Tired of paying for crappy Jiffy Pots for your seedlings? I was! So, last Spring I decided to make my own starter pots for them.
Step 1: Materials
For each 12"x18" tray (3" deep), you will need:
* a 12"x18" cardboard box (or a piece of cardboard 18"x24", to make the tray)
* duct tape
* half of a garbage bag (at least 18"x24" in size)
* 24 coffee filters (I bought unbleached, but it really doesn't matter much)
* potting soil. You will go through a surprising amount of this. Hypothetically, you will use 0.4 cu. ft. of soil per tray, but they usually sell it by weight, not volume (which means they sell it as wet as possible, to rip you off!).
Some scraps from the box will also be used to make the two positioning guides, which can be reused from tray to tray.
Step 2: Tools
Step 3: Make the Tray
Cut off the bottom 3" of the box, or, if you don't have a box the right size, build a 3"-tall box from a single piece of cardboard. This box will be exposed to a lot of water through the seed-starting season. If you are making it from scratch, glue the sides & corners together, instead of just relying on duct tape.
The tray will need to retain water, so cut a trash bag in half along the seams (making two single-ply pieces of plastic, the same size as the original bag). The bag will need to be at least 18"x24" to protect a 12"x18", 3"-deep tray. Lay the plastic inside, and duct-tape the edges to the edges of the tray (so the duct tape overlaps on the outside of the tray). It doesn't matter how you handle the corners, nor is it really crucial that the plastic is cut exactly to fit (as long as it is big enough). Excess plastic won't be a problem.
Halfway through the seedling-growing season, I realized my trays were retaining too much water, so you might want to take a nail and poke a half-dozen or more holes across the bottom, to allow excess water to seep out. This will mean the cardboard will get wet, but as long as it's supported on shelving, this isn't a big problem.
Step 4: Make the Placement Guides
You will need two of these guides, which help keep the seedling "pots" in place as you fill the tray. These are reused as you fill all the trays you make, so you only need two.
The guides are shaped like the inner, removable partition on metal ice cube trays, if you're old enough to remember those. However, the cross-tabs only extend halfway into the seedling "pots" - for my 3" pots, the tabs stick out 1-1/2" on each side.
Cut two 3"x18" strips, and ten 3"x3" squares, from cardboard. Coat at least one side of all of the two big strips of cardboard with duct tape, to help it resist moisture from the damp soil.
Cut thin slots 1-1/2" high into the midpoints of each the ten squares. Cut five thin slots, 1-1/2" high and spaced 3" apart, into one of the 18" sides of the big strips. You don't need to carefully measure these slot sizes; a little under- or over-sizing won't affect the function.
Interlock the squares into the big strips, and use strips of duct tape to fix them in place.
Step 5: Start Filling the Tray (1st Row)
Place one of the guides into the tray, so that the long strip is about 3" from the edge of the tray.
Open the bag of potting soil by cutting the entire top off. It will be much easier to work with if you can reach in easily. Put the bag on a table or chair, so its top is about belly-high.
Filling the coffee-filter "pots", and filling the trays with them, is only slightly tricky, thanks to the guides we've built. Put one coffee filter in your hand, cradling it with your fingers. Using your cup, scoop out about one heaping cup of soil, and dump it into the filter. Place the filled filter pot into the corner of the tray. The filter sides are now supported on two sides by the tray walls, and on the other two sides by the placement guide. The cross tabs on the guide only stick out half the width of the pot, but that's enough to hold it up.
Step 6: Filling the Next Row
The first row probably pushed the placement guide out away from the tray side a bit while you filled all the positions with pots. That's OK; just take a moment now to gently push the guide back into place, so that the row takes up roughly 1/4 of the tray width. Again, you don't need to measure, or worry about being super-neat here. Over the growing season, watering & plant growth will have their way with the pot shapes, anyway.
Now add the second placement guide, so its cross tabs just meet the tabs on the outside of the first guide. This is why the tabs only extend halfway - so that when both are in place, the entire side of each pot is supported. You could probably make them full-sized, and overlap them, but it's really not critical to have that much support.
Just like with the first row, fill all six pot positions in this row. When done, gently push the second placement guide until it lays along the center of the tray. Now, pull out the first placement guide, leaving both rows of pots touching each other, and move it so that it just touches the outside of the second guide. You will now have formed the pot positions for the third and fourth rows.
Filling them is pretty easy, at this point. When done, remove both guides.
Step 7: Some Final Thoughts
As I said earlier, you really should poke some drainage holes in the box. The seeping water may weaken the cardboard a bit, but as long as it is resting on elevated shelving (that lets the water flow through, like wire mesh shelving), it should stay dry enough to be transportable to your garden when the seedlings are ready. However, it won't be very strong, and you will need to support its bottom during transport.
There's plenty of soil in these pots. I grew some very healthy 6-8" tall seedlings (but forgot to photograph them, darn it!). The paper will break down easily in the garden. I don't know if the unbleached filters break down easier or not, but that was my hope (also, there's no need to add more bleaching chemicals to our water supply, just to make seed pots).
Have fun planting!