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My cucumber plants are doing really well in the greenhouse. When should I pinch out the growing tip please?

I don't bother, I just let them keep on doing what they want, after the reach the top of the cane they just flop over. eventually they will just die off.

Thanks for that Steven. It's just that I noticed that some of the cucumbers are yellowing at the tips so I thought that I had to shorten the growth!