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Plant Profiles

Zwanenburg Crocus



The smaller twigs of 'Winter Orange' lindens are, indeed, orange. What if I underplanted my espalier of them with an early-season bulb whose flowers would harmonize with the twigs while the weather was still cold enough to keep those twigs orange? 'Zwanenburg' crocus has flowers that some suppliers describe as orange. In reality, they are deep yellow, which is no match at all for the twigs of Tilia cordata 'Winter Orange'.




The flowers of Crocus olivieri subsp. balensae 'Zwanenburg' are so showy they are worth growing whether or not you can provide a sympathetic context. For help there, see "Plant partners," below. 




The backs of the deep-yellow petals of buds and just-opening flowers are heavily feathered with a maroon so dark it's nearly brown. By the time the flowers are mature enough to open fully on sunny days (crocus flowers close at night or when the day is cloudy), the maroon feathering has faded substantially. 'Zwanenburg' flowers, then, provide one show when in bud, and another when fully open.


Oh yes, the chicken wire! Many digging and burrowing animals love to eat crocus corms. Wire mesh can help frustrate them. See "How to handle it: Another option—or two!"


The top flower in the picture below is much paler than the bottom. Is there variability in 'Zwanenburg' flowers? (There sure is in the pictures of them used in suppliers' websites.) If I can remember to put a slender marker by this flower, I can dig up its corm in mid-Summer, to transplant to its own spot in the garden. See the top "How to handle it" box, below, for how to lift, divide, and reset crocuses.




Although 'Zwanenburg' crocus isn't orange enough to pair smoothly with 'Winter Orange' lindens, another crocus has recently become available (at least in Great Britain) that should do the job: Crocus olivieri subsp. balensae 'Orange Monarch'. If descriptions and pictures are to be believed, this is the world's first truly orange crocus. My lindens and I are waiting.


Here's how the orange-twigged linden espalier looks in cold weather, when its brightly-colored twigs can coordinate with orange-friendly forms of crocus, epimedium, daffodil, and hellebore. 


Here's how to grow 'Zwanenburg' crocus:

Latin Name

Crocus olivieri subsp. balensae 'Zwanenburg'

Common Name

Zwanenburg crocus


Iridaceae, the Iris family.

What kind of plant is it?

Hardy Spring-blooming corm.


Zones 3 - 8.


Clumping, with foliage and blossoms of uniform height.

Rate of Growth


Size in ten years

Normally, three to six inches high, but can "soar" all the way to twelve. Colonies thicken more than they expand. To my knowledge, 'Zwanenburg' does not self-seed, so you'll need to plant the full extent of your desired area of coverage instead of relying on the crocus colony to explore outward on its own.


The foliage is grassy, while the clusters of upward-facing flowers have a density of hue and jostling intimacy that brings visual warmth to a chilly season.

Grown for

its deeply-hued flowers: 'Zwanenburg' has appeared in some retailers' listings as orange. It is yellow-orange at best. Crocus olivieri subsp. balensae 'Orange Monarch' is a recent introduction (still, to my knowledge, not available in the US) whose flowers are described and pictured as "the first-ever" crocus that is out-and-out orange. I'll believe it when I see the flowers in person.


Similarly, while there are no crocuses with flowers that are entirely burgundy, much of the outer face of the petals of 'Zwanenburg' can be so dark as to seem ebony. This deep shade feathers to orange-yellow at the top edge; the inner face of the petals is solid orange-yellow.

Flowering season

'Zwanenburg' is one of the earlier crocuses; here in southern New England, my colony is in bud by the last week in March.    

Color combinations 

The deep yellow and burgundy of the 'Zwanenburg' flowers combines with these colors' feathering interplay to dictate that neighboring colors should be neutral or similar, not contrasting. Stick with green, dark yellow, orange, and burgundy.

Plant partners

There aren't that many early-season flowers to begin with. Evergreen foliage could be scarce, too. With the additional limitation that nearby plants need to welcome similar soil conditions and exposure to cold and wet weather, sun, and shade, the options for 'Zwanenburg' partners whose colors are inspired, not just extant, can be slender. All the more reason to think creatively.


