Cos lettuce grow-off followup

A while ago I started a cos lettuce grow-off between three different types of container growing. And how did it work out? Well, all three methods ended up working pretty well, although my possibly biased impression is that the lettuces planted in the soil-less mix (50-50 vermiculite and perlite, left in the pictures) and Mexican style SWC (right in the pictures) both seem to do slightly better than those in he 1 part perlite 4 parts potting soil mix (in the middle). The pictures below were taken about 3 weeks ago. This is not a completely scientific comparison since I have been harvesting lettuce from them.

Cos lettuce from above

Soil-less mixture far left, 20/80 perlite/potting soil in middle, Mexican style SWC far right.

Cos lettuce from the front

Soil-less mixture far left, 20/80 perlite/potting soil in middle, Mexican style SWC far right. Notice the smaller cos plants growing in the windowsill. Even in 7.5 cm pots a number of edible leaves were yielded :)

The trouble with hydroponics

About a week ago the lettuces in the soil-less mix suddenly appeared quite limp and weak. I measured the Ph of the nutrient mix in the reservoir and it was about 4.8. A bit low! For interest sake I checked the Ph in the soily container, and it was similarly low, but the lettuce growing there was fine. After leaching with a good quantity of fresh water, the lettuce in the soil-less container perked up almost immediately.

This demonstrates quite nicely the vicissitudes of 100% soil-less hydroponic growing. You actually need to manage the nutrient Ph etc. actively, since you don’t have the luxury of the soil buffering the Ph for you. I had been watering both the soil-less and soily container using hydroponic nutrient solution without bothering to regulate the Ph in the reservoir, but trouble only arose in the soil-less container. This is not an indictment of hydroponic growing, but simply an observation that if you go hydroponic, you really do need to do it properly :)

Happiness and Joy

I must say that I have been pleasantly surprised by the quantity and quality of the lettuce that I have been able to harvest. I have made several (10+) salads already, and they still seem to be growing well. It seems that Cos lettuce grows even more easily than rocket :)

Neilen Marais / cos lettuce, gardening, grow-off & self-watering

18 November 2010 | 2 comments

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.

Neilen Marais / cos lettuce, gardening, grow-off, plantings & self-watering

31 August 2010