Propagate from splitting or they easily take from softwood cuttings in summer. Some cultivars are sterile, with no viable seed. Those with viable seed germinate easily indoors from seed at around 20°C (68°F), press into the compost but do not cover. Germination time is 14 to 45 days. Alternatively, sow outside after the last frosts.
Preferring poorer soil, they are best planted in full sun to partial shade, favouring well drained ground. If planted in rich soil or given too much fertilizer they are likely to grow floppy foliage without structure. Water well until established and some gardening writers also suggest pinching out the tops of young plants to encourage bushier growth, although I have never found that necessary.
It is recommended to divide in spring every few years to maintain plant quality. Supports are often recommended, especially for taller and/or more mature plants as on reaching a certain height, the growth tends to fall away from the centre of the plant, but I prefer to leave them to billow over naturally. Some recommend cutting back by a third after initial flowering to encourage a second late summer flowering. Cutting all the plant back tends to leave it looking sad for the height of the season, whereas just deadheading or pruning out the main spent flowering stems retains structure. This can also prevent re-seeding. They can occasionally be prone to powdery mildew, especially if they become overcrowded. Whilst many gardeners may prune back in Autumn, I choose to cut back the dead growth in the spring in order to help protect the plants over winter and to support wildlife.
You may wish to protect young plants with spiky leafed shrub prunings, or an upturned empty hanging basket or similar structure to protect against some but not all cats, who may be attracted to a number of nepeta varieties, as well as catnip itself (Nepeta cataria). Placing twigs into the centre of the mound of mature plants will help in the same way. Gerard in his Herbal (1633) wrote ‘Cats are very much delighted herewith for the smell of it is so pleasant to them that they rub themselves upon it and wallow and tumble in it, and also feed upon the branches and leaves very greedily’. The attractant is a constituent of the plant’s oil called nepetalactone, causing them to both roll around on the plants, eat them and generally behave quite crazily before sometimes falling asleep. A plan that can work is to grow an area of catnip out of the way for the cats so they may leave the other varieties alone. Dried catmint leaves stuffed into fabric cat toys or rubbed over them should keep cats happy for ages.