Starting Seeds Indoors: A Comprehensive Guide for Gardeners (2024)

It is easy and inexpensive to start (or expand) a garden by starting seeds indoors for spring planting. Here at TGG, we use several methods. Get a head start on your flower bed, your herb garden or even your own veggies by starting your seeds before the last frost.

Growing your own seeds also allows you to try plant varieties that you can’t readily get at your local nursery. Check out our complete how-to on growing seeds and then jump on over to our post on Top Garden Seed Catalogs so you know just where to find the best seeds to plant.

What Is Indoor Seed Starting, and Why Should You Do It?

While you can sow seeds directly into the soil, you have to wait until there’s no chance of frost, and the ground is warming up. By starting seeds indoors, you can keep your seeds in a nice, toasty warm environment while they germinate. It speeds up the process significantly!

Another option is to start your seeds outdoors in containers rather than directly into the soil. This allows you to move the seeds around with the sun, or bring them inside if you’re expecting some particularly chilly temperatures. It’s a good idea if you have too many seeds to start indoors, but generally won’t germinate as fast as seeds started inside.

When Should You Start Seeds Indoors?

Even though you’re starting seeds indoors, you still need to be mindful of the temperatures outdoors. You want your seedlings to be healthy and robust when the outdoor temperatures are warm enough for transplanting.

If you start your seeds too early, they will overgrow their seed-starting containers, and might even start to die off before you can get them in the soil. If you wait too long, you might find yourself transplanting your seedlings later in the season, which could limit your harvest.

Use a Seed Starting Chart to Plan and Track your Timing

The best way to figure out the timing is to work backward. Think about when your last frost date is, and when you can safely transplant your seedlings after that date. Keep in mind the germination time, and allow 2-3 weeks for your seedlings to grow in their containers and harden off before you transplant them. Also note that both germination time and the time until transplanting will vary depending on the plant.

It helps to have a central location to do all the math and track seed start times- so that’s why I’ve created a seed starting chart for you to download for free! Click here to get yours

What Are the Best Seeds for Starting Indoors?

It’s important to understand that not all seeds should be started indoors. Some seedlings don’t transplant well, and do better when planted directly in the soil. Others, especially those with a long growing season, almost need to be started indoors, especially if you’re in a cooler climate.

Best Seeds to Start indoors

These varieties tend to do well when started indoors and can benefit from a bit of jump start. If you’re in a cool climate, you may even need to start these indoors to make sure they have enough growing time to reach harvest:

• Tomatoes
• Peppers
• Eggplant
• Cucumbers
• Squash
• Pumpkins
• Melons

Generally, these seeds do well indoors and transplant just fine once the weather is warmer.For more ideas of flowers and landscape plants to start from seed, check out our list of best plants to start from seed.

Alternatively, some plants are best started indoors so that they can be ready for harvest before it gets too warm outside. This is mostly the case if you live somewhere with relatively hot summers. For instance, many of these plants will not do well over the summer and are best suited to either a late start for a fall harvest, or an early planting for early summer or spring harvest.

• Broccoli
• Brussels sprouts
• Cabbage
• Cauliflower

Which Seeds Should NOT be Started Indoors?

As a general rule, don’t start any root vegetables indoors. Also, some plants just don’t transplant well, and prefer to be started in the ground. I’d avoid starting these seeds indoors:

• Carrots
• Potatoes
• Turnips
• Radishes
• Beets
• Arugula
• Beans
• Peas
• Spinach

You can still try starting some of these seeds indoors if you like (especially the non-root vegetables) as some transplant fine – it can depend on the specific seed variety and your climate.

Preparing Your Indoor Seed Starting Area

First of all, think about where to start your seeds indoors. You want to choose a well-lit area that’s warm and sunny, but also out of the way so you’re not constantly tripping over your baby plants! Make sure your location can be easily reached, as you’ll need to water and inspect your seeds daily.

Do You Need a Grow Light to Start Seeds Indoors?

If you have enough natural light (10+ hours a day), there’s a good chance you won’t need a grow light to grow your seeds indoors. If you’re not getting that much sunlight indoors, a grow light can be a good investment to help your seeds get off to a flying start.

Your seed packet should give you an idea of how much light the seedlings will require. You can create an artificial light system easily and inexpensively with this tutorial from ‘Grow a Good Life’.

What Temperature Do Indoor Seed Starters Need?

