Zone: 3 to 8
Soil: see notes
Light: Full sun to part sun
Bloom colour: Yellow
Bloom period: Late spring
Height: 1 to 3 feet
Moisture: Dry to medium
Attracts: Hoverflies and bees
Notes: Considering the size of these plants, Zizia sp. produce quite large yellow flowerheads for a few weeks in June. Both species mentioned here are native to a large number of provinces and states East of the Rockies.
The flowerheads are called umbels in which each small flower is on a small stalk and spread outwards from a central point. The tiny open flowers are good for small bees as well as hoverflies; the larvae of these latter insects are often beneficial by eating aphids. What visits the flowers of this plant is really determined by the spring weather. Warmer springs will bring in a wide range of insects that can take advantage of the flowers.
There are two species in Ontario. Zizia aptera is a smaller plant that does well in dry sandy soil while Zizia aurea can grow up to 3 feet and grows in clay soil or moist soils. Zizia aptera is rather rare in Ontario. Growing consistently, but slowly, in difficult soil conditions, it takes several years to develop into a sizeable plant. Zizia aurea not only grows faster, but it may also flower for a second time in the fall if you deadhead the flowers.
These plants flower when most of the garden is still quiet. They are quite easy to grow from seed by sewing in late fall. Zizia aptera combines well with Prairie Smoke, which has a similar season of interest. Heartleaf Alexanders are short enough to be grown at the front of the border in a wide strip to make an impact. Golden alexanders are slightly taller and can be grown in the middle of beds or in front of taller perennials. Both of the these plants also do well in containers. Later on, in the summer when the flowers have gone to seed, these plants provide further interest by acting as a host plant for black swallowtails. I find these caterpillars on my patch of heartleaf alexanders every year.
If you want a larger patch of these plants, just let the flowerheads mature so that the seeds develop. There is no need for any extra work as you will find daughter plants popping up in different parts of your garden. Zizia aurea reseeds particularly easily and you might consider cutting off the seedheads before maturation as an easy way to cut down on weeding. Golden alexanders are amongst the most beautiful and garden worthy of the umbellifers and their name certainly does them justice. They should be used as a native alternative to yellow Achillea, which is sold in garden centres on a regular basis.