potting soil for seedlings

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)


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


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.

5 Best Seed Starting Mixes For Explosive Plant Growth

Whether you’re planning for spring or just getting a head start on your gardening tasks in general, starting seeds can be one of the trickiest parts of being a gardener.

If you’re used to buying seedlings from your local nursery and haven’t started seeds before, it can be daunting to figure out exactly what type of seed starting mix is best.

In this article, I’ll get into exactly what you need to know when choosing the best seed starting mix for your garden and seedlings.

If you just want my top recommendations, check them out below.

Other Good Options:

What is Seed Starting Mix?

Although the name sounds fancy, a seed starting mix is simply a specific mixture of soil that is designed to give seeds their best chance at germinating and growing into healthy young seedlings.

Seed mixes are typically finer and lighter than typical garden potting soil, making them easier for young roots to navigate.

What is a Soilless Seed Starting Mix?

When I first started gardening, I was confused by soilless seed mixes. How could a plant grow without soil?

It’s a reasonable question, but what I didn’t realize is that seeds get almost all of their early nutrients from the seed itself! They don’t need to draw any nutrients from the soil until later on in life.

Soilless mixes like coconut coir or peat moss can be a good option if you want to be 100% sure that there are no contaminants or pathogens in your seed starting mix. Unless your mix has been sterilized, you can’t be totally sure that it is free of pathogens.

Should You Get Sterilized Seed Mix?

When buying seed starting mix, you’ll often see the word “sterilized” on the package. This means that the manufacturer of the mix has heated the soil past the point of survival for many bacteria and harmful pathogens.

While it’s not necessary to get a sterilized mix, it’s highly recommended. Mold and fungal issues can destroy delicate seedlings. This is an especially sensitive issue if you’re growing microgreens, which are only grown to the seedling stage.

How is Potting Mix Different From Seed Starting Mix?

You’ve probably heard of potting mix before — it’s a staple for flowers, veggies, raised beds…basically any type of gardening.

But is it good for starting seeds?

In general, not really. Here are the general characteristics of potting soil:

  • It’s coarser than seed starting mix and composed of larger particles
  • It’s often too rich in nutrients
  • It doesn’t drain as well as a seed starting mix

Here are the general characteristics of a good seed starting mix:

  • It’s much more lightweight than potting mix
  • It’s composed of finer particles, making it easy for roots to navigate
  • It doesn’t contain any fertilizer

You might think that not containing any fertilizer would be a point against seed starting mix, but seeds contain most of the nutrition they’ll need for the first few days of growth.

Once your seedlings get to the point of growing their first set of true leaves, all you need to do is transplant them into a potting mix with more nutrients in it and they should thrive.

Choosing A Seed Starting Mix

Now that you know why seed starting mix is used instead of other types of soil, let’s get into what makes a seedling mix perfect for your plants.

Lightweight and Retains Water

The best seedling mixes are lightweight but still retain water well. They’ll include either vermiculite or perlite for aeration, and either sphagnum peat moss or coconut coir for water retention.

Sphagnum Peat Moss vs. Coconut Coir

Almost every seed starting mix will have either peat moss or coco coir as their base ingredient. They both provide the water retention that young seedlings need, and don’t have too many differences when it comes to that property. But there are other factors you may want to consider.

Peat moss has come under fire for being less sustainable than other options like coco coir due to the fact that it is mined from bogs and is thus non-renewable. The management of these bogs is pretty good though, almost to the point of peat moss being classified as a renewable resource these days.

Coconut coir is growing in popularity as both a base for seed starting mixes and in hydroponic use due to how similar it is to peat moss. It can retain over eight times its weight in water, making it fantastic in seedling mix. Better yet, it comes in dehydrated and compressed bricks, making it easy to ship!

Perlite vs. Vermiculite

Both perlite and vermiculite add aeration to your seedling mix, making them essential ingredients for young seedlings struggling to establish themselves.

Perlite looks like tiny white puffy balls. It’s a natural material that is extremely lightweight, making it great in seedling mixes, but only in small amounts. If you use too much, it’ll just blow away!

Vermiculite is also a naturally-occurring material but has a flaky and reflective appearance. It provides less aeration than perlite, but more water retention, making it a good choice if you have less water retention in your base of coconut coir or peat moss.

Diatomaceous Earth

Diatomaceous Earth is sometimes added to seedling mix. If you’ve never heard of it before, don’t worry — I hadn’t either when I started gardening. It’s a mineral that is made up of fossilized plants called diatoms. It has the unique property of destroying almost all insects that could bug your little seedlings, which is why it’s added to seed starting mixes.

While seed mixes are usually sterilized by the manufacturer, it’s a good idea to add a bit of diatomaceous earth to the mix just to give your seedlings a better chance at survival.

Organic vs. Conventional Seed Mix

The debate around organic vs. conventional produce is still raging, but does it apply to seedling mixes as well? Make no mistake, manufacturers of seedling mixes are responding to the increased demand for organic and putting all sorts of organic seedling mixes on the market.

When it comes to seedling mix, my personal opinion is that it doesn’t matter much if you choose organic vs. conventional. Think about it – you’ve got peat moss, coco coir, perlite, and vermiculite making up the majority of the ingredients. Most of these are naturally-occurring materials that by definition are “organic” because they’re minerals. They can’t be produced in a more organic manner than they already are!

If you decide to buy an organic mix, make sure it’s certified organic.

Correct pH Levels

Because all of the ingredients in seed starting mixes have different pH levels, manufacturers often add lime to adjust the pH of the overall mix to a level that is perfect for young seedlings.

In general, your seedlings will do well with an acidic pH level between 5.5-6.5. Keep in mind that adding anything to your seedling mix will affect the pH of the entire mixture, so be careful what you add!

The Best Seed Starting Mixes

Best Organic Seed Starting Mix

Starting seeds can be tricky, so why make it harder by choosing a bad seedling mix? In this guide we look at the best seed starting mixes you can buy.