Generally speaking, most seeds germinate best when the seed is between 70-75 degrees F. It depends on the specific plants you’re growing, and you should be able to find more detailed information on your seed packets. Some people use a heating pad under their seeds, but this isn’t usually necessary.

How to Start Seeds Indoors: 4 Easy Methods

Once you’ve chosen your seeds and a nice sunny spot to grow them, you’re ready to start! Gather all the supplies (depending on which of the below methods you choose to use), as well as some seed starter soil (it really does work better than regular potting mix) and a spray bottle for easy misting. Choose your favorite method and get those seeds in the soil!

Method #1 – Use a Seed Starting Kit

Purchase an inexpensive dome-style seed starter from your local garden center or discount store. These are usually pretty cheap, and contain everything except the seeds to get you growing, including a humidity dome to keep in heat, and soil or soilless cubes.

The only downside to these is that the growing blocks are usually pretty small, so if you are going to plant fast-growing annuals such as sunflowers, morning glories or squash, you might want to wait until just two weeks before the last frost. Otherwise, you will have to transplant your seedlings into larger containers as they outgrow the seed dome.

This method works very well for growing perennials, since they are slower growing than annuals as a rule.

Make sure the growing medium is moist, place the seeds at the depth recommended by the packet, and place the dome on. You will see moisture condense inside the dome. This is great for starting out, as the heat and moisture is trapped in.

Once seedlings start to appear, you MUST remove the dome to prevent “damping off” a fungal disease we’ll discuss below. Add water as necessary to keep the soil moist, but not wet. This seed starter greenhouse is from ‘Gardeners Supply‘.

There is a large selection of dome seed starter trays and germination trays available at Amazon. Check them out here.

Method #2 – Use recycled pots

The second method is the most economical one. This is where we plant seeds in reusable or recycled containers. These containers must have drainage, and be able to be moved easily. Tupperware, egg cartons or pots made from recycled newspaper are several popular ideas.

Placed on a tray, such as an old cookie sheet, they make great planting flats, if not too terribly attractive. It helps when using this method to enclose the entire tray in a clear plastic bag until seedlings appear. This does the same job as the dome above, keeping in heat and moisture. ’Lovely Greens’ has 12 ideas for using recycled materials for growing seeds! Here is just one! Use egg shells to make biodegradable pots!

Or, use newspaper to make pots you can plant right into the ground when they are ready. Check out the tutorial over at ‘Fine Gardening’.

Method #3 – Use a greenhouse

This is my preferred method for growing seeds indoors- use a small, portable, greenhouse. This makes it very easy to move the entire set up outdoors for daylight, and the panels keeps the heat in even when it hovers near freezing outside. You can easily grow several hundred seedlings in this setup. I caution you against leaving it outdoors during windy conditions, however.

If you’ve checked out our DIY indoor greenhouse ideas post, any of those ideas could also function great for an indoor seed starting setup!

Also, keep in mind during sunny days it can heat up inside the greenhouse quite quickly, so make sure you open up one side and occasionally monitor the temps in your greenhouse. This one from Amazon even comes with grow lights, but there are many other options to choose from, too!

Method #4 – Soil Blocking

This method has gained some major popularity recently, mostly with small flower farmers. The soil blocking method involves using a tool to create blocks out of your soil. These blocks replace seed starting trays entirely- no plastic needed! It’s definitely a great sustainable option. However, it’s a lot less efficient to soil block for a small garden because you have to mix and wet soil just for a small batch of seeds. When you’re planting a small commercial crop, however, it makes a lot of sense. Either way, it’s a fascinating method, and many farmers note that it even helps produce healthier, stronger seedlings come time to plant. Learn more about soil blocking with this guide by ‘Gardenary’.

Tips for Starting Your Seeds Indoors

Once you’ve chosen your method, add your seed starting soil mix to your containers of choice. I like to water my soil before I add the seeds, so that I can water it thoroughly without the risk of flooding the seeds out.

Plant the seeds at the depths indicated on the seed packets, and be sure to label your trays in a way that makes sense to you – you’ll be amazed how quickly you can forget what you planted where!

Once your seeds are in the soil and you’ve covered them up, water them once more with a spritz from a spray bottle. Over the coming days, continue to mist your seeds to keep the soil damp, but don’t water them excessively.

How to Care For Your Germinated Seedlings

Once the seedlings are up, they should have very bright, though not direct light. If you didn’t pick yourself up a greenhouse, then using a windowsill during bad weather is acceptable. But to grow healthy and strong, seedlings should be placed in either artificial light, or on a protected porch for much of the day. Make sure you bring them in at night, and don’t leave them out in frosty weather.

