I have been growing brussel sprouts on my allotment for more than 40 years,and indeed for the last 20 years on the same spot.I went to a talk by a secretary of the national vegetable growers society some years ago and he said that he had grown brussels in the same spot for yesrs.He said that the important things are
(i) pot on the seedlings finally into at least a 5" pot so as to get a good rootball
(ii) lime the soil heavily in January
(iii) put a trowelful of lime into the planting hole and plant by end of May
(iv)firm in the plants all round with your heel
(v) water in well
(vi) put some collars round the stem
(vii)put a little Derris around the stem.

Then water until established and then feed with chicken manure pellets or fish,blood and bone just once and earth up around the stems.

Well,I have been following this procedure for many years,and it has been reasonably successful.However,every year and independent of the weather,I always have about 20% of the plants which do not fully grow or succumb to cabbage root fly-very few get club root.In particular I have found it difficult to get reliable results from the cultivar Wellington F1,which in my view produces sprouts with superb nutty flavour.My brussel sprouts have generally been the envy of fellow plotholders who get poor plants for one reason or another.

This year I have tried one more thing on about 30 Wellington F1 plants.At the time of planting and after carrying out the above procedure,I have placed an upturned 8" pot with the bottom cut off (rather like a smaller version of the type of ring used to plant greenhouse tomatoes,but empty in this case).
The theory behind it is to stop wind rock on young brussel seedlings,and so minimising the chance of a hole developing for cabbage root fly to get in and lay its eggs-also it provides a slightly warmer environments for the seedlings to get established,although the former is in my view the most important.

Well the results have been almost unbelievable.Every,and I mean every,plant is thriving and they are all of almost the same height.The plants are now 3-4 feet tall,and the rings seem to provide some support- I usually stake them by now.
Tiny sprouts are beginning to form and I am hopeful of a magnificent crop.
Some say it might be the season,but many of our ploholders are having the traditional problems as usual.So I am pretty convinced that the open rings-which is the only different thing I have done-are the reason for these very healthy plants.

Michael