There are four widely-grown rockery plants which, if left to their own trailing habit, will quickly take over and even smother others around them. These are aubretia, alyssum, London Pride and Snow in Summer, all of which have finished flowering and now need attention.
By cutting them back the plants will remain compact, have improved flowering and, more importantly, kept within bounds. If left untouched they become leggy and untidy.
The purple-flowering aubretia is added to many rockeries and benefits from being cut back.
The rockery form and perennial alyssum is not to be confused with the annual white form used in summer displays. A. saxatile provides a splash of yellow in the spring rockery and, like aubretia, will become leggy quite quickly if not dealt with using secateurs.
London Pride to many people, saxifraga its proper name, is so easy to grow it becomes invasive. Its rosette-like habit quickly takes over, smothering nearby other plants. The masses of small starry flowers are quite attractive but not so once they have faded and are best removed. Taking out any unwanted growth is easily done merely by pulling it out.
Arguably the most invasive of all rockery plants but nevertheless still very popular, surely has to be cerastium, which most people know as Snow in Summer and is either liked or loathed. This silvery-leaved rockery plant produces masses of pure white flowers, hence its common name. That’s the best part, its downside is it will take over very quickly and, if left untouched, will cover ground at a tremendous rate to the detriment of other plants. At this time of year the foliage can be cut down to ground level and it will soon produce new compact growth.
n Penstemons are available now on containers and in flower. This plant produces flowers spikes of similar form to antirrhinums, only the individual tubular heads are larger. Penstemons will not survive the winter, so either treat them as an annual and expect to discard them at the end of the season, or taking cuttings now and keep them over the winter in a frost-free greenhouse. The former is probably the better option.