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How to Start Seeds Indoors

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Gardening can be an expensive hobby if you purchase all your plants as potted nursery specimens. Fortunately, most vegetables and ornamental plants can be started from seeds, which offers a much less expensive way to populate your garden. Many vegetables and annual flowers are especially easy to grow from seeds. Perennial flowers can be harder to start from seeds, but the cost-savings can be even greater since perennials tend to cost considerably more when purchased as potted nursery plants.

Many fast-growing seeds can be planted directly into the garden, but in cold climates, slow-growing species may not have enough time to reach maturity if they are planted outdoors. Tomatoes, for example, require warm soil to germinate and take a long time to reach maturity, and thus they are usually started indoors well before the last frost date. A package of seeds will usually announce if the plant should be started indoors, with instructions that include phrases such as “start indoors 8 weeks before last expected frost date in your area.”

Each type of plant has its own particular needs for starting it indoors. Seed depth, type of growing medium, and water and light exposure needs will all vary depending on the species. But the general process is the same for germinating seeds and growing seedlings you can transplant into the outdoor garden.

Everything You Need to Know About Starting an Edible Seed Garden

Reading a Seed Package

The printed instructions on the back of a seed package will give you a lot of information on how (and if) you should start the seeds indoors. The information printed there will not only tell you if the plant is a good candidate for indoor starting, but also what conditions you’ll need to supply and what to expect as the seeds germinate and grow into seedlings. Among the most important information to look for:

  • Planting time: Most seed packets will tell you quite clearly if the seeds can or should be started indoors. For some species (tomatoes, for example), it is virtually mandatory to start seeds indoors in cold-weather climates. For other species it may be optional, and for other fast-growing species, there may be no indoor starting information at all—these plants are best planted directly in the outdoor garden.
  • Days to maturity: This will tell you how long the plants take to produce edible fruit or ornamental flowers. Fast-maturing plants can usually be planted right in the garden, while slow-maturing plants are better candidates for starting indoors while outdoor temperatures are still cold. Some tomato plants take as much as 100 days to reach fruit-producing maturity. If you want tomatoes in July, this means the seeds need to be started in early April.
  • Light and water needs: The seed package will tell you if the seeds need lots of light. If so, starting them indoors may require a fluorescent grow light—or you may need to reserve your sunniest window for seed-starting.
  • Soil needs: Some seeds can be started in ordinary potting soil, while others require a porous, fine-grained seed-starting mix. The package may also suggest an optimal soil temperature for seeds to germinate. Seeds that require 70-degree soil to germinate will clearly need to be started indoors in cold-weather climates since the soil does not get adequately warm until late into May.

The seed package will also give a wealth of other information, such as days to germination, fertilizing needs, planting depth, and transplanting techniques.

What You’ll Need

Equipment / Tools

  • Marker
  • Grow light (if necessary)

Materials

  • Planting trays and small containers
  • Plant seeds
  • Seed-starting mix or potting mix
  • Labels
  • Plastic bags or tray covers

Instructions

Prepare the Growing Medium

There are many good commercial potting mixes available that are suitable for starting seeds. Although they may be called “potting soil,” they actually contain no garden soil at all. Instead, they are a soilless mix containing materials such as peat moss, perlite, vermiculite, compost, pulverized limestone, or fine sand. This ordinary potting mix, the same type used for houseplants, is fine for starting many seeds. Since new seedlings don’t require fertilizer until they sprout their first true leaves, you don’t really need a mix that has additional fertilizer mixed in.

Some seeds—especially those that are very tiny—may do better in what is known as a seed-starting mix. Seed-starting mix is a special form of soilless potting mix that is especially porous and fine-grained. Seed-starting mix typically uses smaller particles of vermiculite and sand, and it omits the organic materials found in standard potting soil. This is because seeds do not require the nutrients provided by organic material to germinate and sprout. If you start seeds in a seed-starting mix, however, you generally need to transplant the seedlings into a standard potting soil as they begin to develop into larger plants.

For many plants, a seed-starting mix is the best choice, because the organic material in standard potting mix can lead to fungal problems. Avoid starting your seeds in outdoor garden soil, which can become compacted. And outdoor soil often contains weed seeds and disease pathogens that interfere with seeds germinating and sprouting.

Loosen and dampen the potting mix before you put it into seed-starting trays or individual containers. This process helps to achieve a uniform level of moisture. Dampen the mix to the consistency of a wrung-out sponge. It should be wet, but not dripping, with no dry lumps.