The pure white of Galanthus flowers isn't so much a clash of hue as of, so to speak, language: There is just no conversation possible between snowdrops' pale, pristine, and chilly palette and the deep warm tones of 'Zwanenburg'. Similarly, the bright yellow of Eranthis hyemalis is too intense and uncompromising to mix with anything as subtle and intricate in coloring and pattern as 'Zwanenburg'.


Cultivars of Hamamelis x media as well as Cornus mas, however, are likely to be in flower at the same time as 'Zwanenburg'. Alas, those of the Cornus are a mild yellow; they're compatible at best, but not interesting. There's enough range of color among the Hamamelis that you can plan for harmony that's at once bold as well as nuanced. Look among the darker-flowered forms, such as 'Aphrodite', 'Bernstein', 'Gingerbread', 'Jelena', 'Orange Encore', and 'Vesna', which combine petals in shades of yellow-orange or apricot with calyces that are brownish-red.


Hellebores are another possibility for an end-of-Winter show of flowers that would be synergistic with 'Zwanenburg', not just simultaneous. Depending on your climate and the severity of any given Winter, the pale orange flowers of Helleborus orientalis 'Apricot Beauty' may be out when those of 'Zwanenburg' are.


Early-season flowers can partner with the continuing show of cold-weather twigs and leaves, which will still be going strong when crocuses start suggesting that Spring, not Winter, should have the upper hand. True, the copper twigs that my 'Winter Orange' lindens sport in cold weather have not proved to be collaborative with the flowers of 'Zwanenburg', because there's not a hint of yellow in them. (The shocking hot pink of these twigs' leaf buds doesn't help.) Winter twigs of a coppiced Salix cardinalis would provide the necessary blend of yellow and orange; those of Salix udensis 'Sekka', the necesssary contrast of maroon. 

Where to use it in your garden

These plants' shortness means that 'Zwanenburg' must be at the front of the crowd if they're to be seen at all. But there's more: The interplay of the ebony feathering on the outside of the petals of 'Zwanenburg' with the petals' yellow-orange base color is, to my eye, this plant's most intense experience. The inner face of the petals is solid yellow-orange, and the flowers lack the colorful zinger that some other crocus forms (see "Variants," below) provide: Strikingly long stigmas that are of an equally-striking saturated color (red, say) that contrasts riotously with the color of the petals.


The best view of the blossoms of 'Zwanenburg', then, is of their exterior, when the flowers are still in bud or when already-open flowers have closed during cloudy weather or for the night. The show can be only vaguely appreciated when the observer is standing erect. Kneeling is just the preliminary option for maximizing your viewing pleasure. Use a kneeling pad, which will lessen the chances, both, of knee-shaped depressions in the ground, and cold wet knees from moisture-saturated soil.


Those who are flexible enough might then sit back (on another little pad) with their knees bent outward and legs crossed at the ankles, and then lean forward far enough that their eyes are, at last, at the necessary elevation of mere inches above ground while also keeping the crocus at one's vision's clearest focal length. The rest of us (me included) must remaining kneeling as we bend over farther and farther, supporting ourselves on one or both hands, while craning our head higher and higher to keep looking out at the crocus, not down at the ground.


Awkward, indeed. If space, time, dryness of the ground, and decorum permit, better viewing will be possible if you can lie on your side, parallel to the 'Zwanenburg' colony, or lie on your stomach, resting your chin on the backs of your hands.


All of this lumbering and repositioning is the other reason that it's essential to site 'Zwanenburg' directly at the front of its bed. Only then is there room for you to adjust your limbs and overall height as needed. I've sited my 'Zwanenburg' colony directly under one section of an espaliered linden bordered all along one side by a grassy walkway four feet wide. There's plenty of room and privacy, both, for even this tall and only moderately limber gardener to get into best-possible viewing position.


