Blooming trees that are hard to see

March 21, 2010

Happy equinox and happy tree watching!! Some trees in your area may have already bloomed, and others may be still to come. Some trees put on such a lovely show that no one can miss their blooms. For others, it takes a sharp eye and sometimes a hand lens to see the blooms. I’d like to show you the details of a few of them. For this post, I’ve chosen ash and elm trees.

Elms, genus Ulma, have their own family, Ulmaceae, which is part of the Rosales order in the rosid branch of flowering plants. They are very early bloomers, and as a result they may have their blooms frozen. If they are able to form fruits, these grow before the leaves. The trees have an early display of “spring green” fruits, which then turn brown and blow away as the new leaves are emerging.

The staminate and pistillate flowers of Siberian elms open while the weather is still freezing.

What’s the advantage in blooming so early and risking a late frost? These trees are wind-pollinated. The leaves would block the wind from the branches where the flowers form. Instead the trees take the risk and produce large numbers of fruits (“elm seeds”) as a trade-off.

The flowers of elms are tiny clusters of staminate and pistillate flowers that have tiny tepals, not showy petals. The stamens have dark anthers and the pistillate flower has two tiny, furry stigmas, usually light-colored. The ovaries start to enlarge soon after the trees bloom, and you can see the enlarging green disk that will become the fruits.

The samaras of elms grow quickly and mature before the leaves open. This is a Siberian elm.

The fruit is known botanically as a samara, which literally means “elm seed” in Latin. A samara is a winged fruit that is wind-dispersed. Elm fruits have a single seed surrounded by a membranous ring.

The flower clusters of the American elm are more open. Each flower has a long stalk.


The developing ovaries of the American elm are covered with short hairs.

Ash trees, genus Fraxinus, belong to the olive family, Oleaceae, which is part of the Lamiales order in the asterid branch of flowering plants. They bloom later than elms, but still quite early. Ash trees are dioecious, which means that they have their staminate and pistillate flowers on separate trees. The staminate or male trees are often planted as street trees. They have the advantage of not producing fruits, so not requiring a lot of clean-up. Their distinct disadvantage is that they produce abundant allergy-triggering pollen.

The staminate inflorescences bear two stamens per flower, like all the olive family. They start as tight, globular clusters. The more open inflorescences of pistillate flowers show the tiny green ovaries that rapidly enlarge. The fruits are another samara.

Early on, the immature stamens form tight clusters as they emerge from their buds.

Later the stamens mature as the cluster opens. The pollen flies as the anthers split open.

This pistillate ash tree has the old, dried fruits (samaras) from the previous year, along with the green inflorescence of this spring.