For what it is worth, it seems to me that all of the comments on how to keep
the hive entrance open are appropriate and will work.  Three more ideas that
I have used are:

1. Roundup(tm).  A wonderful herbicide that is as safe as Ivory Snow.  A
thin spray now will keep your entrances grass free for the rest of the
season, with no damage to soil.  I was introduced to this be a commercial
queen breeder who told me not to even worry that getting bees drenched will
hurt them...it will not!  I use it exclusively.
2. Tarpaper.  A lot less expensive and just as effective as carpet.  Get it
at your local Home Depot or roofing supply store.
3. A cup of vinegar to a gallon of water.  Works just like Roundup(tm).

Hope I have helped.

Lloyd
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Lloyd Spear Owner, Ross Rounds, Inc.  The finest in comb honey production.
Visit our web site at http://www.rossrounds.com.