By Julia Frisbie
posted March 1, 2022
I used to start heat-loving seeds on my kitchen counter in soil blocks under shop lights, but when covid hit, that same countertop became my husband’s background for zoom church. He’s the pastor, so we can’t have him backlit. My seed-starting habit needed a new location.
We didn’t have room for a greenhouse, so we built an outdoor heated propagation bench instead. Think of it like a tabletop greenhouse. I got the idea from my market gardener friends Tony and JP of Green Heart Gardens in Portland, Oregon, who start all the seeds for their CSA in a similar setup. It works great for them and for me. If you’ve outgrown your indoor grow-light setup, but can’t commit to a greenhouse, it might work for you, too.
STEP BY STEP INSTRUCTIONS
- Measure your seedling trays and decide how big of a table you need! Design it to fit the dimensions of the trays you plan to use. For example, my 3 by 4 foot tabletop can accommodate up to six standard 10” x 20” trays at a time. Tony and JP built a much bigger table because they needed to start way more plants at a time.
- Build a table that’s strong enough to hold hundreds of pounds of weight forever. Tony and JP used a sheet of plywood set on top of concrete blocks. I put hefty legs on a shipping pallet.
- Build up the edges of the table 4-6 inches, so that the top of the table becomes a shallow box like a raised bed. This shallow box will eventually contain all the following stuff:
- Cut a piece of rigid foam insulation to the exact dimensions of the inside of your tabletop box, remove about one square inch from each of the corners to allow for drainage, and cram it in there. This keeps the heat where the plants can use it.
- Staple a heavy-duty plastic liner to the inside of the box so that moisture doesn’t sit directly on the foam, and then cut drainage holes through the plastic in all four corners of the table (directly over the cutaway parts of the foam) so that water can escape out the corners of the table and onto the ground.
- Put pea gravel into the lined box, 1-2 inches deep. It will act as a heat sink. Tony and JP used sand instead of gravel at first, but it was too dense and held too much water; their seedlings’ roots grew straight through the bottoms of the trays and into the sand. Now they use gravel, because it holds heat but not water, which encourages plant roots to air-prune themselves instead. That gets them off to a better start when it’s time to transplant.
- Cut a piece of hardware cloth to the dimensions of the inside of the box. Zip tie a heating cable to it (following the spacing instructions that came with the cable) so that the entire area will be heated.There’s a sweet spot for germinating tender annuals right around 77 degrees; most horticultural heating cables are pre-set to maintain this temperature as long as their heating probe is positioned correctly. Lay the hardware cloth and heating cable into the box, and zip tie the heating probe into place.
- Put another layer of pea gravel on top of the cables, 1-2 inches deep. This will increase the thermal mass of the growing area, and allow the radiant heat from the cables to be evenly distributed to the entire tabletop.
- Attach flexible PVC hoops to the outside of the tabletop.
- Optional: zip tie irrigation tubing to the underside of the hoops. Place emitters where they will create a fine mist over the entire tabletop. If you’ll be home for the entire seed-starting season and you prefer to water by hand, you don’t need to bother with this. (I like to water my seedlings from the bottom by dipping them into trays of dirty duck water and letting them wick it up.)
- Clip or tie clear plastic over the hoops. BE CAREFUL to vent this plastic cover on sunny days, or else your plants will cook. Later in the season you might even leave the ends open all the time, or switch it out for a fabric frost blanket.
- Plug in the heating cable, stick a timer on the irrigation tubing, and you’re ready to start your seeds!
Stacking functions: I also use my propagation table to dry herbs once the weather warms up. Here are some nettles for tea, alongside trays of cucumbers, tomatoes, and squash. In summer and fall, I unplug the heating cable, but I still use this hot/dry spot to dry down seed pods.