It's especially convenient, then, that some crocuses can be planted directly into areas of turf—in other words, surrounded by grass, and not in a bed at all—so that flat-out, fully-prone viewing is possible. With the right combination of crocus (C. tommasinianus is preferred because it is early-flowering as well as less enticing to hungry critters); reduced sunlight (high shade to keep the lawn from growing too thickly); lack of lawncare (no fertilizing, watering, or aerating, and—most important—no mowing until early Summer, so that the crocus foliage can mature fully); and annual planting of still more crocuses, a crocus lawn can be a permanent highlight of your early Spring garden. Although not quite as early as C. tommasinianus, 'Zwanenburg' is early enough to be another crocus-lawn possibility. See "Quirks," below.


Full sun is best, although crocuses will tolerate part shade. Regular to rich soil, as long as drainage is average to good. Camassia and Leucojum are two of the few bulbs that tolerate poor drainage.)

How to handle it:

The basics

Crocuses are normally purchased in the Fall, and can be planted any time the soil is still warm enough to be diggable. Plant corms deep enough, with the pointy end up, that there is three to four inches of soil atop them.


Plant as densely as you can, to twelve to fifteen corms per square foot if you have enough. When planting en masse, especially when creating a specific shape for your drift, make your first priority to indicate the perimeter of the desired colony size, filling in (perhaps with additional plantings year by year) with the balance of the bulbs. 'Zwanenburg' doesn't self-seed the way some species of Crocus can (C. tommasinianus in particular), but individual bulbs soon mature to tight clumps that merge with those of near neighbors to form a carpet of bloom. Widely-spaced corms might eventually increase to discrete colonies six inches to a foot in diameter.


After flowering is completed in Spring, do not attempt to deadhead or to clip away foliage. Let foliage mature on its own schedule. If planting in lawn, do not mow while foliage is still green; delay mowing at least until late June.


Corms propagate on their own, producing thick clumps. Division is not necessary, but to expand coverage, clumps can be dug up, separated into single corms, and replanted with the usual "new planting" spacing of two to six inches apart—and still flower well the very next Spring. Lift clumps anytime, late Spring into Fall, after the foliage has turned yellow.

How to handle it: Another option—or two!

Burrowing and digging animals, such as voles, groundhogs, and squirrels, will eat the corms of most forms of Crocus. (C. tommasinianus is reported as being less appealing.) If you have access to crushed seashells, spread a half-inch layer beneath and atop the corms as you plant: The shells' sharp edges discourage digging.


Another tactic is to layer wire mesh with half-inch openings beneath and atop the Crocus colonies. Or use a double-thickness of chicken wire: The resulting openings are much smaller than those of a single layer. (If you have the time and resources, use both the shells and the mesh; start and finish with the mesh layer.) Cover the top of the shells and/or mesh with three inches of soil. If planting Crocus in lawn, then top with sod. Be prepared to top with more soil as your planting settles naturally; don't tamp the soil down through the grid of the mesh or into the interstices of the shell layer.

Quirks and special cases 

For any early-season plant only inches tall, there are few choices for a groundcover that is lower still, active as early, with a growth habit that is thick enough but not so dense that it hinders the crocus shoots, and with a mature height high enough to mask the maturing crocus foliage without blocking its exposure to sun: Moss is evergreen, but prefers shade and soil that is too shallow for most other plants, including Crocus. Lysimachia nummularia isn't active early enough the Spring. A mat of ivy or euonymus will be too thick and high unless you give it a haircut yearly.


Lawn is an almost perfect partner. Each year, the crocuses precede the lawn into growth, but the lawn usually greens up in time to provide a fresh but still low background by the time the crocus flowers have opened. After the flowers are done, the lawn keeps growing to mask the corms' yellowing foliage. After the crocuses are fully mature and dormant, with their grassy foliage collapsed and incorporated into the thatch of the true grass that surrounds the crocus clumps, the lawn is fully walkable. (That said, crocuses will not thrive in grass that receives heavy foot traffic.


