Cos lettuce grow-off

Some time ago, I saved seeds from cos lettuce plants that had bolted. Back then, they did not grow so well, but they did produce a heck of a lot of seed. I decided to do a bit of a grow-off by different methods. The first two contenders are 7l self watering containers, the one filled with 1 part perlite to 4 parts Culterra potting soil, the other a 50/50 mixture of perlite and vermiculite. The third is a low-budget kinda-SWC a la Mexico.

Why the grow-off?

The 1 part perlite (or sometimes with 1 part vermiculite also) with 1 part potting soil mixture seems to be the de-facto standard for SWCs. I find it somewhat unappealing since the potting soil is good for a max of 2 years, or usually just one growing season. The (somewhat expensive) perlite that is mixed with the soil is also discarded, since there is no easy way of recovering it. The purely soil-less vermiculite/perlite mixture is very inert, and can actually be sterilised and re-used many times. The Mexican style SWC really only uses organic waste material that is available for free, so that seems quite attractive too. Seeing which one yields the best crop is of interest too!

Special needs

The vermiculite/perlite mix is completely inert, hence no normal soil mechanisms. This means that hydroponic nutrients need to be used. They are surprisingly cheap from the gro-shop. They are also the cheapest place for perlite that I have found so far. For the Mexican style container, you need to save some urine! Well, you could use other fertilisers, but I thought, you know, what the hell :)

So far

I germinated about 20 seeds, and then selected the best seedlings. I planted three in each of the 7l SWCs, and 5 in the somewhat bigger Mexican style SWC soon after the seedlings formed their first pair of true leaves. Since the Culterra potting soil includes a slow release fertiliser, I decided not to add nutrients to the soil/perlite SWC to start with. I used maple leaves from my back yard to fill the bottom part of the Mexican SWC. Since I also used the Culterra soil for the Mexican style SWC I have not added any additional urine yet. I say additional since I did use some urine for the initial wetting of the maple leaves before I added the soil on top. The nitrogen in the urine is useful for starting the decomposition of the leaves that should be going on below the soil.

All three Cosses in a row
Cos lettuce in 7l SWC with 1 part perlite to 4 parts potting soil mix
Cos lettuce in 7l SWC with 50/50 perlite and vermiculite mix
Cos lettuce in Mexican style SWC

About 60% of the volume is dead leaves, the rest is filled with potting soil. Aparently the leaves below should turn into suitable soil to use next year, so I should not need the potting soil again.

In the pictures I tried to take the close-ups of all three containers from about the same distance. Since the Mexican SWC is somewhat larger than the other two, it does not quite fit in the frame. So far (about 3 weeks after planting out the seedlings) the Mexican container and the soil/perlite container seems to be doing somewhat better than the vermiculite/perlite container. All the seedlings look quite healthy though. I think the slower growth in the vermiculite/perlite SWC may be because I started out with a very weak hydroponic nutrient mix, while the seedlings in the other two containers benefit from the included fertiliser in the potting soil. I’ve recently put a stronger nutrient in, so we’ll see if that evens things out a bit.

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