If you’ve ever shopped for perennials you’ve undoubtedly seen the plant tags that growers stick in the pot. You know the ones. On one side they have a colorful photograph and on the other side they tell you to plant your plants in full or partial sun, in moist well-drained soil.
Chances are good that the tag also includes information on the height of the plant, its bloom time, and “proper” spacing. Here’s a tip for using that information on plant spacing: ignore it.
I’m willing to bet that most sources you consult will tell you to space your perennials approximately 18-24” apart. Don’t do it. If you do, you’ll probably regret it. Plan instead for a spacing more like 8-12 “ apart.
The farther apart you space your new perennials, the more you will find yourself battling weeds. The two feet that Walmart or White Flower Farm wants you to put between your Black-eyed susans is two feet of prime territory for weed seeds: lots of light, plenty of water, and no competition for soil nutrients.
Farmers figured this out decades ago: increased plant density, as a means of reducing weed pressure, has been standard practice in agriculture for many years. Study after study shows that the easiest way to prevent invasive weeds is to avoid giving them an opportunity to germinate to begin with. In prairie plantings, spacings of less than one plant per square foot (i.e. 12″ spacing) had greater susceptibility to weed invasion.
Planting your perennials more densely has additional benefits: the garden or meadow will look “filled in” much more quickly and, more importantly, will begin supporting wildlife much more quickly. Many of the mammals, insects, birds, and amphibians that wildlife gardeners want to encourage depend on having relatively dense cover. These animals much prefer the cover of your carex or goldenrod to empty exposure of mulch that would result from placing the plants two feet apart.
For the most part, when we are dealing with well-chosen native plants, you can forget the “plants need room to grow” myth: it just isn’t true. Claudia West, of North Creek Nurseries, likes to say, “Plants are social”. In nature they grow right beside each other, and have evolved strategies such as different bloom times, different root depths, and so on to do so. I’ve worked with Claudia on designs where the specification is one plant every 8 to 10 inches, and this approach almost always leads to a higher rate of planting success.
In fairness, the deep-rooted and relatively inexpensive landscape plugs that North Creek Nurseries sells make this kind of density feasible: a spacing of 8” requires nine times as many plants as a spacing of 24”. And many prairie or meadow installations work just fine with a more economical 12” spacing, which still results in using twice as many plants as the 18-24” suggested by most sources.
But even for a small project, if the budget is small the outcome is probably going to be better if you concentrate on getting the right density in a smaller area rather than spreading a small budget over too large of an area.