Starting Your Own Seedlings — Part 2: Soil and Containers

Starting your own seedlings is a wonderfully rewarding experience, giving you access to a much greater variety of choices. And the basics are easy: Put a seed in some soil and let it grow. But what about when you want some specifics to refine your practices a bit? For example, what soil should you use, and which containers work best?

Soil

Every seed-starting gardener has his or her own beliefs about the perfect starting medium, but most are at least similar. The only hard-and-fast rule is not to use garden soil directly unless you sterilize it. It’s wonderfully full of microbial life, but this also means it has mold and bacteria present that, while typically held in check in the garden, can overly flourish in the wet conditions you’re creating for your seedlings indoors and cause problems.

Potting soil (not topsoil) and seedling mixes right from the store work fine. Some gardeners, however, find pure potting soil and many seedlings mixes to be “too rich” (i.e. contain too much added synthetic fertilizer) for their seedlings and so mix them with something else. For example, we mix up roughly equal parts potting soil and peat moss (which is great at retaining moisture, but not very rich in available nutrients) and add a little sand (2-3 pints per 2.5 gallons) and vermiculite (just a couple handfuls per 2.5 gallons). Especially green-thumbed friends of mine all have their own, mostly similar, mixes. Enjoy finding your own and comparing notes with other gardeners. It’s one of our pastimes. 😉

When you place your planting medium in your containers, compact the soil lightly with your fingers or thumbs as you add it. Normally, gardeners try to avoid compacting soil, but this is one exception.

Containers

When just getting started, use whatever you have and can afford for containers: yogurt and cottage cheese containers, the bottom half of plastic soda bottles, paper pots, peat pots, etc—just be sure to poke holes that are big enough to drain water and not get plugged up with soil. Your observations of the pros and cons of your chosen containers will likely get you trying something new each year until you settle on one you really like. Let those experiences be your guide. The journey and the wisdom you gain in the process are worth the experiments.

For what it’s worth, in our own experiments, we tried everything from yogurt containers and homemade, newspaper pots to larger blocks of soil and purchased fiber pots. Come planting or garden clean-up time, however, we always found a reason to change for the next year. The yogurt containers were either too big or too small, unnecessarily using too much of our seedling planting medium or binding our plants’ roots. The fact that the pots were bio-degradable was intriguing, and the newspaper ones were super easy to make with a nifty little, inexpensive pot-maker we bought. But, upon further inspection at the end of the season, both they and plant fiber pots didn’t degrade enough to let enough roots through, causing the plants to be extremely root-bound all season by these non-degraded, bio-degradable pots. Willing to give it another try, at planting time the next year, we tried both removing the seedlings from the pots and the pots from the seedlings. However, since the newspaper pots had softened just enough to allow significant penetration by the roots and roots suffused the fiber pots by the very nature of these pots’ bird-nest-like construction, it caused major root damage no matter which way we tried extricating them from each other.

Similarly, we found that planting in large blocks of soil meant that we ended up severing significant portions of our young plants’ roots when dividing these larger blocks into smaller, plantable chunks of soil around each seedling come planting time. The seedlings’ roots always had grown far in all directions, intermingling inextricably throughout the large block of soil.

Soil block makers solve this issue. They’re small metal presses that compact wetted planting mix into small blocks of soil. Kept on trays a small distance from each other, the air around each block keeps roots from extending beyond the edge of the soil, inherently keeping the plants’ roots from twining around the outside of the soil block or growing into adjacent soil. They’re great.

However, we finally settled on the perfect-sized (for us) terra cotta pots (about 3.5”x 3”). They drain and absorb water well and don’t infringe on the roots much when transplanting, and their tapered shape makes it easy to remove plants for transplanting. They’re more expensive upfront, ($.50-$1 per pot), but, used every year, we figured it was worth the benefits so just broke down and bought enough for all of our seedlings. We’ve used these for years and love them. They’re easy to fill, plant, water, and move, and we don’t have any problems with our seedlings being root-bound unless we really mistime our planting dates and keep the seedlings in their pots for weeks too long. It made things easier to find sturdy trays to hold the pots, but that’s the case for any containers.

Recently, we’ve also really come to like Winstrip trays. When it came time to do the 13 experimental beds (much more on these later), we knew we didn’t have enough pots to hold all of the seedlings we planned to grow—45 pepper plants, 18 tomatoes, and tons of lettuce, spinach, collards, and kale—so we decided to try Winstrip trays sold by Neversink Farm.

Winstrip trays operate on a similar principle to soil blocks but are much easier to use. Looking somewhat like a much heavier-duty version of the common plug planting trays found at garden centers, they’re actually quite different. Open bottoms and slits up two corners of each plug create air gaps that keep roots from wrapping around the inside of the plug, preventing the seedlings from becoming root-bound. Each plug is slightly tapered with small tabs on the bottom to keep soil from falling out the bottom, and filling requires only gently bouncing the tray, meaning the soil isn’t compacted at all. Each Winstrip tray is fillable in less than a minute, giving 72 or 128 spots for seedlings that work so well that it’s hard not to use them for just about everything.

The trays are only available in one place, however, and can be a little hard to find. You can find the trays and a video of how simple they are to fill and use here: https://www.neversinktools.com/products/winstrip-trays.

Really, use whatever containers makes sense to you based on your budget and curiosity. Try what interests you and learn from it. There are no hard-and-fast rules here other than 1) make sure the container can drain and 2) keep the plant from becoming root-bound as much as possible.

If you’re looking for the details, there’s a little more to starting your own seedlings than what soil and containers to use. In the next post, we’ll explore options for obtaining the ideal planting depth and thinning, some finer points on watering, options for sources of light, and how to add heat to seedlings that need it.