Keys to Nature
Key to Asclepias of the Chicago Region

© Christopher Noll- Asclepias syriaca - Keys to Nature Project

Key to Asclepias of the Chicago Region
John & Jane Balaban, Bil Alverson, Marlin Bowles
This key identifies both common and rare milkweeds of the Chicago region. Since flower color is variable for many of these species, there may be more than one path through the key when identifying a species. Jennie Kluse selected representative images for this key and managed photo collections. If you wish to share your own photos, please contact the authors of the key.


Key Structure:
1. Flowers yellow to orange; full sun in dry to moderately moist sandy or loamy soils of prairies and woods - A. tuberosa
1. Flowers green to white - 2.
1. Flowers pink to deep rose or purple- 3.

2. Flowers more green than white - 4.
2. Flowers more white than green - 5.

3. Leaves long and narrow (more than four times as long as wide); umbels with many, small flowers (to about 8 mm long); open sunny wetlands - A. incarnata
3. Leaves wider (usually two to three times as long as wide) - 6.

4. Leaves narrow (typically 5 to 10 mm wide); the several to many umbels stalked and erect; sandy soils of dry prairies and fields and sometimes in fine-textured soils - A. hirtella
4. Leaves wider (the larger ones at least 10 to 15 mm wide) - 10.

5. Leaves narrow, grasslike (typically 1 – 2 mm wide); plant short (only 30 – 50 cm tall); common in sunny pastures, roadsides, and abandoned fields - A. verticillata
5. Leaves broader (typically 10 mm wide or more); plant taller – 12.
5. Caution: Some normally pink-flowered species can have white flowers. If your plant doesn't look like either of the choices to the left, key it as if it were pink by selecting this option - 3.

6. Leaves with very wavy margins; lobes of leaf bases overlap ("clasp") the stem; species of open, sandy, disturbed ground – A. amplexicaulis
6. Leaf margins not conspicuously wavy; leaf bases not clasping - 7.

7. Flowers strikingly deep rose to purple; plants to about 100 cm tall; single umbel at top of plant, or sometimes with additional axillary umbels; uncommon species often in mesic savannas, fine-textured soils – A. purpurascens
7. Flowers generally light pink, plants 150 – 200 cm tall, some of the umbels in the leaf axils - 8.

8. Leaves without petioles (“sessile”); leaves have bright pink midveins and are usually held at an ascending angle; full sun on clayey or loamy soil, in moist prairies – A. sullivantii
8. Leaves with petioles (though sometimes short, only 2 – 3 mm) – 14.

10. Leaves without petioles; umbels solitary, terminal, and drooping; extremely rare species of dry-mesic prairies- A. meadii
10. Leaves with short petioles – 11.

11. Plants short (10 – 30 cm tall), covered with long, tan hairs that stand out from the stem; umbels solitary and terminal; flowers smaller (to 8 mm); very rare species in gravel soils of dry prairies – A. lanuginosa
11. Plants taller (30 – 100 cm), sometimes covered with short white hairs that lie close to the stem; umbels two or more and axillary, stalkless and drooping; flowers larger (at least 9 mm long); uncommon species of both fine-textured and sandy soils – A. viridiflora

12. Flowers only about 6 – 7 mm long; swamps and wet ground – A. perennis
12. Flowers usually 10 mm long or longer – 13.

13. Tall plant (100 – 150 cm) with larger leaves (5 – 10 cm wide and 10 – 20 cm or more long) and several drooping umbels; uncommon in partial shade of moist woods – A. exaltata
13. Small plants (20 – 70 cm tall) with smaller leaves (1 – 4 cm wide by 4 – 10 cm long); rare species in prairies - 18.

14. Flowers small (to about 8 mm long); open sunny wetlands – A. incarnata
14. Flowers larger (10 – 20 mm long, or more) – 15.

15. Umbels drooping, on thin stalks (pedicels); flowers often white or greenish; uncommon in partial shade of moist woods – A. exaltata
15. Umbels held erect on thick pedicels; flowers usually creamy, reddish, or tinged with pink; – 16.

16. Stalks and branches of umbels, upper stems, and outside of flowers covered with a very dense mat of white hairs; flowers large (typically about 20 mm long); species introduced from states west of Illinois, uncommon in moist prairies – A. speciosa
16. Flowers and foliage with some hairs but not in dense mats; flowers smaller (typically 10 – 14 mm long) – 17.

17. Plant to only 60 cm tall, with single, slender stem; leaves to 7 cm long; very rare species in rich, sandy prairies – A. ovalifolia
17. Plants usually 100 – 200 cm tall; leaves usually more than 7 cm long; umbels usually several; our commonest milkweed, found even along roadsides and farm fields - A. syriaca

18. Leaves with petioles; umbels mostly erect, solitary to several; very rare species in rich prairies - A. ovalifolia
18. Leaves without petioles; umbels drooping, solitary, terminal; extremely rare species of dry-mesic prairies- A. meadii


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This page was last modified on: Thu, 29 Oct 2009 13:40:53 -0600.