The challenges are to delay the start of mowing until the crocus foliage is fully mature; wait until late June or even, if you can stand it, the fourth of July. And don't encourage the lawn with fertilizer or water, which could stimulate growth so thick that the crocuses will have trouble emerging in early Spring, as well as remaining fully dormant during what, to them, would be an unnaturally wet Summer. If possible, site a crocus lawn where it receives high shade from deciduous trees, which provides early-season sun for the crocuses while restraining the thickness of the lawn's growth in the Summer. As this source says, the lawn most successfully transformed into a crocus lawn looks like the grass of the old property whose garden isn't maintained: Enduring but not enthusiastic.


The kinds of grasses that make up a typical lawn are tolerant of occasionally poor drainage—so much so that it's best practice to grade any adjacent beds to be higher than the lawn. Then excess water drains from the beds onto the lawn. But crocuses require good drainage, so a crocus lawn must shed excess surface water just as quickly as its surrounding beds do. Only "crocus" a lawn that doesn't sheet with water after heavy rain. Lawn grass doesn't thrive in the fast-draining soils that crocuses prefer, so the easiest solution is to crocus only lawns that have enough slope to shunt precipitation away promptly.


To establish a crocus lawn, you could scatter small colonies of crocus into the lawn: Lift up a patch of lawn, plant the corms (with any needed protection of mesh, shells, or both), and top by returning the divot of lawn. But you might have opportunity to create the lawn itself at the same time as the crocus lawn. Do this in late Summer and Fall, which are the best seasons to establish both. Especially if you are "crocusing" the entire lawn, protect the corms just with shells, not mesh. It's likely you'll want to add more crocuses someday (some crocus-lawn afficionados do this every year!); if the mesh were underneath the entire lawn, that would be impossible


Squirrels and other rodents can develop a real affection for the corms; I wouldn't risk planting Crocus unless they were thwarted. For suggestions, see the second "How to handle it" box, above, as well as the "Quirks" box.   


So many Spring-flowering bulbs, such as snowdrops, tulips, and daffodils, exist in a thrilling and, indeed, overwhelming array of species and cultivars. With ninety species and countless named forms arising from them, the Crocus genus only adds to the bounty. While most crocuses flower in late Winter and early Spring, there are also forms that flower in the Fall. Some so-called Autumn "crocuses" might also be one of the larger but similar-looking species of Colchicum, in the Colchicaceae family.


Crocus flowers can be white, yellow, orange, pink, blue, burgundy, or deepest violet, and almost any hue along the way. Although the outside of the petals of some forms, such as'Zwanenburg', can be feathered with maroon, there isn't yet a crocus whose petals are solid maroon. There also isn't any crocus whose flowers features just the red components of burgundy. (For early-season red in a Spring bulb, there are plenty of tulips to consider.) The petals of many forms of crocus can be netted on one or both sides with a darker color; usually, the combination is of blue or violet netting over a lighter or even white base. Other forms flaunt lighter flaring on the inside of the petals as they plunge down into tube. The stamens (usually three) and pistols are often an excitingly-contrasting orange or red.


On my wishlist are several of the forms of Crocus that flower in Autumn. The blooms of C. banaticus have unusually narrow petals that give the flowers a greyhound-like sleekness. Those of C. speciosus 'Oxonian' are deep blue, with vivid orange styles that are divided several times over. The flowers in the forms of saffron, C. sativus, that I find most desirable are deep- or at least sky-blue, with a trio of bizarrely-long stigmas that seem to loll out of the flower like the tongues of cartoon Lotharios. Their red color is in stunning contrast to the petals.


Another must-have is Crocus tommasinianus. It is the earliest to flower (sometimes as early as late Winter, hence, one of its common names: snow crocus) and, so, is the first choice for crocus lawns. See "Where to use it in your garden," above.




By division of the clumps in late Spring and early Summer, after the foliage has become dormant.

Native habitat

Crocus olivieri subsp. balensae is native to Greece, the islands in the Aegean Sea, and western Turkey. C.G. van Tubergen Ltd. was a bulb-importing wing of Zwanenburg Nurseries, a Dutch firm that was in business in the 19th and early 20th centuries.


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