Have you installed gutter guard?
How long has it been since you looked under it?
You may be surprised at what is growing in your gutters. The tell tale signs of grass growing out the top is a result of wind and birds dropping seeds on the guard and it falls through the holes, only to find a lovely growing ground in the silt that collects in your gutter.
Or you may have a situation where moss has collected and spread through out your gutter. Gutter guard can provide the perfect climate for moss, a dark and moist space. If you reside under a canopy of trees then, in winter especially, you may not see much sun and again it is a perfect environment for moss.
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