Can You Use Potting Soil to Start Seeds?

You might be wondering if it's okay to grab that leftover potting mix from last season to get seeds going. In this article, gardening expert Melissa Strauss explores the use of potting soil for seed starting.

A tray filled with small green sprouts emerging from circular brown planting material.

Contents

Spring is coming, and we are getting ready! Ordering seeds and gathering supplies are on the agenda. Before you set the timer on your grow lights, you’ve got to lay down a good foundation for those new baby plants. 

When it comes to gardening, some things are flexible, like the containers you use and the brand of your favorite shovel. Others, like sun exposure, nutrient content, and frost tolerance, are a bit more narrow in their requirements. Anyone who left their poinsettia on the porch during a frost in December can tell you how important that one is.

When it comes to potting mixes and soil content, I tend to err on the flexible side. I don’t worry too much about putting a philodendron in succulent soil if it’s fresh. But when it comes to seed starting, is there a hard and fast rule, or can we just use any medium? Let’s dig in. 

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Epic Seed Starting Grow Lights Standard

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Epic Seed Starting Grow Lights Standard

Seed Starting Trays

EPIC 6-CELL SEED STARTING TRAYS

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Epic 6-Cell Seed Starting Trays

Heat Mat

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Epic Seed Starting Heat Mat

The Short Answer

You can start seeds in potting soil, but it’s not the most effective medium. You may find that you get better results, including a higher germination rate and better root development, with a more specialized product or mixture. Try a commercially available seed starting mix, or combine ingredients to create your own.

The Long Answer

Close-up of a planting tray divided into squares, each containing dark planting material with small white pieces visible on the surface.
You can use regular potting soil or just about any medium for seed starting.

Technically, you can use regular potting soil or just about any medium for seed starting. But just because you can do something, or it’s been done before, doesn’t mean it’s going to offer the best results. Let’s explore the potential benefits and disadvantages of using different mediums for starting your seeds.

Advantages and Disadvantages

Hands scooping dark planting mix into a container surrounded by blooming purple and white flowers.
It can contain compost, manure, coco coir, vermiculite, bark, and sometimes, actual soil.

All potting mediums are not the same, and it would stand to reason that different plants have different needs. This is never more apparent than when you’re working with tiny, vulnerable seeds. 

Some mature plants are fairly tolerant of different environments. That’s not to say that all seeds need a special set of circumstances to germinate. I think I could get a zinnia seed to sprout in a sandbox. Others are less forgiving and more delicate about their needs.

The advantages of using potting soil to start seeds are that it is easy to come by and usually inexpensive. If you’re a seasoned gardener, you’ve probably got half a bag rolled up someplace and an emergency stash, if you will. I wish I could list other perks, but that’s about the long and short of it. 

When it comes to disadvantages, we will need to get a little more in-depth. The composition of potting soil it what makes it an inhospitable environment for some of the more sensitive and delicate seeds. Potting soil is typically a mix of fine and coarse particles. It can contain compost, manure, coco coir, vermiculite, bark, and, often, actual soil. 

Because of the high organic material content, bacteria and fungal pathogens may be present. Some of these can cause damping off or rotting of your seedlings. If you’ve ever gone to check on your seedlings one morning to find that they have wilted into their cells overnight, you’ve experienced damping off

Seeds and seedlings can be more vulnerable to these pathogens, whereas more mature plants can tolerate them. That’s one reason why potting soil is good for mature plants but not great for seed starting. Some tougher ones, like peas and sunflowers, will survive in it, but for most, it’s not ideal.

Potting soil also tends to be rather heavy and dense and holds more moisture. Of course, you can cut it with perlite or peat moss to improve the drainage, but once you’ve gone that far, why not just use a seed starting mix? Young roots have a harder time developing in heavy soil and rot easily with wet feet. 

Ultimately, if you want to experiment with it, you’ll probably get mixed results depending on what you’re planting. But let’s explore and consider the alternatives, anyway.

Better Alternatives

Hands holding a rectangular block of compressed brown fibers, with loose fibers scattered on a white surface.
Coco coir or peat moss makes a good foundation for your formula.

If you want to go the easy route, a better alternative is a pre-formulated seed starting mix. These are easy to find, especially in late winter and spring at most retailers that sell seeds. They have some definite advantages over potting soil. 

For one thing, these mixes typically contain no actual earth, and they are sterile. They won’t contain bacteria or fungi that can get to your seedlings before they have a chance to develop a second set of leaves. 

The texture of these formulas is perfect for tender, young roots. It’s loose, porous, and lightweight. There’s no chance of a rogue weed popping up with this type of starter medium. Most of the time, they’re composed of peat and vermiculite, which are light and airy and allow for excellent drainage. 

Another thing you won’t find in these mediums is fertilizer. You don’t need to bombard your seeds with nutrients right up front. Most of them contain some stored energy to get them growing. Potting soil can sometimes contain fertilizers that are not necessary for this purpose. 

If you like to DIY when it comes to growing media, you can mix in different materials to create a mixture that you are comfortable with. Pre-mixed starters can be expensive, considering the small amount in most packages. If you’re growing a lot this year, it may pay to create your own.

Coco coir or peat moss makes a good foundation for your formula. They are sterile and hold moisture well, but are lightweight and don’t compact easily. Perlite and vermiculite are great additions. These improve the texture, drainage, and aeration of the medium. 

If you decide to add some organic material to your mixture, I highly recommend worm castings. Worm castings contain microbes that can protect your seedlings and improve the germination rate. They add some nutrients, enabling you to let your plants grow a bit larger before transplanting them. 

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Other Useful Equipment

Tiny green sprouts growing in brown planting material under bright pink grow lights. A drip irrigation line is visible in the foreground.
Grow lights make seed starting much easier.

Before we go, I’d like to wrap up with some notes on other equipment that might come in handy. Getting your seedlings off to a strong start is key in planting a vigorous garden where most, if not all, of your plants will survive. 

First, sturdy cells are a great investment. When I think about all those thin plastic trays I’ve purchased over the years, I could kick myself for not investing in long-lasting ones sooner. The lightweight ones are cheap, but you get what you pay for. One wrong step, and you’ll have to replace the whole thing.

Another useful tool is a heating mat. Most seeds germinate best in the range of 65-85°F (18-29°C). If you do it indoors and keep your home pretty toasty, you might be able to swing this. A heated seed starting mat will raise the temperature of your soil by about 10-20 degrees. This means faster, stronger germination

Grow lights make seed starting much easier. They allow you the flexibility of placing your trays in just about any space, even a windowless garage. Direct sunlight is too harsh for tiny seedlings, and it can be a challenge to find a convenient spot with bright, indirect light. Grow lights solve that problem and make your trays more portable.

Finally, plastic germination domes are another good investment. These are excellent for any plants that need some extra humidity. Indoor humidity levels can be sub-optimal for growing, especially in winter with the heater running. Plastic domes that fit over your trays hold in moisture, and are good for seed starting as well as propagating.

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