I planted one 'Walker's Low' clump in my zone 5 front border, right under the kitchen window. I see it every morning over coffee. Six years on, it has grown into a soft blue cushion about three feet wide. It has never once jumped its spot.
So the honest answer depends on which plant you grow. The catmint spreading habit is slow and tidy for ornamental types. True catnip is a wilder story. Many gardeners ask is catmint invasive before they plant it. For a named garden cultivar, the answer is no. You can put one in a tight border and trust it to stay there. The trouble starts only when you plant the wrong kind by accident.
Here is the mechanism behind that. Most ornamental catmints are a sterile catmint type, like 'Walker's Low'. A sterile plant sets little or no live seed. It can only grow wider from its own crown. So it spreads just a few inches a year at the edge of the clump. You can map out where it will sit in five years and be right. That slow creep is easy to live with.
True catnip works the other way. Nepeta cataria is fertile and drops a lot of seed. That catmint self seeding turns into baby plants all over your bed. One parent can scatter seedlings into paths and lawn edges. It can reach the neighbor's yard too if you let the flowers ripen and dry. You will spend part of each spring pulling the volunteers out by hand. That is why so many people grow catnip with a bit of caution.
University of Wisconsin Extension lists 'Walker's Low' as a sterile cultivar that will not set seed. That sterility is the main reason ornamental catmint stays well-behaved while N. cataria can turn weedy.
This is why I keep true catnip in a pot and let the ornamental kind run free in the soil. A container traps the seed heads in one place. Any stray seedlings then sprout where you can pull them in seconds. Catnip in open ground works the opposite way. It can take over a corner in just two or three seasons. If you love the herb for your cats, grow it the same way I do. Set the pot on a hard surface so seeds cannot reach bare dirt below.
If you want a plant that stays neat, your choice is simple. Buy a named sterile cultivar. Skip the generic seed packet labeled only catmint or catnip. Those plain packets often grow the seedy kind. Named picks like 'Walker's Low', 'Cat's Meow', and 'Junior Walker' give you the slow clump habit instead of a self-sower. Check the plant tag at the nursery for that exact cultivar name. A pot with no name on the tag is a gamble you do not need to take.
Even a tidy clump goes woody and bare in the center over time. You can fix that with a simple split. Dig the whole plant up every three to four years in early spring. Cut the crown into two or three pieces with a spade. Replant only the healthiest chunks and toss the tired middle. This one chore controls the spread and keeps the plant full and blooming hard. You also get free plants for the rest of the bed. Share the spare pieces with a friend or line them along a path.
A good ornamental catmint will not run away from you. The catmint spreading habit stays slow when you start with the right plant. Pick a sterile cultivar to begin. Divide it on a regular schedule. Do those two things and your clump stays a clean blue mound for many years. My own front-border plant is proof enough. Six springs in, it still holds its shape and never asks for more room than I gave it.
Read the full article: Nepeta Plant: Care, Varieties, and Growing
