When once is not enough--the art of interplanting

04/19/2007
When once is not enough--the art of interplanting

Nonstop waves of bloom in perfectly orchestrated color schemes--that's what we're all dreaming of when we plant perennial gardens, right?  Yet, by now you've probably figured out it's not as easy as it looks to keep all  of your garden in bloom all the time.  After all, to every plant its season.  And were we to disqualify all perennials that have a relatively short period of bloom, we'd be doing without a lot of splendid plants. 

Besides, I've noticed that some recently introduced perennials, bred for non-stop bloom, have lives proportionately as short as their flowering is long.  These plants seem to exhaust themselves with all this flower production, and have a hard time going dormant when winter approaches, making them prone to winter-kill.  As the old-time TV ad used to admonish:  You can't fool Mother Nature. 

But you can work with her.  One of the most creatively challenging ways I've found of maximizing the bloom period of any given patch of border is by interplanting.  Interplanting is a my name for planting two different plants, in a sort of checkerboard pattern, in a given area.  The idea is to choose an early and a later blooming perennial to cohabit the same spot, thus prolonging its flowering period.The early bloomer provides the spring or early summer show, and then as it fades, the later bloomer takes up the flowering slack.




At first blush, this may sound simple.  And in a manner of speaking, it is.  Successful interplanting can be as easy as falling in love, but for it to last--as in marriage--the partners have to be compatible.  For instance, if one of your partners is, say, gooseneck loosestrife (Lysimachia clethroides), and the other is candytuft (Iberis sempervirens), at the end of the first season, you will be left with simply gooseneck loosestrife, whose aggressive, stoloniferous, mat-forming habit will have choked out the mild-mannered candytuft.

The main key to successful interplanting is invisible in the garden, unless you take out a shovel and look underground.  For two plants to be compatible partners in my scheme, they must have compatible roots.  A plant with delicate, fibrous roots like the peach-leaf  bellflower, for example, cannot successfully cohabit with an aggressive mat-former like beebalm.  In fact, plants with stoloniferous, spreading roots are in general not good subjects for interplanting, even with each other.  The best candidates for interplanting are first plants with fleshy taproots, and secondarily, plants with fibrous roots which remain in tidy clumps.

Besides their root habits, the size and vigor of your partner plants will dictate whether they can cohabit successfully.  For instance, a towering New England aster would quickly overpower a more diminuitive and less vigorous partner such as an erigeron.

Finally, perhaps you've already been interplanting without thinking of it as such.  If you've planted bulbs in your perennial garden, you've engaged in interplanting.  Spring-flowering bulbs are one of the best-adapted plants to use in this technique.  It's just that you can carry it far beyond the widespread practice of interplanting a border with bulbs.


Let me give you an example of a great interplanting combination:  pasqueflower (Pulsatilla vulgaris) and butterflyweed (Asclepias tuberosa).  Pasqueflower blooms very early--around Easter, as its name suggests--with large, usually violet-blue blossoms.  After the flowers finish, the plant remains attractive throughout the growing season with its tidy clump of handsome ferny foliage.  Butterflyweed, on the other hand, is late to emerge, and blooms from July through August with umbels of fiery orange.  Its linear foliage isn't particularly interesting and so it profits from the pretty leaves of the pasqueflower surrounding it.  Both plants have tidy, fleshy taproots which cohabit underground in perfect harmony.


Now here's another tricky part of interplanting, which poses a delightful challenge to the experienced gardener.  You'll need to choose the neighboring plants for your interplanted combination to complement each of the individual partners.  So, the set off the pasqueflower, you may want to plant some basket-of-gold (Aurinia saxitilis) nearby.  Meanwhile, a 'Lochinch' butterflybush (Buddleia x 'Lochinch') would be handsome behind the butterfly weed, providing good contrast both in form and flower color.

Finally, this may be self-evident, but you must choose partners for your interplanting with similar light, soil, and moisture requirements.  In the above example, both the pasqueflower and the butterfly weed require full sun and light, well-drained soil.

While complementary bloom periods are the main motivation for interplanting, you can interplant for other reasons as well.  You can choose complementary, simultaneously blooming partners which provide a closely-integrated, attractive plant combination.  An example would be wandflower (Gaura lindheimeri) and white or pink oriental lilies.  The lilies would provide bold contrast to the delicate blossoming stems of the wandflower.  A late-blooming partner can also help camouflage an early-blooming plant which becomes untidy-looking  later in the season.  And a plant with superlative foliage can complement its partner all season long.  For example, a purple-leaved heuchera would make a dramatic partner with quite a different effect for the orange-flowering butterflyweed.


A final factor to consider is the plant's posture.  Perennials that flop are inappropriate partners as they will cover and shade out their neighbors.  The flip side of this is that plants that are extremely sensitive to crowding and shading over, such as all alpine plants and most silver-leaved plants, also make poor partners.

Here are a couple of lists of potential candidates for successful interplanting.  I have grouped them by their root habits, as this is one of the most important considerations.  Tap-rooted plants can almost always be partnered with each other and with fibrous-rooted clump formers.  Fibrous-rooted clump-formers can be paired with each other provided their vigor is compatible. 

A final note:  you can partner vigorous matformers, such as Nepeta, Nepitella, certain asters, and so on, as long as their vigor is more or less equal.  They will form entwined mats of stolons which will give rise to mixed blooms.  I have not included these plants in my lists.

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Products of Interest:
Shade-tolerant--Greater celandine
Average to wet soils--Valerian
Average to dry soils--Jupiter's beard
Shade-tolerant--Wild columbine