There’s something deeply satisfying about seed sowing , the quiet act of filling trays, tucking seeds into soil, and watching the first green shoots unfurl. Over the years, I’ve tried just about every method: plastic punnets, biodegradable pots, even recycled yoghurt containers. But one tool has quietly become a favourite in my greenhouse, the soil blocker.
At first glance, soil blockers look a little unusual. They’re hand-held tools that press damp potting mix into small cubes, each with a neat indentation on top for your seed. Instead of sowing into plastic cells, you’re creating free-standing blocks of soil. It’s simple, almost old-fashioned, and yet remarkably effective.
The biggest advantage is in the roots. In traditional pots, seedlings often circle round and round, becoming “root bound.” Then, when you transplant them, they can struggle to adapt - what’s called root shock. With soil blocks, roots grow right to the edge of the block and stop, waiting until they’re planted out and given fresh soil to explore. This means stronger, healthier seedlings and much less stress when it’s time to move them into the garden.
There’s an environmental benefit too. Soil blockers remove the need for endless trays, pots and plastics. Once you’ve bought the tool, it lasts for years, and you can press out hundreds of blocks season after season. For me, there’s a small joy in seeing rows of neat, earthy cubes lined up in a tray, tidy, sustainable, and ready for life to begin.
Using them is straightforward. The key is in the mix: your potting soil needs to be moist enough to hold its shape when pressed, but not dripping. You dip the soil blocker into the mix, press firmly, and release onto a tray. A little practice and you’ll get blocks that are perfectly square and sturdy. I sow directly into the indent, cover lightly if needed, and then water gently with a fine mister to keep the blocks intact.
I use different sizes depending on the crop. Smaller blocks are ideal for quick-growing flowers like cosmos or zinnias, while larger ones are perfect for plants that take longer to establish, like delphiniums or snapdragons.
Soil blocking might not suit every gardener. If you’re short on time or space, or you prefer a less hands-on approach, traditional trays might be easier. But if you enjoy the process, value sustainability, and like giving your seedlings the best possible start, it’s worth trying.
For me, soil blockers have become part of the rhythm of seed sowing each season. They feel purposeful and practical, but also carry a little magic - taking a simple block of soil and turning it into the beginning of a garden.
Soil Blocking: Step by Step
What you need
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Soil blocker tool in your chosen size. Micro for tiny starts, mini-5 for larger starters and slower growers.
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Two trays. A mesh or ridged insert for airflow, and a waterproof tray for bottom watering.
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Seed-raising mix with enough fine particles to bind.
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Labels, a small scoop, watering can with a fine rose.
1. Mix the medium
Moisten your seed mix until it reaches the texture of a squeezed sponge — holding its shape but not dripping.
2. Prime the blocker
Dip the blocker in water, then plunge it into the mix several times to pack it firmly. Scrape the base on the rim of the tub for clean edges.
3. Make the blocks
Press the plunger to release blocks onto your mesh tray. Leave a fingertip of space between each block so air can circulate. Each block should have a neat dimple on top for sowing.
4. Sow the seed
Place one seed per dimple. Cover lightly to the seed’s own depth, or use vermiculite for very fine seeds. Label as you go.
5. Water gently
Set the mesh tray inside the waterproof tray and bottom-water until the block tops darken. Let the blocks wick water, then pour off any excess. Keep surfaces moist but never soggy. This is best practice but I can be lazy and instead use a fine misting spray over top.
6. Give them light and air
Remove any cover as soon as seedlings appear. Bright light and gentle airflow keep stems sturdy and blocks firm.
7. Feed and tend
Once seedlings have true leaves, feed weekly with a mild seaweed or fish emulsion. Rotate trays, pick off stray seedlings, and keep blocks evenly moist.
8. Pot on, if needed
Start slow-growing seeds in micro blocks, then nest each into a larger block as they grow. Quick growers can remain in a single block until planting out.
9. Harden off and plant out
Give seedlings 7–10 days of hardening off. Plant the whole block into prepared soil, firm gently, and water. Handle the block, not the stem.
10. Clean down
Rinse tools and trays, dry, and lightly oil the blocker. Good hygiene keeps seedlings healthy and blocks crisp.
Troubleshooting
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Blocks crumble: mix too dry or coarse - add water and a little fine material. Pack firmly.
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Green algae/fungus: too wet or still - increase airflow, bottom-water only.
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Leggy seedlings: too little light or too warm - adjust light and temperature.
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Slow growth: check roots, feed lightly, ensure consistent moisture.
Seasonal note for colder gardens
In cooler climates, hold warm-season plantings until mid-October when the risk of frost has passed. Soil blocks make waiting easier, seedlings grow strong, without root circling, and can be slipped straight into the garden the moment the soil is ready.