I have tried to grow various lettuce plants over the past 6 to 9 months and the funny thing is that some volunteer lettuce plants have done best growing in of all things the crushed rock of my driveway.
Last spring we had two nice lettuce plants grow on their own in a 10" plastic pot that must have seeded somehow from the prior year. I tried to grow some other types of lettuce in containers and had some limited success, but most of them seemed to bolt rather quickly. The two volunteers gifted us with a good bit of greens and then went to seed. The pot somewhere in this process got moved to the edge of the driveway. Several months later a number of lettuce plants stated sprouting out of the well compacted crushed rock several inches thick. I didn't notice them at first, but when I did I transplanted some of them and they have grown, but they also are beginning to bolt on me. Ones that I never got around to carefully prying out of the rock bed did so well that my wife put some protection over them. Unfortunately this seemed to immediately bring them to the attention of our local ground hog who demolished them in one afternoon. One plant that came up later survived a 15 degree freeze in the rock bed before I noticed it and transplanted it into my moveable beds that go into the garage at night. It is way healthier and very compact producing nice leaves for greens.
Since the lettuce seems to germinate well in the rocks and seems to grow well in it, I am seriously wondering if I am using the wrong grow medium for lettuce. I am thinking of using it to try and start lettuce plants under lights and see what happens.
Lettuce seeds need to be chilled off in order to germinate, perhaps the escapees are achieving that degree of cold, then finding a microclimate that suits – the gravel keeps the roots warm and moist as a mulch, and the bits of debris that collect around the gravel over time probably act like compost. The gravel would also provide the plants with micronutrients from the inevitable rock dust. Add to that good drainage.
So, mimicking those attributes in the formal vegie garden or pot plants should be successful – water with some well diluted fish or seaweed liquid fertilizer and the plants should just about grow legs and walk themselves to the kitchen table.
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The broadest thing to say about lettuce seeds is that they like disturbed soil and are sensitive to bolting from heat/stress/transplanting.
Bolting is also a response to a medium with insufficient nutrients (even though its drainage qualities might be good for encouraging the seed to germinate).
Disturbed soil means they'll often sprout when you give up on some seeds and empty out the container. (Simulating wild conditions of erosion or animal disturbance)
I've grown about 15 varieties of lettuce and tried to get them all to grow through the Australian summer heat.
There's only one I found which grows year round, is indifferent to heat, incapable of bolting and resistant to transplant-shock.
It's the 'Tree Lettuce' - middling taste, difficult to source, but productive and worth hunting down.
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