It seems like a small question, but how many seeds you drop into each cell of a seed tray makes a real difference in how your transplants turn out. Plant too few and a dud seed leaves you with an empty cell and a gap in the garden. Plant too many and you get a crowded tangle of weak seedlings competing for light and root space. There is a simple logic to getting it right, and it changes a bit depending on the crop.
The logic is the same anywhere, but it matters a little more in Montana because our season is short. An empty cell or a weak start is hard to make up for when there is little time to sow again. A strong, well-spaced transplant gets your garden off to the head start it needs, while a crowded, leggy one sets you back before you have even planted out.
A Common Approach: Sow Two or Three, Keep One
Many gardeners sow two or three seeds per cell and then thin to the single strongest seedling. This is insurance. Not every seed germinates, so planting a few per cell makes it more likely you end up with a healthy plant in each one rather than empty gaps. Whether you need that insurance depends on the seed’s age and germination rate, what the seed costs, the crop, and the packet directions. University of Minnesota Extension suggests sowing fresh seed one per container and saving the two-or-more approach for older seed with lower germination. Once the seedlings are up and you can see which is strongest, you remove the others and let the best one have the cell to itself.
This works for tomatoes, peppers, and many other crops you start indoors, especially when the seed is older or its germination rate is modest. If you do sow extras, thin them promptly. Leaving all three to grow gives you three weak, crowded plants instead of one strong one, and crowded seedlings stretch for light and develop poor roots.
Big, Reliable Seeds: One Per Cell
Large seeds that germinate dependably do not need the insurance of extra seeds. Beans, squash, cucumbers, and other big-seeded crops sprout reliably, so one seed per cell is usually enough. These seeds are also bigger and more expensive by count, so there is no reason to waste them sowing extras you will only pull out.
If you are worried about germination, you can sow two and thin to one, but with fresh, good-quality large seed, single sowing works fine and saves seed. These crops are also often direct sown rather than started in cells in the first place, which sidesteps the question entirely.
Tiny or Old Seeds: Sow More
At the other end, very small seeds and older seed call for sowing more per cell. Tiny seeds like those of some herbs and greens are hard to place one at a time and do not all come up, so a small pinch per cell is common, thinned later to the strongest one or two. The same goes for any seed you suspect is past its prime. If a packet is a few years old and germination is likely lower, sow extra to make sure each cell produces a plant. With tiny seeds especially, practice varies by crop, so the packet directions are worth a look.
When in doubt about old seed, a quick germination test before you sow tells you how thickly to plant. If only half the seeds sprout in a test, you would need to double up in each cell to land a healthy seedling, and fresh seed is often the better bet.
How to Thin Without Hurting the Keeper
Thinning trips people up because pulling extra seedlings can disturb the roots of the one you want to keep. The trick is not to pull at all. Once the seedlings have their first true leaves and you can see which is strongest, snip the others off at the soil line with small scissors. This kills the unwanted seedlings without tearing up the roots of the keeper, which stay undisturbed.
It can feel wasteful to cut off perfectly good seedlings, but giving one plant the full cell makes a far better transplant than letting several fight it out. A strong, single seedling with room to develop roots is exactly what you want heading into our short season.
Get the seeds-per-cell right and thin without delay, and you set yourself up with sturdy transplants ready to make the most of every warm day we get. For when to start each crop, see the seed-starting schedule for Missoula gardens.
Sources
- University of Minnesota Extension, Starting Seeds Indoors
- University of Illinois Extension, Will My Seeds Still Grow?