Interspecific Hybrids in the Landscape Garden

Some of the most reliable plants on the garden center bench are not species at all. They are the offspring of two different species in the same genus, deliberately crossed by a breeder. That little × in the middle of a botanical name, as in Heuchera × Tiarella, is the only clue most shoppers get. Behind it is a plant that often outperforms either parent: hardier, longer blooming, better colored, or simply less fussy. Here is how interspecific hybrids come about, and which ones you are probably already growing.

How interspecific hybrids work

Every plant you buy has a two-part Latin name: the genus, then the species. Echinacea purpurea and Echinacea paradoxa share a genus but are distinct species, separated in the wild by geography, bloom time, or pollinator preference. An interspecific hybrid is simply a cross between two such species. (A cross between two cultivars of the same species, like most tomato breeding, is not interspecific at all.)

Species in the same genus usually carry enough shared genetic machinery that pollen from one can fertilize the other, but the barriers are real. Breeders hand-pollinate thousands of flowers, grow out the seed, and discard nearly everything. The rare seedling that combines the best of both parents gets propagated by cuttings or tissue culture so every plant you buy is an exact clone of that one winner.

Two things make the effort worthwhile. The first is hybrid vigor: the offspring of two genetically distant parents are often sturdier and faster growing than either. The second is trait stacking. One species brings the color, the other brings the hardiness or the compact habit, and the hybrid carries both.

There is a catch that works in your favor. Mismatched chromosomes often leave interspecific hybrids sterile or nearly so. A plant that cannot set seed keeps pouring energy into flowers instead, which is why so many of these hybrids bloom for months and never self-sow into the lawn.

On the tag, a hybrid between two species is written with the × before the species epithet: Lavandula ×intermedia. A hybrid between two genera gets the × in front of a blended genus name: ×Heucherella. Many modern hybrids skip the Latin entirely and appear under a trademarked series name, so the × is not always there to find.

The hybrids you will meet at the garden center

Coneflower (Echinacea). The orange, yellow, and red coneflowers that arrived in the 2000s are crosses between purple E. purpurea and yellow E. paradoxa. Early ones were short-lived; current series are far more reliable in Midwest winters.

Echibeckia (×Echibeckia). A cross between coneflower (Echinacea) and black-eyed Susan (Rudbeckia), sold mostly under the Summerina series. It carries the big, warm-toned flowers and sturdy stems of a coneflower with the vigor and fast, dense habit of a rudbeckia, blooming from early summer well into fall. Hardiness is less proven than its parents, so treat it as a Zone 6 perennial or a long-season annual here.

Foamy bells (×Heucherella). A cross between coral bells (Heuchera) and foamflower (Tiarella). You get the colored foliage of the first with the shade tolerance and fuller flower spikes of the second. Excellent in dry shade under trees.

Hardy geranium 'Rozanne'. Possibly the most-planted perennial of the last two decades. A sterile cross of G. himalayense and G. wallichianum that blooms violet-blue from June to frost precisely because it cannot set seed.

Lavandin (Lavandula ×intermedia). 'Grosso', 'Phenomenal', and 'Provence' are crosses between English and Portuguese lavender. They are larger, more fragrant, and more tolerant of humidity than straight English lavender.

Catmint (Nepeta ×faassenii). 'Walker's Low' and 'Cat's Pajamas' are interspecific and sterile, which keeps them tidy and blooming instead of seeding around.

Hardy hibiscus. The dinner-plate hibiscus in the Summerific and Luna series blend H. moscheutos with other native species for darker leaves, more compact plants, and reliable hardiness to Zone 4.

Shasta daisy (Leucanthemum ×superbum). Luther Burbank's famous cross of several daisy species, still the standard white perennial daisy.

Foxglove (Digiplexis). A cross between common foxglove and a Canary Island relative. Treated as an annual here, but it reblooms all summer rather than once.

Woody plants. Witch hazel (Hamamelis ×intermedia), forsythia (Forsythia ×intermedia), saucer magnolia (Magnolia ×soulangeana), glossy abelia (Abelia ×grandiflora), and Burkwood viburnum are all old interspecific crosses that outsold their parents a century ago and still do.

Annuals. Petchoa, a cross of petunia and calibrachoa, has the big flowers of one and the weather resistance of the other. Interspecific impatiens (SunPatiens, Beacon) combine New Guinea and common impatiens for sun tolerance and mildew resistance.

What it means for your garden

An interspecific hybrid is not a guarantee, but it is a strong hint that someone did a lot of selecting before the plant reached you. Expect longer bloom, less seeding, and often better disease resistance. Expect, too, that you cannot save seed from it and get the same plant back. Buy it, divide it, or take cuttings. And next time you spot that × on a tag, you will know you are looking at two species doing something neither could do alone.