Fill the Containers

Use the pre-dampened potting mix to fill your chosen seed-starting trays or containers about two-thirds full. Tap the container on the tabletop to help the potting mix settle.

Gently firm the top of the mix with your hand or a small board. Don’t pack the potting mix tightly into the container—you want it to remain fluffy and aerated.

Gardener’s Tip

Seed-starting containers can be any small leftover containers you have around the house, such as old yogurt containers or six-pack seedling containers from nursery plants you purchased. Just make sure the container has holes in the bottom for drainage.

Plant the Seeds

Once you have your containers prepared, you can begin planting the seeds. Make sure you read the seed package for special instructions. Some seeds may require a period of pre-chilling or soaking, and some seeds need exposure to light in order to germinate.

Small seeds can be sprinkled on top of the potting mix. Larger seeds can be counted out and planted individually. Use at least three seeds per container, since not all seeds will germinate and not all that do germinate will survive. You can thin out extras later.

Finish Planting

Cover the seeds with some more dampened potting mix and then gently firm again.

Re-check your seed packet for information on how much potting mix should go on top of the seeds. Generally, the smaller the seeds, the less you need to cover them. There are a few seeds, such as lettuce, which require light to germinate and should barely be covered with potting mix.

Water the Seeds

Although the potting mix was pre-dampened, it is still a good idea to sprinkle some additional water on top of the newly planted seeds. This ensures that the top layer of mix won’t dry out and it also helps to firm the potting mix and ensure good contact between the seed the mix. With very small seeds, the best way to moisten them is with a spray mist bottle.

Control the Environment

The hardest part of starting seeds indoors is providing the optimal temperature, light, and humidity levels for them to germinate and sprout into seedlings.

Start by covering the trays or containers with clear plastic. This can be provided by rigid plastic domes or covers, as is included with commercial seed-starting trays, or with clear plastic bags if you are using repurposed containers for starting your seeds. The plastic covering serves to hold in heat and moisture.

Next, move the container to a warm, draft-free spot where you can check it daily. Most seeds germinate best when the temperature is between 65 and 70 degrees Fahrenheit, but check the information on the seed packet for specifics. The top of a refrigerator is an ideal spot, or you could consider purchasing heating mats specially made for germinating the seeds. Heating mats go under the potting containers and heat the soil from below. You will usually need to water more frequently when using heating mats. Caution: Only use heating mats certified for seed-starting use.

Remove the plastic as soon as you see a seedling emerging and move the containers into indirect light. In general, seeds will not need light until they emerge. From this point forward, be sure the potting mix stays moist, but not wet. Overly damp soil can lead fungal disease. This is a critical point in the seedlings growth, as they need both slightly damp soil and good air circulation. Improper conditions can lead to damping off disease, a fungal disease that quickly kills seedlings. You can minimize the chances for damping off disease by watering the containers from below, and by providing good air circulation once the seedlings have sprouted.

Monitor Seedling Growth

Once your seedlings begin poking through the soil, they will start to straighten up and unfurl. What looks like two leaves will appear. These are leaf-like structures, called cotyledons, that are part of the seed and serve as food sources until true leaves are formed and the plant becomes capable of photosynthesis. This is the point at which you should move your seedlings under a light source.

Your seedlings will need between 12 to 18 hours of light each day. This may seem extreme, but artificial light and even the low rays of the winter sun are not as intense as the full summer sun. The best way to ensure regular, long doses of light is to attach a fluorescent or high-intensity plant lights to an automatic timer.

Begin Feeding

As the seedling grows, the cotyledons will wither and the first “true” leaves will form. This is when your seedling begins actively photosynthesizing. Since it is growing in a soilless mix, you will need to give it some supplemental feeding at this point. Use a balanced fertilizer or one high in nitrogen and potassium to encourage good roots and healthy growth. Excessive fertilizer will overwhelm the seedlings, so use a water-soluble fertilizer diluted to one-half the normal strength. The seedlings should be lightly fed every two weeks.

Seedlings can remain in their original containers until you are ready to plant them in their permanent spots. However, it’s common to move the seedlings into a larger pot once several sets of leaves have formed and the seedling is a couple of inches tall. This is called “potting up,” and it allows the roots more room to develop. Three to four-inch pots are good sizes to pot up to, allowing plenty of room for root growth.

If more than one seedling is growing in the same pot, either separate the seedlings into individual pots or cut off all but the strongest seedling. Don’t try to pull out the extra seedlings, since this may damage the roots of remaining seedling.

