Rutaceae | The Michigan Nature Guy’s Blog

I was asked to identify a fruiting elm shrub. It was actually a Hop-tree or Wafer-ash (Ptelea trifoliate). Ptelea is the ancient Greek word for elms. The fruit resembles a large elm fruit (1.9 to 2.5 cm [3/4 to 1 inch] in diameter) but the veins of the wings are more pronounced than an elm and are continuous, not notched as an elm is. They have been used as substitutes for hops in beer brewing.

The leaves are compound with three leaflets and are entire or sometimes with fine teeth. They are alternate which helps to distinguish it from the opposite-leaved Bladdernut (Staphylea trifolia) if one only has the leaves.

The greenish-white flowers appear with the leaves. They are small for a flower that is insect pollinated but their aroma attracts insects. I do not care for the aroma but some people think it resembles lime.

The slender twigs have short, stubby hairs. Buds are hidden under a membrane but expand and break through it in late winter. The leaf scar almost surrounds the bud and has three vascular bundle scars. The terminal bud is absent.

Hop-tree is a member of the Citrus Family (Rutaceae). Our other native citrus is prickly-ash (Zanthoxylum americanum). Hop-tree is found in southern Michigan. Keep an eye out for it this winter. It holds its seeds so it can easily be identified. Look for it in southern Michigan on the sand dunes of Lake Michigan, or in open sandy fields, and also along rivers.

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For the last forty years (on and off), I have searched for the larva of the Giant Swallowtail (Papilio cresphontes) which feed on Prickly-ash (Zanthoxylum americanum.) Prickly-ash occurs in Tenhave Woods, Oakland County and I have seen adult Giant Swallowtails in the woods for many years. I was helping identify and measure trees in the swamp forest when I found three larvae, the first that I have seen. They resemble bird droppings. A few days later, I found another group of Giant Swallowtail larvae at a highway rest area in Genesee County. This seems to happen to me often, that once I find an insect or plant for the first time I will start finding it at other places.

Giant Swallowtail Larva

Prickly-ash is not an ash but a member of the Rue family (Rutaceae) that includes the citrus. The odor of the flowers and fruit is similar to that of lime and the unripe fruit looks like a tiny lime. The plant is armed with prickles and a stand is painful to walk while doing fieldwork. The twigs and the main leaf stalk have prickles. Prickly-ash grows mainly in the Lower Peninsula but recently a few stations in the Upper Peninsula have been discovered.

Prickly-ash flowers and fruit

The adult Giant Swallowtails have an impressive six-inch (15 cm) wingspan. When it alights on a flower it must continue to flap its wings or the flower stem bends under the butterfly’s weight. This makes the butterfly easy to identify and difficult to photograph. The Giant Swallowtail ranges north to Saginaw. Go out and look for it in southern Michigan.

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Tag Archives: Rutaceae

Hop-tree: A Michigan Citrus

Prickly-ash and the Giant Swallowtail