Cutting a hedge freehand often results in a line that wanders slightly from one end to the other, even with a steady hand and plenty of care, simply because it is difficult to judge a perfectly straight edge by eye alone over any real distance. A length of string, pulled taut between two posts or canes at the desired height, gives a clear, reliable guide to follow, making a straight, even finish far easier to achieve than cutting freehand.
Setting up the guide properly
Push a sturdy cane or post into the ground at each end of the section to be trimmed, checking that both stand firm and upright before tying the string between them. Use a spirit level, or simply step back and check by eye from a distance, to make sure the string sits level along its whole length, since any slope at this stage will carry through into the finished hedge line. Pull the string taut enough that it does not sag in the middle, particularly over a longer stretch of hedge.
With the guide in place, trim just below the line of the string, working in smooth, even strokes along the hedge rather than jabbing at it from different angles. Step back regularly to check progress against the string, adjusting your angle slightly if one section is running higher or lower than the rest. Moving the string along in sections for a longer hedge keeps the guide accurate throughout the whole length, rather than trying to judge distant sections without one.
Once the top has been levelled off using the string, the sides can be tackled next, often more easily judged by eye once the top line is already crisp and even. A hedge with a level top generally looks tidy even if the sides are slightly less than perfect, so it is worth prioritising this step above all else.
With a taut string as a simple guide, even a fairly long hedge can be trimmed to a straight, professional-looking finish without any specialist equipment. It is a small extra step that makes a noticeable difference to the finished result.
