Catnip is easy to grow and slightly too easy to keep. Give it sun and indifferent soil and it will thrive. Give it a garden bed with no barrier and it will be back next year in places you did not plant it.
The plant is Nepeta cataria, a hardy perennial. The oil you want, nepetalactone, concentrates in the leaves and flowering tops. Everything about siting, feeding, and harvest is in service of that oil, not of a handsome shrub. If you want a handsome shrub, you want catmint.
Light, soil, and why rich compost is a mistake
Full sun, at least six hours. Shade makes lanky stems and weaker-smelling leaves. It is hardy in USDA zones 3 to 9, which covers most temperate gardens.
Drainage matters more than fertility. Sandy or loamy soil, pH somewhere around 6.0 to 7.5. Poor-to-average ground is better than a well-fed bed. Fertiliser pushes leaf at the expense of oil, so you get a bigger plant that smells of less.
Water young plants until they are established. After that, wait until the top couple of inches of soil are dry. It is drought-tolerant once it has a root, and wet feet are how you get mildew.
Put it in a pot if you can
Catnip spreads by seed and by underground runners, the same way mint does. A single plant in open ground can own a bed in two seasons.
A pot at least 30cm deep is the simplest answer. The taproot needs the depth, and the rim is a barrier the runners cannot cross. If you must plant in the ground, sink a bottomless bucket or a root barrier, and deadhead the flower spikes before they set seed.
Space plants 45 to 60cm apart if you have more than one, for airflow. Pinch the centre stems once the plant is about 15cm tall, so it bushes out rather than going up as a single pole.
Start from seed indoors eight weeks before the last frost, or sow direct once the soil has warmed. Nursery plants skip that wait.
When to cut it
Harvest at the start of flowering, usually mid-summer. That is when the oil is highest. Late morning, after the dew has gone and before the afternoon heat starts driving the scent off.
Cut the stems about 15cm above the ground. The plant will recover, and you often get a second cut in early autumn. Hang the bunches upside down in the dark, with air moving around them. Sunlight bleaches the leaves and destroys the nepetalactone. You want green, not brown.
Store the dried leaf in a jar, in a cupboard. The same rules as a toy: air, heat, and light are what kill it.
If neighbourhood cats find the young plants, a wire cage until they are established is not overkill. A 10cm seedling does not survive a dedicated rolling session.
What to do with the harvest
Loose dried catnip is for pinches, for rubbing into a cardboard scratcher, and for the jar you keep a toy in. A cotton toy left in a jar with a spoonful of your own herb for a few days will take the oil up, which is the cheapest way to revive one that has gone quiet.
Growing it does not replace a well-filled toy. Homegrown leaf is only as good as the drying and the storage, and a pinch on the carpet is not the same as 30g inside something the cat can actually kick. If you want the filled version without the gardening, that is what Le Croissant is for: organic catnip buds and Silver Vine fruit, at least 30g, in reinforced OEKO-TEX cotton, hand-stitched in the South of France. Use the garden for topping up and for the windowsill. Use a proper toy for the session.