Step-By-Step Guide To Starting Seeds Indoors (Plus a sample setup)

soil blocks of kale seedlings

Starting seeds indoors is a great way to start gardening from the very beginning of the growing season, even when it’s still cold outside. However, it can seem like a daunting task if you’re new.

In my experience, there are two main reasons why people avoid seed starting:

  • Seed starting doesn’t seem very appealing – they think that it requires too much work and effort
  • Seed starting feels like it’s something that only experts do

The truth is that seed starting is pretty straightforward. Once you’ve mastered the basics, it becomes an extension of your gardening hobby.

You can start seeds indoors with a minimum of equipment. All you need are seed trays or containers, seed-starting soil mix, seeds, and a shop or grow light. With this gear, you can start your garden 2-3 months before the weather warms, giving you a headstart on the season. 

This guide will help you understand how to start seeds indoors so that you can get started right away.

Why start seeds indoors?

Aside from being cost-effective and convenient, starting seeds indoors has several other benefits:

Greater variety of crops

You can grow many varieties from seeds that you can’t find at the garden center as transplants. You can start perennial flowers from seed and expand your flower garden without buying plants.

If you want to try out annual flower varieties, there’s no end to the seeds you can buy. If you need some reputable sources for seeds, check out my favorites here: 10 Best Places To Buy Quality Flower Seeds Online.

More control over the environment

One of the many benefits of starting seeds indoors is that you have a greater degree of control over the environment. When you start seeds outdoors, you are at the mercy of the weather conditions – which can be unpredictable. It may be sunny one day, and chilly the next, or heavy rains can wash away seeds.

You can create an environment conducive to germination by starting seeds indoors. Steady, warm temperatures and strong light will lead to healthy seedlings that you can transplant to the garden.

Get a head start on the growing season

Another benefit to starting seeds indoors is that it extends your growing season. When you start seeds outdoors, you are typically limited to the growing season within your geographic region.

However, you can extend the growing season by several weeks or months. This can be especially helpful if you are trying to grow a specific crop that needs more time to mature than your growing season provides.

For example, I can’t grow tomatoes in my area without starting them indoors a couple of months before transplanting them out. My season is just too short for the tomatoes to mature in time. The same goes for tender perennials such as black eyed Susan. Starting seeds indoors makes it possible for me.

It’s fun!

There’s nothing like checking in on your seeds every day, waiting for that little pop of green to tell you that they’re alive and well. You’ll feel like a proud plant parent in no time!

9 steps to start seeds indoors

  1. Set up grow lights
  2. Decide which type of supplies to use: seed trays, soil blockers, or individual containers
  3. Prepare seed starting mix and fill containers
  4. Sow seeds in seed-starting mix
  5. Keep seeds moist and warm until they germinate
  6. Thin seedlings if necessary
  7. Pot up seedlings when they get bigger
  8. Harden off outdoors
  9. Transplant into the garden

Here’s a picture of my seed starting setup and the nine steps in action. It’s far from glamorous, but it works well for my needs!

home seed starting set up
A heat mat with a thermostat, lights on a timer, and all the miscellaneous things I need to start my seeds. Shall we call it shabby chic?

1. Set up grow lights

When you start seeds indoors, one of the most important things you can do to help them grow is to provide them with light. Grow lights provide artificial light similar to the light from the sun, encouraging growth and strong stems.

There are a few different types of grow lights available on the market, so you may want to do some research before choosing which type is right for you. Some popular and affordable options include fluorescent lights and LED lights. You can find both kinds at your local garden center, hardware store, or online.

Types of grow lights:

Fluorescent lights: These bulbs are what you typically find in shop lights. They are a more affordable option, but they do not last as long as LED lights, and the bulbs need to be replaced every 12 months or so.

LED lights:  A little more expensive, but LED lights last a long time and are the most energy-efficient option. Plus, they don’t get warm with use, so they can’t scorch your plants. 

How to set up grow lights:

After you have decided which type of grow lights to use, the next step is to set them up.

For both fluorescent and LED lights, you should hang them about 1-2 inches above your seedlings. To make this easy, you can attach whatever hardware or hangers come with the light kit.

If you need to lengthen the chain, try using zip strips or buying an additional length of chain so you can lower the lights further.

Your seedlings need 12-16 hours of light per day. Setting a timer makes this easy to provide, and you don’t have to watch the clock. 

2. Decide which type of supplies to use: seed trays, soil blockers, or individual containers

When you’re starting seeds indoors, one of the decisions you’ll need to make is what type of supplies to use. There are three main options: seed trays, soil blockers, or individual containers. Each option has benefits and drawbacks, so you’ll have to decide which one is right for you.

Seed trays

Seed trays are a popular option because they are easy to use and allow you to sow many seeds at once. They come in various sizes, and many have removable trays so you can transplant the seedlings when they get bigger. They take up more space than other options.

