How-to Guide
Soil Blocking: Starting Seeds Without Pots
The short answer
Soil blocking starts seeds in compressed cubes of growing mix instead of plastic cells. A handheld block maker presses a very moist mix into firm blocks that hold together on their own, each with a seed dimple in the top. The two real advantages, per extension write-ups: roots stop growing when they reach the air at the block's edge (no circling, no root-bound seedlings, less transplant shock), and you stop buying and throwing away plastic trays.
The technique lives or dies on mix consistency — noticeably wetter than normal seed-starting, closer to oatmeal than to crumbly — pressed firmly so blocks keep crisp edges. Drop one seed in each depression (or a couple if germination is doubtful), keep blocks warm per crop type, and bottom-water gently; overhead watering erodes the cubes. At transplant time the whole block goes in the ground, roots undisturbed.
Make and sow blocks
- 1Wet the mix thoroughly — Moisten a fine seed-starting mix until it is much wetter than you would use in trays — it must compress and hold shape.
- 2Press and release — Set the blocker on a sturdy flat tray, press down with steady pressure into the mix, then release the blocks in neat rows and lift the tool straight up. Good blocks stand alone with defined edges and a seed indentation on top.
- 3Sow one seed per dimple — Drop a single seed in each depression; sow a second only if the packet's germination is low. Most seeds need no covering beyond the dimple.
- 4Keep warm, water from below — Iowa State's guidance: around 65–70°F air temperature for cool-season crops like lettuce, kale, and broccoli, and 70–80°F for warm-season crops like tomatoes and peppers. Water by wicking from the tray, never by pouring on top.
✔ Leave a small air gap between blocks. That gap is the mechanism — roots hitting air stop and branch instead of circling, which is exactly what you want at transplant time.
Is it worth the fuss?
Soil blocking trades a small learning curve (mix moisture, gentle watering) for healthier root systems and zero plastic cells. It shines for gardeners starting many seedlings each year, and for crops that resent root disturbance. If you start a handful of tomatoes annually, cell trays you already own are perfectly fine — the blocks earn their keep at volume.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Iowa State University Extension — Starting Seeds in Soil Blocks — Block-making technique (press, release, lift straight up; defined edges and seed indentation), one seed per depression, air-pruned non-circling roots and reduced transplant shock, plastic reduction, temperature guidance 65–70°F cool-season / 70–80°F warm-season.