The trefoil with it's nitrogen fixing properties is benefiting the perennial broccoli which I planted amongst it. I'd like to say that but actually the truth is it was a happy accident. I sowed the trefoil first as a green manure crop and as it didn't appear I thought it had failed to germinate. So I planted the broccoli there instead, which of course is when the trefoil sprang into life. I can't say for sure if the trefoil is benefiting the broccoli as I haven't got a control crop to compare it with but it doesn't seem to be doing it any harm.