Both methods hold a seedling until it is ready to plant out. The difference is what happens at the edge of the root ball. In a plastic cell the root reaches the wall and turns, and it keeps turning, so by the time the plug is ready the root system is circling. In a soil block the root reaches air, stops, and the plant pushes out a new branch further back. The result is a denser root system that establishes faster after planting.
That is a real advantage and it is not the whole story, because a block is also a lump of damp compost with no container around it. It dries from the top, the sides and the bottom, which in a bright greenhouse in spring means a block can go from moist to critically dry inside a day.
The comparison
| Measure | Soil blocks | Plug trays |
|---|---|---|
| Root architecture | Air pruned, no spiralling | Spirals at the cell wall |
| Transplant shock | Minimal, nothing is pulled | Small, some root disturbance |
| Time to fill a tray | Slower, mix and press | Fast, fill and strike off |
| Media required | A specific sticky blocking mix | Any fine seed-starting mix |
| Watering frequency | Higher, dries from all faces | Lower |
| Best watering method | From below, into the tray | From below or a fine rose |
| Ongoing plastic use | None beyond the tray | Cells, replaced periodically |
| Cost to start | One blocker plus trays | Trays alone |
| Suits an absent grower | Poorly | Better |
The mix is the part people get wrong
A standard bagged seed-starting mix will not hold a block. Blocking media needs to be wetter than feels right, closer to a thick mud than a damp crumble, and it needs enough fibre to knit. Coir based mixes block well because the fibre length holds the shape, which is why a Burpee Organic Coconut Coir Seed Starting Mix, 16 qt Expanded ($10.39) is a reasonable starting point for a blocking mix rather than a bag of general potting compost.
If the first blocks crumble when they leave the BlumWay 8-Cell 2 in Soil Blocker ($29.99), add water rather than changing product. Almost every failed first attempt is a mix that was too dry.
Watering decides which method suits you
Blocks are watered from below, by standing them in a shallow film of water in a solid tray and letting them draw up what they need. Overhead watering erodes the block face and knocks small seed out. That means the trays underneath matter as much as the blocker, and a thin flexible tray that folds when lifted full of wet blocks will cost you a whole sowing. Heavier trays such as Bootstrap Farmer Shallow 1020 Trays, No Hole, 10-Pack ($62.99) are worth the difference here specifically.
Plug trays tolerate more. A 1020 Seed Starting Trays with 7.3 in Humidity Dome, 5-Pack ($59.99) set with a tall dome holds humidity through germination, and the cells buffer a missed watering by a useful margin. If you work away from home two days a week, that buffer is the deciding factor and nothing about root architecture outweighs it.
Cost, honestly
A hand blocker costs about the same as a set of trays and lasts indefinitely, so over several seasons blocks are cheaper. The saving is real but small, and it is not the reason to choose them. Choose blocks because the roots are better and because you would rather not buy plastic cells every few years, and accept the watering discipline that comes with them.
Bear in mind the volume question too. A hand blocker makes eight blocks at a time, so a sowing of three hundred modules is a long afternoon. Plug trays fill in minutes.
The decision, summarised
- Fewer than about 200 transplants a season, at home most days: soil blocks, and the roots will show it.
- Large sowings, or crops raised in a hurry: plug trays, every time.
- Crops that resent root disturbance, such as cucurbits: blocks or peat and coir plugs, not bare cells.
- Away from the greenhouse for a day or two at a stretch: plug trays with a wicking tray underneath.
- Trying to remove plastic from the process: blocks, with heavy reusable 1020 trays that last years.
