Because trees persist throughout the year, books often use winter keys (based on bark and buds), summer keys (based on leaves and other forms of growth), and flowering keys (based on the spring flowers appearing more-or-less before the leaves as is the case with many trees). Rhoads and Block offer two keys for Birch:
A. mature bark white
B. leaves triangular
C. branches not pendulous - B. populifolia Gray Birch
C. branches pendulous - B. pendula European White Birch
B. leaves ovate
D. leaves 5 to 8 centimeters long - B. papyrifera Paper Birch / Canoe Birch
D. leaves 3 to 5 centimeters long - B. pubescens European White Birch
A. mature bark not white
E. twigs aromatic
F. bark blackish-brown, twigs strongly aromatic - B. lenta Black Birch / Sweet Birch
F. bark yellowish-gray - B. alleghaniensis Yellow Birch
E. twigs not aromatic - B. nigra River Birch
A. mature bark white
B. buds <5 mm long - B. populifolia Gray Birch
B. buds 5-10 mm
C. branches erect to slightly drooping
D. bark peeling - B. papyrifera Paper Birch / Canoe Birch
D. bark not peeling - B. pubescens European White Birch
C. branches pendulous - B. pendula European White Birch
A. mature bark not white - see second A in summer key above
For full keys and species descriptions consult Rhoads and Block's book The Plants of Pennsylvania
One confusing detail with Birch taxonomy is the use of the species name nigra for River Birch while the common name for Betula lenta is Black Birch (nigra means black). But anyone who has spent time trying to identify plants knows that taxonomy isn't perfect and it should not be difficult to memorize this inconsistency with the names. You might also take notice that other Betula species names are quite descriptive. For instance populifolia refers to the Poplar-like triangular (flat-bottomed) shape of the leaf.
Although the key describes Yellow Birch bark as "yellowish-gray" I usually describe it as golden, as it is quite shiny and to me appears more golden than yellow. Yellow Birch is very useful for fire-starting. The thin peels of bark will light right up when struck with a good spark. Additionally, Yellow Birch is quite significant as the host of the famous Chaga fungus that is revered as a medicine and also used for fire-starting and fire carrying.
Gleason & Cronquist's Manual of Vascular Plants of Northeastern United States and Adjacent Canada lists a few additional species (shrubs) of Betula. Here is an abbreviated version:
1. Large shrubs or usually trees; bark usually separable in layers
2. Bark yellowish-gray to dark reddish brown
3. Leaves rounded to subcordate at base; crushed twigs with the flavor of wintergreen
4. Bark yellowish-gray - B. alleghaniensis Yellow Birch
4. Bark dark reddish-brown - B. lenta Black Birch
3. Leaves cuneate at base; not aromatic - B. nigra River Birch
2. Bark whitish
5. Leaves pubescent in the vein-axils beneath - B. papyrifera Canoe Birch
5. Leaves glabrous beneath - B. populifolia Gray Birch
1. Shrubs with close bark
6. Pistillate scales 3-lobed; leaves mostly 1-3(-5) cm
7. Resin-glands none - B. pumila Swamp Birch
7. Resin-glands copiously scattered on the young stems and leaves - B. glandulosa Dwarf Birch
6. Pistillate scales oblong; leaves 0.5-1 cm - B. michauxii Newfoundland Dwarf Birch
These dwarf species are mostly out of our area, but are included for those out of the area and for reference. As I constantly mention, keys are limited and often it is useful to use more than one. This one, for instance, is (even besides the dwarf species) arranged slightly differently from those above. Again, for the full keys and species descriptions, see the text. I also include this key for reference regarding medicinal uses. Birch species in general have many medicinal uses. Black Birch, though underutilized at this point is perhaps one of the most important and famous medicinal plants of our area. Even the root beer formula is a medicinal recipe; Black Birch, Sassafras, and Sarsaparilla all being important "blood cleansing" and anti-rheumatic herbs of early America. The Iroquois also highly valued Black Birch as medicine because they associated it with the deer, which sustained their lives as a staple food.
