How to grow Nepeta
• Species of Nepeta, such as N. racemosa, N. x faassenii and N. grandiflora and the garden hybrids that come from them, seem to thrive on neglect and are best grown in nutrient-poor, light soils. Avoid overfeeding or overwatering them, or else their soft, silky foliage will be too much for the thin, square stems to bear and they will flop. This is often the case when they are grown on richer loam or clay soils. They need full sun and a welldrained soil to be at their best. Drainage is particularly important in winter, as they can succumb to too much wet.
• A hard cut-back straight after flowering is often necessary to keep plants from opening up in the middle. On dry silt or sandy soil it is best to just deadhead, otherwise the plant may struggle to reflower. During the RHS trial, each of the three plants of each cultivar being grown would be pruned differently: one would be left untouched, one would be deadheaded and one would be cut to the ground after flowering. It was noted that many of the N. racemosa cultivars often opened up in the centre leaving a skirt of flowers at the edge, which was infilled with new foliage and flowers without any pruning, covering the old flowers at the edge in so doing – making it useful for low-maintenance planting schemes.
• Clump-forming species need dividing regularly, leaving the clumps no longer than three years for best results and to keep the group vigorous and healthy. You can take stem-tip cuttings in early spring, or from fresh growth after the summer cut-back. Sow fresh seed in spring.
• N. nuda and its cultivars seem content on almost any soil pH. There is no need to cut them back after flowering as they do not reflower. Stem-tip cuttings of cultivar types in early spring is the best way to increase numbers. Seed is viable and easy from spring surface-sowing in a seed tray, although there is some variability in the quality of the seed-raised offspring.
• N. subsessilis from Japan and N. cyanea from the Caucasus form a handful of exceptions in the Nepeta tribe, preferring cool, damp spots in partial shade, although I have noticed over the years that they can cope with sun if they have a moist soil. Their serrated, green leaves turn autumnal hues of orange and yellow, while the seedheads of these species are worth leaving for extra interest in winter. I have had great success with cuttings taken from them in spring while the stems are short and before they hollow out in the centre in summer.