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Native Plants

 

February 2025 Native Plant of the Month –

Osoberry (a.k.a. Indian Plum)
(Oemleria cerasiformis)


by Marilee Henry

 

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“No other westside shrub better celebrates the spring rebirth of our lowland landscapes.” Arthur Kruckeberg, in Gardening with Native Plants of the Pacific Northwest, 1996, regarding Oemleria cerasiformis.

 

Within a few short weeks, maybe even days, the brilliant greens of osoberry’s bursting new leaves and pendulous clusters of greenish-white blossoms will be enlivening the dull winter tones of forests and open woodlands at low elevations throughout western Washington and parts of the Columbia Gorge.  This early blooming characteristic makes osoberry easy to spot in our region in the still chilly temperatures of February, as it will be the first native shrub to flower.

 

Osoberry is an erect deciduous multi-stemmed shrub or small tree, ranging from British Columbia to northern California, that grows up to 20 feet in height.  Its bright green, alternate, lance-shaped leaves, 2 to 5 inches in length, have smooth margins, pointed tips, and duller green undersides.  When crushed, young leaves can smell like cucumber or watermelon rind.  Its stems of purplish-brown bark are dotted with obvious yellow-orange lenticels, soft tissues that aid in the exchange of gases and water, facilitating photosynthesis and stem hydration.  The chambered pith inside its stems provides storage space for water and/or nutrients, an advantage during summer drought or seasonal low-nutrient periods, as well as adding structural flexibility and strength.

 

Fragrant, faintly almond-scented flowers having 5 greenish sepals and 5 whitish petals hang in clusters of 10 to 20 blooms from the ends of leafy branch tips.  Flowers may open just before or as the young leaves are emerging in late winter.  Since osoberry is dioecious, each individual plant will have either male or female flowers containing 15 stamens or 5 pistils respectively – you can see the differences here.  Though the female flowers may appear to also have stamens, they are usually stunted and non-functional.

 

One-seeded whitish-tan fruits (“drupes”) form only on the female plants, turning orange to red to purple as spring progresses.  They are bitter to the taste at this stage, containing small amounts of cyanide compounds to discourage consumption before seeds are mature.  (Warning: unripe fruit ingested by pets or children may be toxic. )  The fruits become palatable only when they are fully ripe and blackish-blue in color; in western Washington this may occur as early as May.  Their flesh is said to then taste like watermelon or cherries.

 

Osoberry is a wonderful plant for wildlife, hosting at least 10 species of moth  and butterfly larvae in our region.  Its early blooming habit provides critical late-winter-to-early-spring sources of nectar and pollen for bees, bumblebees, flies, moths, butterflies, and hummingbirds.  Its early fruit is so popular with many birds, foxes, coyotes, deer, bears (“oso” means “bear” in Spanish), and other mammals that they quickly disappear upon ripening.  

 

Indigenous peoples had numerous uses for this native.  The ripe berries were consumed raw, cooked, or dried in small quantities.  The strong yet flexible and fine-grained character of its wood made stems ideal for bows and arrows, and for small carved objects such as combs, spoons, and knitting needles.  Bark and twigs could be seeped for tea or included in medicines.

 

Osoberry is very interesting botanically in its being dioecious and precocious.    Approximately 6% of all flowering species are dioecious.  The advantage of dioecy, forcing cross-pollination between different plants of the same species, results in greater genetic diversity in the offspring.  However, it has its disadvantages as well.  Dioecious plants require pollinators to move pollen from male plants to female plants for germination, whereas in non-dioecious plants, germination can result from the movement of a pollinator from any flower to another or even within a single bloom.  Also, in a given population of dioecious plants, only half their number can produce seeds, whereas in non-dioecious species any plant can reproduce.  So one might think that in the struggle for survival, dioecious plants are at a great reproductive disadvantage to non-dioecious species within their habitats.  But in evolving toward dioecy, species have also developed precocious strategies to overcome these disadvantages.  In the case of osoberry, an understory woodland species, its leaves and flowers emerge in late winter before the overstory trees leaf out to shade it.  So it gets the sunlight and early pollinators all to itself.  Its male plants produce more biomass in flowers and pollen than females in flowers, attracting more pollinators first to themselves and thereby increasing the direction of pollen movement from male to female plants.  Osoberry starts producing fruit at a young age, as early as 2 years after planting, maximizing reproductive life span.  Producing fruit that both matures earlier than that of other shrubs and trees and is tasty to many animals ensures widespread dispersal of its seeds.  Research shows that given these precocious strategies, osoberry populations can be maintained in roughly the same numbers as non-dioecious species.  For more details, read this.

 

In restoration projects osoberry is important for its wildlife values as well as for its fast growth, its ease of inexpensive propagation by cuttings or seeds, and for its fibrous roots that enhance slope stability and erosion control.   Though it prefers part shade and fertile, moist, well-drained soils, it is tolerant of most soil types and moisture regimes, the exception being water-saturated areas. 

 

In urban landscaping, osoberry is most striking planted on the borders of conifer groupings where its early leaves and flowers create maximum contrast in late winter.  Osoberry can also be placed in the understory in full shade, though it flowers and fruits best in sunnier spots.  Supplemental watering during the first 1 or 2 years while getting established may be required due to its rapid growth and shallow roots, especially when planted in full sun.

 

Because male plants produce larger, showier flowers than females, some gardeners prefer to plant only males.  But in doing so, they will be excluding the highly prized fruit that attracts birds and other wildlife to their backyard habitats!  So plant groups of 3 or more osoberry together, spacing them 8 to 10 feet apart and having at least one female for every 2 or 3 male plants in borders with morning sun for numerous blooms and good fruit production.

 

Remember,  “You are nature’s best hope!” - Doug Tallamy.

 

Native plant sale events coming soon:

 

Snohomish Conservation District plant sale

Online ordering continues through Feb 7, 2025

Pickup at Evergreen State Fairgrounds, Monroe, March 1-2, 2025

 

Arboretum Spring Plant Sale April 6, 2025

 

Oxbow Farm & Conservation Center Spring Native Plant Sale April 20, 2025


WNPS Native Plant Sale April 26, 2025, BBG

FHNA, Finn Hill Neighborhood Alliance Kirkland, WA
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