Raspeberry Pi are now doing an IR version of their camera board for about £20. You could easily set up a night vision motion detection system and catch the thief. You’d need an IR light source though.
Yes, those are ridiculously tiddly cauliflower and the broccoli probably come in at about £5 each, but that allotment has been a fantastic place to deal with a pretty below-par year, and I have grown my own veg!