Three Systems for Indoor Seed-Starting
All over the country, growers are switching on greenhouses, light carts and germination chambers. Seed-starting season is here!
The Hensley's team starts thousands of transplants for our variety trials. Here we share the pros and cons of the three main seed-starting systems we use.
How growers start seeds indoors depends on their goals, needs and experience. Here's a look at the three systems we carry and what works for us at trial scale, so you can test them and decide which gives you the best professional-quality seedlings.
1 • Conventional Trays & Cell Flats
Molded plastic seedling trays are called 1020 trays because they measure roughly 10 by 20 inches.
Cell flats (also called plug flats) sit inside 1020 seedling trays, leak-proof trays, or lightweight mesh trays, and come in many sizes, named for the number of cells per flat.
For vegetable transplants the usual sizes are 50, 72, 128 and 200 cells. The square cells sit very close together so you can fit as many plants as possible in the greenhouse.
For bigger crops there are 38- and 24-cell flats, with larger, round cells set farther apart so roots have more room and leaves more space to spread.
You can also use a 20-row seed flat, where seeds germinate in rows and are then moved up ("potted up") into cell flats once true leaves appear. Seed flats save space on your seedling heat mat or in the germination chamber.
Advantages. Most large vegetable farms start seeds in trays and cell flats because:
- They fill fast.
- They're easy to move.
- They work with mechanical transplanters.
- They last many seasons if handled with care and stored out of direct sun.
Disadvantages. Plastic trays and flats have some downsides.
- They have to be cleaned and stored between uses.
- Seedlings can get root-bound in plastic cells, and there's some transplant shock when they're pulled out.
- They wear out eventually, and not all of them can be recycled; those that can't usually go to landfill, which has an environmental cost.
To sanitize trays and help prevent disease, we suggest a peroxide-based disinfectant such as OxiDate, an OMRI-listed product allowed in certified organic production.
Whether plastic trays, flats and domes can be recycled depends mostly on the plastic they're made of and on your local recycling center's rules and capacity. If you use plastic seed-starting containers, ask your local recycler what it accepts.
At our scale, we mainly use 1020 trays for greenhouse seed starting. Here's how we start specific crops in cell flats:
- Lettuce is sown straight into 200-cell flats, two to four seeds per cell, and covered with vented plastic domesto manage humidity and temperature. Domes come off as soon as seeds sprout, and seedlings are thinned to one per cell with scissors.
- Peppers and tomatoes are sown in 20-row flats, covered with plastic lids, and placed in a germination chamber. Once seedlings start emerging, lids come off and flats stay on heat mats until everything has sprouted. When the seed leaves are fully open, seedlings are moved into 50-cell flats.
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Cucurbits — Cucumbers, melons, pumpkins, squash, and watermelons are sown directly into
50-cell flats, two seeds per cell, then thinned with scissors to one per cell soon after they're all up. - Flowers are sown in 20-row seedling flats and moved up to 38-cell flats.
2 • Biodegradable Pots
Biodegradable pots such as Fertilpots are OMRI listed and suitable for certified organic growers. Each pot is planted right along with the seedling, then breaks down in the soil, letting roots, water and nutrients pass through.
Advantages. Biodegradable pots greatly reduce transplant shock and are very easy to plant, since seedlings never come out of the pot. They also leave no hard-to-recycle plastic behind, which makes them popular with home gardeners, so commercial transplant growers often use them to help sell plants.
Disadvantages. On the downside, you have to buy new pots each season, they need a carrying tray, and they take up more greenhouse space. And as with plastic pots, seedlings left too long before transplanting can become root-bound.
3 • Soil Blocks
Soil blocks are cubes of potting mix shaped with tools called Soil-Block Makers. The seedling grows in a freestanding block of mix with no container around it.
As with cell flats, there are several sizes for different crops. Small ¾" mini-blocks work like 20-row seed flats, for germinating seeds and then potting them on into larger blocks. Swappable dibbles make a small round hole for small seeds, a deeper round hole for larger seeds or cuttings, or a ¾" square recess to hold a mini block inside a bigger one.
More on starting seeds in soil blocks, based on Eliot Coleman's methods…
Advantages. Soil blocks grow excellent plants thanks to "air pruning": when root tips reach the edge of the block they dry out, which prompts new side roots that grow until they also reach the edge. This repeats, filling the block with a strong, evenly spread root system, so seedlings settle in very fast after transplanting.
Disadvantages. Soil blocks are simple to use and give dependable, fast-growing seedlings, but making them takes much more labor than filling flats or pots. That can make them less cost-effective for larger growers or anyone short on help or time.
Seed-Starting Supplies & Expertise
Those are the basic pros, cons and supplies for the three main indoor seed-starting methods the Hensley's team uses for our trials.
Depending on your goals and experience, you might copy these methods, adapt them, or come up with your own. Hensley's carries the seeds, seed-starting supplies and growing info to back you up, from your first tray of seedlings to your twentieth season.
Learn More
Seed starting date calculator…
This information applies to most common crops and should help your seedlings get off to a strong start. Paying attention to each crop's and variety's specific needs will help them do even better. The resources below have more detail on growing your own transplants from seed.
- Growing information for vegetables, fruit, flowers, herbs and farm seed • Index
- Seed starting date calculator
- Transplanted vegetables • plant quantity and typical yield chart, plus row feet per acre • CHARTS
- Starting seeds indoors: tips and troubleshooting for healthy seedlings
- Soil blocking: a better way to start seedlings
- Choosing a grow light • PDF tech sheet
- Potting mixes for certified organic growing • ATTRA (PDF)
