A few weeks ago, we built a device that allows us to partially mechanize seeding flats in the greenhouse. Here it is under construction:

I am drilling holes through a piece of thick plexiglass mounted at the midpoint inside a rectangular wooden frame. These holes are sized for a particular type of seed, and arranged in the same spacing as the cells in a plug flat, which we use to produce vegetable transplants. Next, I attached a piece of plywood to seal off the bottom, creating a chamber underneath the plexiglass. After drilling a circular hole in the side of that chamber, I'm able to connect it to a vacuum cleaner and apply vacuum inside that chamber, resulting in suction on each of those holes drilled in the plexiglass.

So here's how it works: turn on the vacuum cleaner, pour a bunch of seeds onto the surface of the plexiglass, and swirl it around so the seeds slide all over. Each one of the holes will grab a single seed. Then, with the vacuum still on, pour off the excess seed into a container, and flip the box over onto an unseeded flat with dibbles (the technical term-honestly-for depressions in the potting mix to hold a seed) made in each cell. Then turn off the vacuum and the seeds will fall out, one in each cell of the flat.

Here are a few photos of the process. In this case, I'm using pelleted onion seeds, meaning the seeds are embedded in a clay coating, forming little BBs. This makes them easier to handle and more uniform. We plant onions in clusters of 2 to 4 plants, depending on the variety.

Vacuum applied, one seed stuck to each hole.

The vacuum seeder in front, and the flat waiting to be seeded behind.

This flat already has two seeds in each cell.

The seeder is turned over onto the flat. Vacuum is then turned off, and the seeds drop.

Ta da! Now there are three seeds in each cell.

We have a separate seeder to use with smaller seeds, like broccoli, cabbage, and kale seeds. This tool doesn't work as well with irregularly shaped seeds, such as pepper or tomato seeds, since they're more likely to have multiple seeds get stuck to each hole. But we are happy to use any tool that make tedious tasks, like seeding flats, a little bit faster.