Some perennials have major territorial issues. Give them an inch, and they’ll take a yard – or at least a good chunk of it. Allow them a toehold, and their rampant root systems or prolific seedlings will likely haunt your garden for years to come.
Of course, such perennials don’t limit their thuggery to the garden; they also can spread (usually by seed) into nearby natural areas, out-muscling native vegetation. For the scoop on the worst offenders in your region, check your state’s list of banned invasive species. But keep in mind that many species with serious boundary issues don’t appear on most state banned lists. Even if it’s not listed by your state (as is probably true of the species described below), the perennial that’s captured your heart might have designs on taking over your garden.
Just about any list (state or otherwise) of takeover perennials is likely to include a few that go by the common name “loosestrife.” Purple loosestrife (Lythrum salicaria) – the perennial that ate the Northeast (as well as several other regions) – is the textbook example. Close behind, however, are several species in the genus Lysimachia. Many a gardener has regretted falling under the spell of the arching white spires of gooseneck loosestrife (Lysimachia clethroides). Lovely in bouquets, this East-Asian native is a rambunctious bully in the garden, spreading rapidly via fleshy white underground shoots (known as rhizomes). A far wiser (and better behaved) choice for perennial borders is milky loosestrife (Lysimachia ephemerum), which forms 3-foot-tall, gray-leaved clumps surmounted in summer by candles of frothy white flowers.
Also too vigorous for most gardens are yellow loosestrife, Lysimachia punctata, and fringed loosestrife, Lysimachia ciliata (usually grown in its purple-leaved form, ‘Firecracker’). Both make good candidates for damp, isolated niches where they have room to romp. Yellow wild indigo (Baptisia sphaerocarpa) and its hybrids (including ‘Carolina Moonlight’) would be a better choice for situations where good manners and 3-foot-spires of bright yellow, early-summer flowers are desired.
Quite a few other yellow-flowered aggressors are commonly grown (and virtually ineradicable) in gardens. Almost all perennial sunflowers (Helianthus), for example, are rapid colonizers with tenacious questing rhizomes. If the mellow yellow blooms of Helianthus ‘Lemon Queen’ beguile you, you might want to opt instead for Silphium mohrii, which produces summer daisy-flowers of an even softer pastel-yellow, but on 4-foot-tall plants that stay in place. Another popular splashy yellow summer-bloomer to avoid is the double-flowered Rudbeckia laciniata ‘Golden Globe’ (also known as ‘Hortensia’). Take a pass on this common pass-along plant (there’s always plenty of it to share thanks to its romping rhizomes), and seek out its mannerly look-alike, ‘Goldquelle’. Also often passed along are some of the more assertive yellow-flowered members of the evening primrose tribe (including Oenothera fruticosa and Oenothera tetragona). These might best be passed by in favor of arguably the largest-flowered and loveliest hardy Oenothera, Missouri primrose (Oenothera macrocarpa).
In contrast, goldenrods (Solidago) often get tagged with the “invasive” label, even though many of them are model citizens with arresting late-summer flowers. Stiff goldenrod (Solidago rigida) holds dense, flat heads of lemon-yellow flowers above handsome clumps of gray-green foliage, while the equally fetching showy goldenrod (Solidago speciosa) carries steeples of bright yellow blooms on burgundy-red stems. As for the canard that goldenrods cause hay fever: they don’t.
The common name of Physostegia virginiana – obedient plant – also gives the appearance of a canard, given the relentless rhizomes of this Central U.S. native. The moniker, however, refers to the mauve-purple, turtlehead-shaped flowers that line its 3-foot-tall spikes in late summer. Push an individual flower into a new position, and it compliantly stays put. The white-flowered cultivar ‘Miss Manners’ departs from other physostegias in possessing obedient flowers AND rhizomes. Its flower color is also more compatible.
The domed flower heads of garlic chives (Allium tuberosum) share the adaptable, milk-white coloration of ‘Miss Manners’. This prolific ornamental onion isn’t so good at sharing space, however. Neglect to deadhead its late-summer blooms, and it will populate much of your garden with seedlings. The somewhat earlier blooming Allium ramosum also bears showy (and sweet-scented) heads of white flowers atop 18-inch stems, but without the resulting seedling swarm.
Three more invasive perennials to steer clear of (and suggested substitutes) include:
- Plants sold under the botanical name Adenophora, which almost always are the fiendish, tuberous-rooted Campanula rapunculoides. Use peach-leaf bellflower (Campanula persicifolia) or great bellflower (Campanula latifolia) instead.
- Yellow archangel, Lamium galeobdelon. Rather than unleashing the garden-variety species on your yard, substitute its cultivar ‘Herman’s Pride’, which offers even handsomer silver-splashed foliage, sans the infinite spread.
- Butterbur (Petasites). Yes, the romping colonies of immense, heart-shaped leaves are captivating, but the thick rhizomes will not stop until they’ve occupied every square millimeter of available soil. A Ligularia or Rodgersia will give the same foliage effect without commandeering the whole neighborhood.
Whenever you plant a mannerly perennial in your yard, be sure you know its soil needs. Fortifying soil with needed amendments will result in better overall performance. We suggest Fafard Premium Natural & Organic Compost, for starters. Although these and quite a few other perennials are too rampant for most garden areas, they might work in an isolated niche (such as a driveway island) where nothing else will grow, or in an informal planting (such as a cottage garden) that features plants that can fend for themselves. “Right plant, right place” is a garden maxim that never goes out of style.