Wildlife charity the RSPB is appealing for people not to cut their hedges until the end of the month.
The charity says nesting birds are at risk if people get the hedge trimmers out before September.
The unpredictable weather this year is expected to have delayed the breeding season, making the problem worse.
An RSPB representative said: “A dry day and an unruly hedge can be enough to tempt even reluctant gardeners to get the shears out.
“However, the RSPB is urging residents of West Sussex not to be tempted into cutting their hedges too soon, as they may disturb nesting birds and their young.
“Warmer and drier August weather means people are more likely to get out and tackle any overgrown hedges.
“But, the RSPB recommends avoiding cutting hedges from March to late August in order to allow nesting garden birds to breed safely.
“It’s especially important this year as the wet and cold weather in the early part of the year may mean that some species have bred later than normal and are still using their nests and feeding their young.”
Samantha Stokes of RSPB South East, said: “The humble hedge is a place of safety for many birds, especially in urban areas, as they may be the only suitable nesting places for birds.
“Although it’s tempting to trim up hedges at this time of year, our advice is not to.
“It’s against the law to intentionally damage an active nest or prevent parent birds access to their nests, and while most of us would never dream of causing intentional damage to a family of birds, an innocent over-keenness to keep a tidy garden can be harmful too.
“We’re eager to make people aware of the risks.
“By early September most young birds will have fledged, so it should be safe to start trimming. It’s been a particularly tough breeding season with unseasonable cold and wet weather, so birds need all the help they can from us.
“In this case, the best way to help is to do nothing at all, just get into a holiday mood by relaxing and enjoying the wildlife in the garden.”