A Different Sort of Seeding

I opted to change how I planted the seeds this time. Instead of just broadcasting the seeds as I did last time, instead, I scraped the gravel back to the “wet zone”. The wet zone is the high-water mark of the growbed during the flood part of the flood-and-ebb cycle. It’s about 15 mm (0.6 in) below the surface of the pea gravel.

The reason we don’t flood the bed all the way up to the surface is to keep blown-in weed seeds from getting started, and to prevent mildew and / or algae from starting. None of that is healthy or welcome for an aquaponics farmer.

There seems to be a split decision in the Internet aquaponics community about which way to go, if you are germinating seeds right in the growbed. Some folks will tell you the right answer is start them in rockwool cubes out of the grow bed, like seedlings destined for a more conventional soil garden. Once the roots are well developed and the stalks are 2 to 3 cm (0.8 to 1.2 in) tall, then transplant them into the grow beds.

There is one particular reason to go this route, beyond the obvious; planting density. In aquaponics system, seedlings are often spaced much more densely than in dirt. Often the spacing will be 1/2 to 1/3 the normal expected. You can do that in aquaponics because of the nutrient and moisture availability. This shapes the root zone to be deeper than and narrower than in soil gardens. As a result, the grow beds will just support a higher density than soil.

So, if you’re working with sprouted seedlings, you can place them to get the density and distribution in the grow media that you want. With broadcast seeding, or even drop-in-spot seeding, you aren’t going to get that. The seeds will move with the ebb and flow, ambient vibrations, or simply bounce off the gravel and land not-quite-there to start with.

On the other hand, it’s extra infrastructure, cost, and process to germinate to seedlings, verses putting seeds into the grow beds themselves to germinate. So, I started out experimenting with this approach. As I said, the first time I planted, I just went with broadcast seeding. I made a little screen to keep the seeds within about an inch of where I wanted them and sprinkled merrily. I wetted the surface of the grow beds daily until I saw seedlings sprout up and then trusted that they were as far down as up and could collect moisture themselves.

In point of fact, it worked really well. Almost everything I put in germinated and started to grow before the pepper plants overtook everything. So, that’s a viable approach. This time, as I said, I tried the other approach.

The results were less than stellar. With the seeds carefully placed at the wet zone and then re-covered, I’ve had about a 10% success rate. I know the biology in the grow bed is OK, because the sage and basil that I planted the week before are growing well, the lone stalk of maligned celery has become a bush, and a couple bits of lettuce and one kale have germinated.

The culprit, I suspect, is too much moisture. The grow beds are about 2/3 the depth of what most aquaponics gardeners build to. I also run the beds “fast”; a five minute cycle time, instead of the 15 or so minutes that “bigger” set ups run on. For an established plant, this is marvellous. For a seedling, I suspect that it’s a deluge.

That’s it for this post! As always, comments and suggestions are welcome!