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David, I'd think some overhead protection from wet ( maybe in summer as well as winter!!!) would be a good idea. Something along the lines of Ian's favoured method of squares of weldmesh with a perspex lid would be a good answer. These are easily made, don't blow around and can be quickly deployed in grotty weather.Will have a look for a link to what they look like, for anyone who doesn't know what I'm talking about.....
Here's a link to a Bulb Log with the "rain hats" : http://www.srgc.org.uk/logs/logdir/2010Aug041280927654BULB_LOG__31comp.pdf -they can be made any size, from single small plant size to large cushion or whole trough cover - very versatile.