Posted: 29/07/2014 at 20:55
You need to evaluate your plants.....basically many late flowering perennials benefit from being cut back......sedums and heleniums are good examples. But timing is important....too late and growth will be poor. Done early June and strong sturdy, albeit later flowers, will result. However, asters like frikartii Monch, rudbekias, anemones, dahlias and phlox then, no. Plants like argyranthemums, dianthus, campanulas, violas, etc., benefit from late "Chelsea chop" being sheared back right now to re energise flowering from early/mid August through to autumn.
Have fun though .....it may seem a shame to hack floriferous cosmos back but lightly sheared of all buds will quickly produce masses of buds quickly.