There is so much to write about Birches, it far exceeds what I could possibly put in this article. Here, I can only mention a few medicinal uses such as recorded in Native American Ethnobotany by Daniel E. Moerman and a little from my experiences. A thorough examination of these special trees would also include fungal associations. I have already mentioned Chaga, which grows on Yellow Birch. Much more familiar in our area is the Birch Polypore, a common sight on Gray Birch, which is edible (not highly regarded, but a possible survival food when it is young) and medicinal. Further, there are many mycorrhizal (symbiotic) fungal species with birch, including edible mushrooms. I once found Slippery Jacks growing near Black Birch that even tasted of root beer!
stand of Gray Birch, close-up of Gray Birch, and a Gray Birch being consumed by the Birch Polypore:
Black Birch is another important but underused and mis-understood remedy. I realized that we have forgotten much of its medicinal uses years ago when I began regularly enjoying Birch twig tea, which is an obvious diuretic. I also recall serving it to one of my first study groups and seeing more trips to the bathroom than usual. However, when looking up the medicinal uses in various texts it was not described as a diuretic (which is a property commonly associated with many herbs and lesser diuretics). Additionally, many resources consider Black Birch similar to European White Birch when describing the medicinal virtues of Birch in general derived from European sources - a phenomenon related to the fact that more has been recorded about European herbal medicine than American herbal medicine. Though Black Birch probably has much medicinally in common with Birches in general, it certainly stands out as an aromatic and should not be thought of as the same medicinally as non-aromatic species.
According to Moerman, Black Birch has been used in many ways as a medicine by the Algonquin of Quebec; for dysentery, colds, stomachache, and milky urine by the Cherokee; for diarrhea and lung troubles by the Chippewa; for bad blood, cold menses, fever, gonorrhea, soreness, and fatigue by the Iroquois; and as a spring tonic by the Mohegan. The Iroquois seemed to have a particular relationship to Black Birch (Sweet Birch) in that it "is a highly valued medicine because it sustains the deer, the mainstay of life" and because of its natural range. (See Iroquois Medical Botany by James W. Herrick) They clearly regarded it as a warming herb, which is a typical quality of aromatics as understood by cultures around the world, such as in Chinese herbal medicine. It was used when "blood gets bad and cold" and when women "catch cold with the menses" and as a diaphoretic for fevers. This warming property is also seen in the use of Wintergreen oil, which although was originally made with Wintergreen, it was also made with Black Birch (even though still under the name Wintergreen oil - which persisted even when the medicine was made synthetically). Wintergreen oil was once a common household remedy for aches and pains and has a distinct warming effect when rubbed on the skin or added to a bath. Interestingly, people often guess "Spearmint" when I break open a twig and ask them to guess the aroma. This is because of the association with Mint and Wintergreen (which is not a Mint either). But when I mention root beer, folks can quickly recognize the aroma even though few would associate Mint with Birch. Whether topically or internally, Black Birch definitely has a beneficial effect on arthritic conditions and other pains.
Yellow Birch is also aromatic, but not as strong as Black Birch. It has been used as a diuretic by the Ojibwa and for blood diseases. The Lenape (Delaware) in Oklahoma have used Yellow Birch as a decoction and as an emetic - I assume the bark was harvested in either an upward or downward motion according to the use as was done with Elder. It was used to remove bile. Iroquois have used Yellow Birch as a blood purifier, for "Italian itch", and for lactation.
Cherokee have used River Birch much like Black Birch. The Chippewa have used River Birch for stomach pain.
Paper Birch has been used by the Algonquin for diaper rash and other rashes; by the Chippewa as an enema; by the Cree for burns, rashes, sores, "women's troubles", back pain, gonorrhea, to promote milk flow, for teething sickness, and as a diaphoretic; by the Iroquois as a contraceptive; by the Koyukon in shamanism; by the Menominee for dysentery; by the Ojibwa for stomach cramps and blood diseases; by the Shuswap for pain; by the Tanana to make castes for breaks; and by the Thomsons for colds and contraception.
Glandulose Birch has been used by the Ojibwa for menses and after childbirth.
Downy Birch (B. pubescens) has been used by the Cree as a dermatological aid.
Water Birch (B. occidentalis) has been used by the Blackfoot for contraception.
Bog birch (B. nana) has been used by Eskimos for stomachache.
According to Peter Holmes in The Energetics of Western Herbs Birch (B. alba, B. pendula, B. lenta, B. fontinalis, B. papyracea and other species) is bitter, astringent, pungent, cool, dry, dissolving, and stimulating. He lists that it is used for detoxification for skin eruptions, joint and muscle pains, urinary infections, urinary stones, and obesity; to promote urination for edema, hypertension, (and urinary conditions already mentioned) and to promote bile flow for gall bladder congestion; to promote sweating for fevers and headache; and to promote hair growth and benefit the skin.