How to Thin your Seedlings

If you planted multiple seeds per container and your germination rate was high, you may need to thin out your seedlings so that they’re not starving each other of nutrients. Keep the tallest, strongest seedlings, and discard the others. Rather than pulling them out (which may disturb the fragile roots of the seedlings you’re keeping), pinch them off or cut them just above soil level.

Transitioning Seedlings From Indoors to Outdoors

As soon as your seedlings sprout up, you can start transitioning them to some outdoor time in the sun. Once they have their first set of true leaves, take them out into a sunny, dry location, but bring them in before the sun goes down. When you think your seedlings are ready to be transplanted, you can start hardening them off.This will vary depending on the plant, so pay attention to each individual variety’s growing instructions.

How to Harden Off Your Seedlings

If your overnight outdoor temperatures stay roughly above 50 degrees F, you can start leaving your seedlings outside overnight. Do this for about 5 days to a week before you transplant them, but don’t put them in the soil if their condition starts to deteriorate.‘Garden Betty‘ has some super helpful tips for hardening off seedlings, too!

How to Transplant Your Indoor Seed Starters

Once you’ve hardened off your seedlings, you can transplant them into your prepared garden bed. Make sure your soil has been tilled for aeration and is well-fertilized. If you’re planting your seedlings in rows, you can add a balanced fertilizer between the rows, at around the same depth as your seedling roots. This will encourage root growth, giving you strong, healthy, seedlings.

Preventing Root Shock

Seedlings often experience root shock after being transplanted – a small amount of this is normal and there’s not much you can do about it. To prevent severe root shock, transplant your seedlings later in the day, and water them in thoroughly. You can also add a very dilute seaweed extract to your watering can to reduce the effects of root shock.

Avoiding Frosts

Remember, your last frost date is only an estimate – there’s always a chance you could experience a frost even after you transplant your seedlings into the ground. Make sure you’re prepared for this with frost protection to keep your seedlings safe through the chilly nights.

Struggling to Start Your Seedlings Indoors?

Just like anything in the garden, you might have to do a bit of troubleshooting to get your seeds growing well indoors. Here are a few of the main problems I’ve come across and different solutions you can try if you experience any of these.

Damping Off Seedlings

Seedlings grown indoors are particularly susceptible to damping off, a disease caused by fungi that are commonly found in the soil. Damping off causes seedlings to wilt over and die, or may even prevent them from germinating in the first place. It’s more common in cool, damp conditions.

To help prevent damping off:

• Start with high-quality seeds and seed-raising soil
• Use clean pots and containers
• Keep your seeds warm
• Don’t over-water them
• Make sure they have good drainage
• Only plant outdoors once soil temperatures are warm

If you avoid cold, damp conditions, your seeds have a much better chance of surviving damping off.

Why Are My Seedlings Leggy?

Generally, seedlings will become “leggy” or spindly (long, thin, unstable, and top-heavy) if they’re not getting enough sunlight. Rather than growing strong, they’ll put all their energy into getting as close to the sun as possible. Here’s a great visual example from “Homestead and Chill,” where you’ll also find more helpful tips on how to avoid legginess! You can prevent this by moving your seedlings around the house so they’re constantly in the sun or using an artificial light to increase their sunlight hours.

Seeds Not Germinating Indoors

If your seeds aren’t germinating at all, it might not be your fault! Your seeds might be expired, or poor quality. It could also be that your soil isn’t warm enough. You can check the soil temperature with a thermometer, remembering that 65-70 degrees F is ideal for germinating seeds. If your soil temperatures are too low, move the seeds to a warmer part of your home or use a heating pad.

Damping off can also prevent seeds from germinating – check your soil for visible signs of fungus (usually seen as white, powdery spores) and make sure your seed trays have adequate ventilation during the germination phase.

Looking For More Seed Starting Tips? Get Our Newsletter!

Growing seeds indoors will save you money and allow you to grow plant varieties in your garden that are special and unusual. It’s easy, so try starting your seeds indoors today!

For more gardening tips and tricks, sign up for our newsletter! It’s sprinkled with seeds of knowledge that will lead to a bountiful harvest down the road. And in case you missed it above, don’t forget to download the seed starting chart!

Make sure you check out our posts on DIY Greenhouses, and also Square Foot Gardening! And then jump on over to OhMeOhMy and read 13 Indoor Plant Shelf Ideas!