Soil blocks

Soil blockers are another popular option because they take up far less space, don’t require any seed trays or pots, and the seedlings are less likely to become root-bound.

A seedling becomes root-bound when its roots start circling the container it is in, which doesn’t happen with soil blocks. Once the roots hit the edge of the soil block and feel the air, they stop growing. This is called air-pruning, and it’s a nice side effect of using soil blocks.

One drawback is that you have to monitor the soil moisture more carefully with soil blocks due to exposure to the air, so keep that in mind if you want to be a little more hands-off.

Individual containers

This option gives each plant its pot or container. It is excellent for plants that like to develop their root systems before transplanting into the garden. You can use individual plastic pots, peat pots that decompose in the ground, or recycled containers like a small yogurt cup or milk jug. 

3. Prepare seed starting mix

One of the most important things you can do to help your seeds germinate and grow is to use a sterile seed-starting mix. This mix is light and fluffy for tiny roots, free of weed seeds, and is free from any fungus or disease that could kill your new sprouts. 

You can create your soil mix or purchase a pre-made mix from your local garden center. When creating your mix, it’s essential to include the right ingredients so that your plants get the nutrients they need.

I made my own seed-starting mix with peat moss, perlite, worm castings, and soil amendments. It’s budget-friendly and fun to make my own mix.

What to look for in a store-bought seed-starting mix:

  • Peat moss or coco coir for moisture retention
  • Perlite or vermiculite to help with drainage and aeration
  • Compost to add organic matter and nutrients

Getting the mix right will make the difference in growing healthy, happy plants. A few different recipes are available online if you’d like to try creating your soil mix.

Recipe to make your own sterile seed-starting mix:

  • 1 part peat or coir (coconut fiber)
  • 1 part perlite or vermiculite
  • 2 parts compost or worm castings  (optional)  

Mix all the ingredients in a large bucket or wheelbarrow, then moisten the soil mix. The mix should be evenly damp but not saturated. If you squeeze a handful of soil, it should form a loose ball but not drip water.

4. Sow seeds in seed-starting mix

Fill your choice of a container with the dampened seed-starting mix, lightly pressing the soil down into the container. Place 1-3 seeds in each cell, the cover with soil if needed. If your seeds are fresh, you can sow only one seed. If they’re old seeds, plant a few in case only some of them sprout.

Proper depth for seeds

Check the seed packet instructions to see the recommended planting depth for your seeds. Planting depth ranges from on the soil surface to an inch deep, depending on the type of seed.

In general, the smaller the seed, the shallower you should plant it. Tiny seeds like snapdragons don’t need to be covered at all, while large sweet pea seeds should be planted half an inch or so deep.

For seeds that need to be covered, you can sprinkle a layer of vermiculite over the soil’s surface. Vermiculite holds moisture more evenly than the soil mix, and it helps prevent damping off, a fungus that kills seedlings in the first few weeks.

5. Keep seeds moist and warm until they germinate

One of the most important things you can do to help your seeds germinate and grow is to keep them moist. This means watering them regularly and ensuring the soil doesn’t dry out. You can observe the soil for moisture or test the soil by touching it lightly.

Don’t let seed trays dry out

To keep the seeds and soil moist until germination is complete, you can mist the surface with a spray bottle to gently water the seeds. This provides moisture without washing away soil or surface-sown seeds like snapdragons. 

If your seed trays came with a bottom tray, also known as a drip tray, you can pour some water between the bottom tray and the cell tray, and the soil will wick moisture up through the drainage holes. You can set the trays on a cookie sheet if you don’t have a bottom tray. 

Bottom watering is essential for soil blocks, so they don’t fall apart under the stream of water. Just pour water onto the tray the blocks are on so they can soak it up.

Use a heat mat to optimize germination

Providing your seeds with warm soil will help speed up germination. Most seeds need a soil temperature between 65 and 75 degrees Fahrenheit to germinate. You can use a seed starting heat mat to help maintain this temperature or place your containers in a warm room until they germinate.

If your seed trays are on a heat mat, be sure to remove them once the seed has germinated. Most seedlings don’t need the extra heat at that point, and it will free up space for other trays waiting to germinate. 

6. Thin seedlings if necessary

If you sowed more than one seed per cell or container, you might need to thin your seedlings once they’ve germinated and grown for a couple of weeks. At that point, the seedlings will have at least one or two sets of true leaves, and you’ll be able to see which are the strongest seedlings.

image of seedlings with labels pointing to first and true leaves
Here you can see the difference between the first leaves of a newly germinated seed, and the true leaves, which are what the plant will continue to grow.

Thin your plants by gently pulling the extra seedlings out of the soil, being careful not to damage the roots of the remaining seedling. You can also clip the stems with a pair of scissors, which is a little easier.