Harden Off the Seedlings

By the time the temperature warms outside, you should have stocky, healthy young plants. Before moving them out into the garden, take a week or two to gradually introduce them to their new growing conditions. This is called hardening off. It gives the plants a chance to acclimate to sunlight, drying winds, and climate changes.

Move the plants to a shady, sheltered outdoor spot for increasing lengths of time each day, over a period of seven to fourteen days. Gradually increase the amount of outdoor time, and introduce direct sunlight as they grow accustomed to outdoor conditions. At the start of this period, you will bring your seedlings indoors or cover them at night if the temperature looks like it will dip overnight. By the end of the hardening off period, you can leave them outdoors all night, uncovered, so long as the overnight temperature doesn’t dip below about 50 degrees Fahrenheit.

Once they can comfortably thrive outdoors through the night, your seedlings are ready to transplant into the garden or into permanent outdoor containers. Water your seedlings well before and after transplanting. Try not to transplant during the hottest, sunniest part of the day.

Starting seeds indoors is a good way to save money and get a head start on the gardening season. Learn the techniques for successful seed-starting.

How To Make Your Own DIY Seed Starting Mix (With Recipe!)

Seed starting mix can be expensive to buy, so I came up with my own recipe for a homemade medium. This is the best mix, and it’s super easy to make too! In this post, I will share my recipe, and show you exactly how to make DIY seed starter soil from scratch.

When I talk about starting seeds indoors, one of the first questions new gardeners ask me is about the best potting soil mix to use.

This is a very important question, because the type of soil you use for growing seeds really does make a huge difference – and it can make or break your success!

Using the wrong type of soil for planting seeds indoors is a common mistake. Many new gardeners think that “dirt is dirt”.

So they either buy a cheap potting mix – or worse, try to use garden soil. This my friend is just a recipe for disaster.

Table of Contents

Seed Starting Mix -vs- Cheap Potting Soil

The reason you can’t use cheap potting soil or garden soil to grow seeds indoors is because those types of soils will become compact in containers.

When that happens, it’s extremely difficult (if not impossible) for seeds to germinate, and for the delicate seedling roots to grow.

Your seed starting medium should be porous so the soil stays light and fluffy, which makes it much easier for the seeds to germinate.

A porous seedling mix also allows plenty of air around the roots – which is super important for healthy seedling growth.

In fact, the best potting soil to use for starting seeds indoors shouldn’t even contain soil at all.

What’s The Best Soil For Seed Germination?

The best seed starting medium to use for growing seeds indoors is a soilless seed starting mix that is both fast draining, and also holds moisture (sounds like a funny combo, I know).

You can buy a quality seed starter mix pretty much anywhere you can buy seeds, or you can make your own DIY seed starting mix.

I love making my own homemade seed starter mix, it’s super easy and it gives me the flexibility to modify the ingredients and make it just how I like it.

Plus I can make as much or as little as I need for starting my seeds, no need to have a big bag of seed starting mix lying around if I only need enough for one seedling tray.

Getting ready to make DIY seed starting mix

How To Make Seed Starting Mix

When I came up with my own soilless seed starting mix recipe, it was mainly because I had a bunch of the necessary ingredients laying around from making other potting soil recipes… and because buying pre-made seed starting mix is expensive.

But I also wanted to make sure the ingredients were easy for you to find too, so that I could share my recipe.

These are all common ingredients that can be bought wherever you find potting soil for sale at your local garden center, or ordered online any time.

DIY Seed Starting Mix Ingredients

To make your own seed starting mix, you only need three main ingredients:

  1. Coco coir or peat moss
  2. Vermiculite
  3. Perlite or pumice
  4. Garden lime (only needed if you use peat moss)

Ingredients for DIY seed starter mix

Coco Coir -vs- Peat Moss

You can use either coco coir or peat moss as the base ingredient for your DIY seed starting mix, whichever you prefer. There are a couple of cons to using peat moss rather than coco coir…

One con for using peat moss is that it’s slightly acidic, so if you choose to use peat moss, add some garden lime to the mix to balance out the acidity.

The other con to using peat moss is that it’s not as sustainable as coco coir is. Peat moss is a renewable resource, but it is very slow to renew so it’s not as environmentally friendly.

Starting seeds in coco coir on the other hand is very sustainable, it is the by-product of coconut processing. It’s a wonderful additive to any DIY soil mixes, and I really love the consistency of it. It’s my top choice for adding to my homemade seed starting mix!

