Zone: 3 to 9

Soil: Sand to clay

Light: Full sun to part sun

Bloom colour: Yellow

Bloom period: August to September

Height: 4 to 12 feet

Moisture: Average to moist

Attracts: Bees, butterflies and birds

Notes:

There are 4 species of Silphium in Ontario. Three of them are considered native and the other is a near native that will likely be considered a native in the not too distant future due to climate change. The plants described here look quite different from each other. It is only when you see the flowers that you realise why these plants are related to each other. The composite flowers are large and resemble sunflowers. They differ from sunflowers and many other composites in having infertile disc flowers and fertile ray flowers.

All of these species have fantastic wildlife value and are noted for their longevity. It is worth providing supports for these plants. Heavy storms can cause the stems flop over and because of their size, it can get messy.

All three species are rare in the province of Ontario and the author has only seen cup plant growing in the wild. Cup plant is also the most common species that is planted in gardens. The cup plant is one of the best native plants to grow. It serves as source of food for a wide range of wildlife and its leaves, which are joined at their bases to form a cup, store water that can be accessed by birds. In my garden, it is one of the more popular plants for Monarch butterflies. It is normally found along riverbanks where its deep roots can access the water table. It will also grow in rain fed gardens in clay or sandy soil. The plant will grow well in sandy soil, but it will need additional watering to prevent the lower leaves going brown.

The flowers appear as a spray on top of the plant so it would also be a good idea to plant some other quite tall flowers in front of it. Cup Plant is the most versatile of the three species tolerating a fair bit of shade. One note of caution; cup plant should only be placed in managed gardens. Every year, a plant produces several daughter plants that are easily pulled. In an unmanaged space, these daughter plants will form a colony. This plant can be seen growing out of control in parks where a single specimen was probably planted. If you want a single plant to have beautiful proportions, then you should allow it to grow into a space 5 or 6 feet in diameter.

Compass plant, Silphium laciniatum, is another notable member of this genus. It is easily recognisable due to its large size and deeply indented leaves with pointed lobes. The flowers occur in several or many vertical sprays above the foliage. This plant is not the best looking and it would be difficult to place as a standalone plant; however, at the back of a border behind other foliage it could look impressive. It is of interest because of the huge wildlife benefits and the orientation of the leaves in a north/south direction. The plant can be grown in sand, but it will need extra water for a few years. Over time, the taproot can extend more than 10 feet into the ground giving it good protection against drought. This matches the height of the plant which has flowering stalks over 10 feet high. It is an impressive plant.

Prairie dock is distinctive. The leaves are basal being up to 18 inches and 12 inches wide. Include the leaf stalks, and you get basal leaves that can reach to over 3 feet high. Later on in the summer, almost leafless flowering stalks sprout up to 9 feet high with numerous flowers that attract many pollinators. It is difficult to design a mixed flower garden that includes this plant. Is it a plant that is 3 feet high or 9 feet high? It is probably best to treat it as a 9 foot high plant, surrounded only with plants similar to or less than the height of the leaves. In other words, it must be the most imposing plant in the plot. It is susceptible to drought when it is young, but a mature plant is very tough due to its deep taproot. This plant grows in full sun to part sun.

Wholeleaf Rosinweed is a near native that is easy to grow. It can grow up to six feet high, but compared to the other Silphium species, it has a shorter stature. It grows wild in sandy prairies and along railroads because it has a high tolerance for drought. For these reasons, this Rosinweed is a good choice when designing a managed garden, especially if you are not intending to do much watering. You will still need to give it some room because with short rhizomes it will fill in its allotted space. This species looks great when planted en masse.

Silphium perfoliatum - Cup Plant

Silphium laciniatum - Compass plant

Silphium terebinthinaceum - Prairie Dock

Silphium integrifolium - Rosinweed