Betula pumila (Bog Birch)
|Also known as:||Dwarf Birch|
|Life cycle:||perennial woody|
|Habitat:||sun; open bogs, fens, peatlands, swamps|
|Bloom season:||April - May|
|Plant height:||4 to 12 feet|
|Wetland Indicator Status:||GP: OBL MW: OBL NCNE: OBL|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Male and female flowers are borne separately on the same plant (monoecious), in clusters called catkins. Male catkins are single from short spur-like lateral shoots on 1 year old twigs, ascending to pendent in flower, ¾ to 1 inch long, developing in fall as a slender spike of tightly appressed scales and opening up the following spring. Female catkins are erect or ascending, cylindrical, 1/3 to ¾ inch long from new, spur-like lateral branches on the same branch as the males.
Leaves and stems:
Leaves are alternate and simple in mostly 2s or occasionally 3s on short, spur-like lateral twigs, and singly on the new, elongating terminal branches. The blade is rounded-fan shaped, widest above the middle, ¾ to 1½ inch long, ½ to 1 inch wide, the tip broadly rounded, the base wedge-shaped or a short, rounded taper to the short stalk. Edges have course rounded or sharp teeth except near the base and are sometimes double-toothed. The upper surface is dark green, usually smooth or with fleeting scattered hairs, often gland-dotted; the lower surface similar but lighter green.
Twigs are brown to reddish brown or even deep maroon, with scattered lenticels (pores), new growth is hairy but becoming hairless, smooth the second year. Older bark is gray to reddish brown with pale lenticels, not shedding or papery. Stems are profusely multiple from the base, larger ones up to 1¼ inch diameter.
As its name would indicate, Bog Birch typically inhabits wetland environments, though true acidic sphagnum bogs will not support its growth. It can also be found in low areas of sand dune habitats where there is adequate soil moisture close to the surface. It is typically a 6 to 8 foot, densely multi-stemmed shrub but can be highly variable in leaf characters and height. While a few varieties have been proposed based on various characteristics, they are not all universally accepted and not currently recognized in Minnesota. The two more widely accepted vars are: var. glandulifera with a more northern distribution has leaves glandular and generally less hairy (the var. in Minnesota), and var. pumila with a more eastern and southern distribution has hairier leaves without glands. Confusing all this is its ability to freely hybridize with tree form species; B. x purpusii (a cross with B. alleghaniensis) and B. x sandbergii (a cross with B. papyrifera), both of which have been collected in Minnesota. But even these recognized hybrids are known to further cross and backcross producing potentially a myriad of intermediate characteristcs. Generally anything over 12 feet and is more tree than shrub form is likely a hybrid.
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Photos courtesy Peter M. Dziuk taken in Aitkin, Anoka, Kittson and Lake counties.
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