Seed Starting 101

updated on 02 May 2024

What to plant and when, whether you're starting indoors or out

Guest Contributor: Cassey Anderson, Horticulture Agent, Colorado State University Extension

Extra produce to share? Get the app to support local hunger relief!
Extra produce to share? Get the app to support local hunger relief!

Starting your garden from seed is a great way to stretch your garden dollars. A single seed packet can start a whole bed's worth of veggies, but it takes a bit more than simply dropping seeds into a pot. Not to worry! This article will take you through the basics to get started, and how to start the most popular annual vegetable crops from seed.

A note on timing: The most important thing you need to know for your local area when planning to start your own seeds is to check your last frost date. This is the last date, on average, that your area is likely to experience frost. If your last frost date is around mid-May like here in the Colorado Front Range, then the dates I’ll discuss should be close. Otherwise, adjust accordingly depending on your local last frost date.

The Basics

Basics of seed starting for many crops:

Pre-Germinate: If you’re starting seeds outside you can pre-germinate by soaking your seeds in a wet cloth or paper towel for several hours up to 48 hours. Most vegetable seeds like to germinate in warmth and the cool season crops can then adjust to growing cool.

Know Your Cotyledons: It’s also good to recognize what the first leaves look like on your seedlings. These first leaves are called cotyledons, and they provide early nutrition to the seedling as it starts to grow its roots. They also look quite different to the true leaves, so learn the difference between the cotyledon leaf and the true leaves so you know when best to transplant. The first picture below shows cotyledons of squash, pumpkin, and cucumber. Compare this to the blush and orange tomato cotyledons in the second picture, where you can see some true leaves just starting. Some plants are more tolerant of planting with more true leaves, but many like to be transplanted when they have 2-5 true leaves.

Cotyledons of squash, pumpkin, and cucumber
Cotyledons of squash, pumpkin, and cucumber
Blush and orange tomato cotyledons, with some true leaves beginning to appear
Blush and orange tomato cotyledons, with some true leaves beginning to appear

Indoor Seeds to Start


One of the earliest varieties you’ll want to start are the brassicas, your broccoli, cabbage, cauliflower, brussels sprouts. These seedlings can be started as much as 8 weeks before the last frost, which is around early to mid-March. They can also be transplanted before the last frost as they are frost tolerant. Transplant brassica crops once you’ve got 2-4 true leaves growing2, see picture. You can take 7-10 days to adjust the seedlings to their new world by hardening them off.


Tomatoes and Peppers

Tomatoes and peppers are probably the most popular ones to start early by seed, there are many new and popular options, and you can get more diversity and customize size much better when growing by seed. Start tomatoes 6-8 weeks before the last frost, start peppers 8-10 weeks before the last frost. Peppers appreciate germinating very warmly and may take up to 3 weeks to germinate even in warm conditions so be patient! Depending on how you start your seeds it may be good to transplant them up into a larger pot at least once before hardening off and transplanting into warm soil (at least 55 degrees).



A less common seed to start, but a great one, is the onion. You can broadcast them into a shallow dish with seed starting soil. Grow for 4-6 weeks before transplanting. I usually start onions either with peppers or with tomatoes but transplant them out 2-4 weeks before the last frost as they can withstand light frosts. There are no true leaves to identify when transplanting, but you want the seedling to be strong enough to withstand being teased out of the soil and losing some of its root system if it got too tangled with its neighbor, usually when they’re the size of a toothpick or so.




Peas really benefit from a 24–48-hour soak in warm inside conditions prior to transplanting, but then can be transplanted into soils as cool as 45 degrees. You can plant as early as mid to late March. Seeds can be planted deep since the seeds are quite large, ½ to ¾”. Seedlings will pop up easily. If you have not grown peas before you may look online for pea rhizobia, a bacterium that has a beneficial relationship with nodules on the peas allowing the peas to take atmospheric nitrogen from the air and make it into plant available nitrogen.



Lettuce can be planted either inside or outside. If planted inside you can plant in individual containers or broadcast like with onions. Lettuce seeds are very small so need only be covered by a bare skiff of soil to ensure good germination. They do germinate rapidly even in cool conditions so you may have them transplant ready in as few as 2-3 weeks. Transplanting can occur with 2-4 true leaves, be sure to be gentle with the root systems.



Beans should always be planted in-ground outside once soil temperatures are over 60 degrees. They are a tough seed since they are so large, so can be planted up to an inch deep. Seedlings should emerge within 1-2 weeks. They have very recognizable cotyledons, hence being popular for elementary school seed germination projects. Beans do not need much supplemental nitrogen as they, like peas, are able to fix their own nitrogen.


Let’s talk about those cucurbits

Melons, cucumbers, pumpkin, squash etc. Generally, these seeds are going to do best when planted outside, place 2-3 seeds in each planting hole about ½” deep once soil temperatures are over 60-65 degrees F. However, some like to guarantee germination, or may have pest pressures that make getting seedlings up and growing in the soil a challenge. In this case you can start these seeds inside, but you may only wait 2-3 weeks after germination before transplant. You can transplant with as few as 1-2 true leaves, this will limit the disturbance of the root system and ensure the most success.


Hopefully this helps you get a plan in place for starting some of the heavy hitters of the vegetable garden from seed. If there are other varieties you want to start that we didn’t have time for today please reach out to your local Extension office. Happy growing!

Ready to get started? Get the Must-Have Gear for starting from seed.

Plant to share! See the Top 5 vegetables requested by hunger relief organizations.

Gardening in Colorado? Check out Grow & Give and in particular our Colorado Vegetable Guide for more crop information on all of the above plants.

What if every gardener shared just a little?

One small donation can have a tremendous impact. Just imagine, if every gardener planted one extra plant to share, or donated just a pound from the garden. Collectively, we would have an abundant source of fresh, healthy produce available to be distributed to families experiencing food insecurity in our own communities! The free Fresh Food Connect mobile app connects you to a local hunger relief program, then manages and tracks your donations of homegrown produce throughout the season.  Download the app today!

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