily fry out your seedlings. Purchase a certified heat mat or germination heat mat created specifically for seed germination. 

Seeds started at a constant, warm temperature are more successful. They will:

  • Germinate sooner 
  • Produce healthier roots
  • Avoid damping off

What is damping off?

Damping off is the most common problem in starting seed indoors. It’s caused by a variety of water molds and fungi. Healthy young seedlings can be affected overnight. They blacken and die as stem rot occurs at soil level, causing the plant’s tissue to collapse. Because Mosser Lee’s SucSeed® Seed Starter is made of sustainably harvested, milled long-fibered sphagnum moss, the fungi and molds cannot grow, so damping off is avoided.

What kind of seed trays should I use when starting seedlings indoors?

At Mosser Lee, we are committed to preserving and improving the environment in everything we do. For many gardeners, plastic is the bane of their existence; it holds mulch, soil, plants and seedlings. It seems to contain nearly everything gardeners buy. 

Reuse those plastic containers and seedling trays you’ve already purchased by washing them in a light bleach solution and warm water and then drying thoroughly. Consider using reusable and recyclable sheet pans to hold your seedling containers. 
Avoid peat based products. Peat moss is not considered a renewable resource and harvesting peat from bogs releases 2 billion tons of greenhouse gas into our atmosphere each year. Long-fibered sphagnum moss products, like the ones from Mosser Lee, are harvested sustainably, with the moss renewing itself year after year. It’s compostable, non-toxic and has been recommended by the USDA since 1944.

Plant hardiness zones and Seed Starting

Sometimes called a grow zone, understanding your plant hardiness zone is essential for deciding when to start seeds indoors. The USDA created this standard to help gardeners determine which plants will thrive in their region and, most importantly for seed starting, your average last frost date. 

When you live in Zone 2, the upper reaches of Minnesota in the United States, you could see a frost as late as June 4th. When you live in Zone 6, much of Kansas, Missouri and Ohio, your latest frost comes about April 21. Annuals that have been started from seed should not be transplanted outdoors until the last frost date has passed.

How to Read a Seed Packet

When you purchase seeds from a reputable source, you’ll be given a wealth of information on the packet about how to start seeds indoors. You’ll see planting depth recommendations, soil and light recommendations and, to help determine your seed starting date, days to transplant. For instance, your packet may read “start seeds indoors 2-3 weeks before transplanting.” From your last frost date, April 21 for example, count back 14-21 days to arrive at your seed starting date, determining the best indoor starting dates are April 1 to April 8th. 

Cold Stratification and Scarification

While many types of seeds can be sowed with no preparation, there are some that will need special care. Seeds such as prairie coneflower, pincushion and milkweed need cold stratification. Other seeds will need scarification, like nasturtium, sweet pea and morning glory. When going through all the steps involved in how to start an avocado seed, scarification is number one on the list of things to do.  
Are you hoping to grow milkweed to help support the Monarch butterfly? Click here and we’ll send you a packet of seeds with growing instructions – free!

Indoor Seed Starting Chart

Seed Starting Chart, When to Start Seeds Indoors

Indoor Seed Starting Groups

Starting seeds in groups can be helpful to your schedule and give you a bountiful harvest. Consider starting these plants at roughly the same time. For instance, you’ll start tomato seeds indoors at the same time that you’ll start eggplant. Ratatouille, anyone?

Double check your cold loving plants such as cabbage, broccoli and brussel sprouts. You may want to start those seeds indoors when it’s still snowing outside!

Group A – Start these plants indoors 10-12 weeks (70-84 days) before your last frost.

Group A Seed Starting 12-10 Weeks before Frost

Group B- Start these plants indoors 8-10 weeks (56-70 days) before your last frost.

Group B Seeds, Seeds to be started between 8-10 Weeks before frost

Group C – Start these plants indoors 4-6 weeks (28-42 days) before your last frost. While many of these can be direct sown outside, they can also benefit from a little extra boost to get a strong healthy start.

Group C Seed Starting list- Seeds that start between 4-6 Weeks before frost

Can I start seeds indoors earlier than recommended?

The short answer is yes, but you’ll need to make some adjustments. When you begin earlier than recommended, little seedlings can become big seedlings very quickly and will need to be hardened off and transplanted earlier, while there is still the threat of frost which will kill annual flowers, vegetables and tender perennials. Starting seeds indoors too early makes it a struggle to keep your plants thriving. You may want to invest in hoop houses, cold frames and row covers to extend growing times.

The benefit of starting seeds earlier is that you are moving up your harvest date and can enjoy your favorite veggies and a vibrant floral display a bit sooner. Consider staggering your planting schedule to extend your harvest throughout the season. Many gardeners prefer to divide their quantity of seeds into portions, starting one portion each week for several weeks. This way, plants grow and vegetables mature at different times, ensuring a steady supply throughout the season.

Can I start seeds late?

Absolutely! Your plants will be smaller than plants sown earlier, but when starting indoors in a high quality seed starter, you’ll achieve robust root growth for healthy seedlings.

Which seeds should I direct sow?

Many vegetable and flower varieties do very well when direct sown or planted directly into the ground or outdoor container. Check your seed packet to determine the grower’s recommendation. Some vegetables such as peas and spinach can be sown before your last frost date; that information will also be on the packet.

Vegetable and Flower Chart for direct sow

Find More Gardening Projects:

Healthy Seedlings growing in tray filled with Mosser Lee No Damp Off

How to Start Seeds Indoors

In most northern climates, gardeners have long known the many benefits in starting their own vegetable and flower seeds indoors. It is easy and fun. With this tutorial, you will learn about the joys and feel the pride in growing your own plants from seeds.

How to Plant Seed Stock

How to Plant Nursery Stock

Many Gardeners choose to plant Nursery Stock Seedlings when growing vegetables like tomatoes, and peppers.  Often the journey from seed to soil for Nursery Stock Seedlings can be stressful.  Use Long-Fibered Sphagnum Moss to decrease your new seedlings stress and increase your yield this year.

Finished terrarium

How to Create a Terrarium

Planting a terrarium is ideal for gardening indoors where space is limited for potted houseplants.  Terrarium gardens also are great for those who would like to try their hand at first time gardening.

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