Coir ingredient in seed starting soil mix recipe

Vermiculite

Vermiculite is a naturally occurring mineral, and is commonly found in soil for starting seeds. That’s because it helps the soil retain moisture.

Seedlings won’t tolerate dried out soil, and vermiculite helps to keep it evenly moist so you won’t have to water as often.

Vermiculite DIY seed starting mix ingredient

Perlite

Perlite is a very lightweight soil amendment. It’s the white pieces that look like Styrofoam that you see in many commercial potting soils.

It retains very little moisture, and prevents soil compaction. In other words, perlite helps the soil drain faster, and makes the mix porous (which is ideal for germinating seeds).

You can buy perlite in many garden centers or home improvement stores in the same section as you would find the potting soil for sale.

A great alternative to perlite is pumice, just in case you have a hard time finding perlite in your local garden center.

Perlite ingredient in seed starting soil recipe

Supplies Needed:

Along with your seedling mix ingredients, you’ll need a few other supplies to help you measure and mix your everything. So grab these items before getting started…

  • Measuring container (I usually use a 1 cup measure)
  • Trowel or large mixing spoon
  • Container for mixing (I use a bucket or my table top potting tray to mix mine)
  • Seed starting trays

DIY Seed Starting Mix Recipe

  • 8 parts (pre-moistened) coco coir or peat moss
  • 1 part vermiculite
  • 1 part perlite or pumice
  • 1 tablespoon garden lime per gallon (if you use peat moss)

(a batch using a one cup measure as your “part” is enough to fill one commercial seed starting tray)

What is a “part”? – This is a question that I get asked a lot when I talk about how to make potting soil. A “part” is just a generic unit of measure for portioning your ingredients.

Use anything you want as your part, as long as you use the same thing for each “part”. For example if you use a 1 cup measure as your part, then this recipe would convert to 8 cups coir, 1 cup vermiculite, and 1 cup perlite.

Seed tray filled with homemade seed starter mix

How To Mix Your Homemade Seed Starting Soil

Making your own mix for starting seeds is easy. First, dump all the ingredients into a bucket or bowl…

Combine seedling mix ingredients

Then simply mix the ingredients with a spoon or trowel until they are well blended. Once the ingredients are blended together, you can fill your seedling trays and start planting seeds right away.

Mixing ingredients for DIY seed starting soil

That’s it. Told you it was easy to make your own seed starting mix. You could make a bunch ahead of time and store it for later use, or just mix up small batches as you need it.

I like to mix up a big batch, and then I store it in a plastic bucket in the garage so I always have seed starting mix on hand when I need it.

Storing Your Leftover DIY Seed Starter Mix

Whether you make your own seed starting mix, or choose to buy a commercial soil for starting seeds… make sure to store your leftover soil in an air tight container to avoid attracting bugs.

These air-tight seal lids work great to keep bugs out, and they fit on any standard five gallon bucket.

Store leftover seedling soil mix in sealed container

The best part about making your own soil for starting seeds is that you can experiment with different mixes.

If you find that the soil is drying out too quickly, next time add more vermiculite to the mix. If it’s staying too soggy, then add more perlite to your mix.

Seedlings Growing In DIY Seed Starting Mix

Making your own DIY seed starting mix is easy and economical. Use it right away, or store it for later. There’s no expiration date! Oh, and you can also use this homemade recipe for potting up your seedlings too!

Looking for even more help with growing your own seeds? Then you should enroll in my Seed Starting Course. This fun, in-depth self-paced online course has everything you need to know about growing any plant you want from seed. Register and get started today!

Otherwise, if you just need a quick refresher, or want a quick-start guide, my Starting Seeds Indoors eBook is for you!

Products I Use To Make DIY Seed Starting Mix

More Seed Starting Tips

  • Seed Starting Supplies & Equipment
  • How To Disinfect Seed Trays And Flats Before Starting Seeds Indoors
  • How To Figure Out When To Start Seeds Indoors
  • Tips For Starting Seeds Indoors
  • How To Test The Viability Of Seeds With An Easy Seed Germination Test

Share your favorite recipe for seed starting mix in the comments section below.

This 3 ingredient seed starting mix recipe uses 8 parts coco coir (or peat moss), 1 part vermiculite, and 1 part perlite (or pumice). It’s easy to make, inexpensive, and the best soil for seed germination. Here are step-by-step instructions for how to make seed starting mix yourself from scratch!