Back when I was side-dressing sulphur a couple of weeks ago, I noticed that the embankments of soil I made along my rows were giving rise to quite extreme temperature gradients. The south facing slope of each ridge was heating and drying out rapidly from the sun, whereas the other side remained reasonably cool and damp (far from soggy, though). As one might expect, root development seemed much better in the northerly direction, both in terms of extent and density.
I was thinking mulch might help but the earthworks would've made that a bit tricky. Then I had a brainwave and thought of using strips of light coloured fabric to shade some of the ground and reflect solar energy away without trapping moisture. So I rooted out some spare canvas (light-weight calico) and an old bed sheet that never gets used and put the plan into action.
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Time will tell if/how the plants actually respond but, just going by how it feels now beneath the fabric, it seems to be working as expected, if not better. Looks kinda' cool too, though I doubt those sheets will stay such a brilliant white for very long.
Whether or not this has any bearing on the polyphylla/frenching I've observed, I'm not sure. They say it's commonly associated soil that's wet, poorly ventilated and rather warm. All three of those conditions are present here and there but rarely all together in one place, at least not for long periods, so I dunno'. That does maybe strike a chord with the cases of frenching associated with root intrusion, come to think of it. Either case presents the possibility of plants getting confused by mixed signals from different parts of the root system. Given that my plants are growing vigorously enough in all other respects, it may well be that they have plenty of decent soil available to support healthy growth but unfavorable regions, or just variations in general, are enough to upset them.
EDIT: Oh, I forgot to mention that most of my Little Dutch have started budding now and the Basma look (and smell) like they're getting close.