I use Phacelia, the great thing about like mustard is the height, if you live in an exposed area, a late sowing of Phacelia gives you a nice green carpet. The following year you can then dig it in or leave patches to flower. It's a great green manure because it acts in two ways, locks in the nutrients over winter and stops the heavy rain washing them away and when it flowers the following year it brings in bees, hoverflies and even dragonflies! Tried rye grass but the wind just flattens it around here and you spend more time trying to support the stuff.

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Best wishes
Andrewo
Harbinger of Rhubarb tales