Latin Name: Asclepias cordifolia
Common Name(s): Heartleaf Milkweed
Family: Apocynaceae
Size: up to 4 feet tall.
Life Cycle: Perennial
Distribution: CA, NV, and southwestern Oregon in a variety of habitats, particularly on rocky or talus slopes, open conifer woodland, and chaparral.
Light Requirements: Full sun to part sun
USDA Zone: 7 to 10
Drought Tolerance: Fairly high once established.
Fire Risk: Presumably low while it is growing, given the fleshy leaves. If growing near a structure in a fire-prone area it is probably best to cut it down after it dies back and dries to avoid the seed fluff and dried plant matter fueling any fires.
Cultivation Notes
Once established, most milkweeds tend to be tough and adapt well to the garden. This is not one of those species. I’ve never grown it, but I have heard it is quite challenging to get established and tends to fade away easily unless it really likes the conditions it is grown in. On the other hand, nurseries do grow and sell this plant so definitely keep trying with multiple plants and positions. If you can get the plants established from seed in their permanent location, you can avoid the dangerous process of transplanting milkweeds.
Heartleaf milkweed seems to prefer full sun or part sun and can adapt to a variety of soil conditions, but generally does not appreciate wet soil for long periods of time. Do not disturb the roots if you can help it. Once established the plants should do fine without summer irrigation, but make sure they receive regular water when young. Fertilization not needed, the plants do fine even in poor and gravelly soil. Slug control may be needed when the plants are young, as heavy feeding by non-native slugs can kill young plants. Simply remove slugs in the evening and kill in soapy water.
Ecology
The interesting flowers bloom from May through July and are very attractive to pollinators. Many specialist insects feed on this plant, and this is a preferred host plant of the monarch butterfly. The seeds are consumed by birds in the winter, when few food sources are available.
Uses
Milkweeds are all toxic to some degree, containing cardenolides. This species is not edible and has been linked to livestock and human deaths. It is a great source of fiber for making ropes and nets, however, and they can probably be used as a source of natural latex like their relatives.
Propagation
Seed or by division (more difficult). Seeds are easily collected and can be sown directly in the fall or in spring. Sow just under the soil surface. Germination can be slow and often is irregular. If planting in the late spring, cold stratification will likely be needed for up to two months before germination will take place. I have found milkweeds in general to be finicky when first establishing them. Young seedlings need irrigation, at least for the first year or two. The seedlings are temperamental, so it is best to transplant carefully without disturbing the roots too much. Division is difficult as well, as milkweed resents disturbance, and division is best done in winter when the plant is dormant.