It’s tough to thin seedlings. Just think of the potential flowers you could get! But thinning will help ensure that each plant has enough room to grow and develop properly.

7. Pot up seedlings when they get bigger

When your plants have outgrown their seed tray cells or soil blocks, you’ll need to do what’s called “potting up.” You do this by transplanting the seedlings into a new, larger container.

Generally, once the seedling has two or more sets of true leaves (see the picture above), it’s time to pot it up.

Potting up gives the seedling new room to grow and strengthen its roots. Fresh potting soil will also provide a new source of nutrients for the seedlings, ensuring continued growth and healthy development.

The process is easy: fill a larger container, such as a pot the next size up or a big plastic cup (with drainage holes poked in the bottom!) halfway with more seed-starting mix.

Set the seedling on the surface, then gently fill the space around it with more soil mix until the new container is full. Water the seedling well and move on to the next one. 

8. Harden off seedlings outdoors

If you’re transplanting your seedlings from a warm, sheltered environment like your house or a greenhouse to an outdoor garden, you need to “harden them off” first.

Hardening off is the process of slowly adjusting your plants to the new conditions they will be living in.

How to harden off seedlings

To harden off your plants, start by setting them outside in a shaded area for a few hours each day. Gradually increase the amount of time they spend outdoors, and move them to a sunnier spot each day.

Check them periodically, especially in the first couple of days, to make sure they aren’t drying out too fast or getting exposed to strong sunlight as the sun moves throughout the day. 

By the 6th or 7th day, the seedlings should spend most of the day outside. You can even try leaving them out overnight on the last day in their pots to finalize their acclimation to the outdoors. 

The hardening off period should last about a week. After that, your plants will be ready to go into the garden with the best chance of an easy transition. 

tray of soil blocks
The larger scabiosa seedlings on the left are hardening off before I plant them into larger containers. The smaller seedlings will keep growing as they are.

What happens if you skip the hardening-off process?

If you skip the hardening off process, your plants may not adjust to the new conditions and may die. Hardening off helps prepare your plants for the changes in temperature, sunlight, and moisture that they will experience in the outdoor environment.

Without this process, your plants may not be able to withstand these changes, and you’ll have to start over. 

9. Transplant seedlings into the garden or pots

When your seedlings have grown large enough, it’s time to transplant them into the garden. Do this by carefully removing them from their starter containers and planting them in the ground or containers.

If you’re planting them in the ground, be sure to dig a large enough hole to accommodate the roots. Place the seedling in the hole and cover it with soil, making sure to pack it down well. Water generously after planting.

If you’re planting them in containers or pots, make sure the pot has a drainage hole at the bottom. Fill the pot with soil, leaving about an inch of space at the top. Gently place the seedling in the pot, cover it with soil, and water it well.

For in-ground gardens and containers, don’t plant the seedlings any deeper than they were previously growing. The crown of the plant (the point where the stem meets the roots) should be at the soil level.

Planting too deeply can cause damage to the stem and roots that may stunt growth or even rot the stem due to constant moisture from the soil.

Tomatoes are an exception here since they should be buried up to their first set of leaves. The buried stem will grow roots. But generally, burying a seedling’s stem will cause it to rot, killing the plant.

Wrap up your seed starting

Starting seeds indoors is a great way to get a head start on the growing season and grow plants that are not typically available in your region. By following these nine simple steps, you can start seeds indoors successfully and have healthy plants ready to transplant into the garden or pots when they’re big enough.

FAQ

Should I soak seeds in water?

You can soak some seeds, such as sweet peas to soften the hard outer coating of the shell. This can help speed up germination. Some vegetable seeds can be soaked, such as beets and Swiss chard. 

However, soaking is generally not recommended for most small seeds, as it may cause the seeds to rot before they have a chance to sprout.

Can I start seeds in regular soil?

Seeds are best started in a seed-starting mix because it contains peat moss or coco coir and vermiculite (both of which are sterile), along with fertilizer mixed in. This mixture mimics the nutrient-rich environment of the soil found outside without the risk of bacteria and fungus, which could kill tender seedlings.

Can egg cartons be used to start seeds?

You can use egg cartons to start seeds as a budget-friendly option. Fill each compartment with potting soil as you would any other container. ‘There’s no need to poke drainage holes if the carton is made from cardboard. Styrofoam cartons will need holes.

Be sure to label each egg slot so you don’t forget what type of seeds are growing inside, and keep an eye on the moisture level since the egg carton will dry out the soil more quickly than a plastic pot or cell.

Can I start seeds indoors without grow lights?

It’s possible to start seeds indoors without lights, but the seedlings won’t be as strong as those grown under lights. Seeds grown near a sunny window are often “leggy,” which means they’re tall with weak stems. Seedlings should be strong and stocky as they grow, which signifies a strong and healthy stem.

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