I like to call Ailanthus altissima the tree of good or evil. Depending on your perspective, it is either the “ghetto palm,” an invasive pest, or the “tree of heaven,” a tough, hardy street tree once planted for its beauty.
Introduced into the United States from China in 1748, it was being planted as a street tree by the 1820s. Pollution-resistant, salt-tolerant, quick-growing, and with tropical-like foliage, what was not to love about this tree?
Peter Del Tredici, senior research scientist at the Arnold Arboretum, held a tree mob on July 22 on a specimen of this tree (accession #695-80-B). Before eventually turning brown, most ailanthus trees have fruit that go from green to yellow. The tree that Del Tredici collected goes from green to red, hence the form “erythrocarpa,” which means red fruit. Del Tredici described the red on the tree as a spontaneous mutaton that appears sporadically. Continue reading →
For populations to evolve, mutations are necessary. These changes in the genetic code often go unnoticed but, when they do affect the plant’s development, horticulturists select for the ones that make plants more productive or pleasing to people. In Tuesday’s tree mob, the Director of the Arnold Arboretum Ned Friedman spoke to us about a clearly visible mutation in an eastern redbud tree close to the katsura collection (accession #10-68*B).
Friedman first introduced the botanical term “cauliflory,” a reference to plants that flower from old growth. Redbud is one of these plants. See the photo below for flower buds on the trunk of the tree.
Friedman then pointed out that one branch of the redbud tree had white flowers, in contrast to the reddish-purple flowers on the rest of the tree. Because eastern redbud flowers on old growth, he was able to pinpoint the location on the branch where the mutation knocking out the anthocyanin pigment in the flowers must have occurred. He guessed that the mutation occurred between ten and twelve years ago.
Friedman said that the mutation exists in every cell from that point on the branch onward and, as long as the flowers don’t cross-pollinate with reddish-purple flowers, will be seen in any plants grown from the seeds that develop on the branch.
This tree mob was the first in a series of three tree mobs that Friedman will lead on mutations in plants.
Next to the Hunnewell building, a shrub has been blooming bright yellow for quite a while now. Refusing to wait for spring, certain species of witch-hazel unfurl their petals as early as January. ‘Arnold Promise’ has a little more patience, waiting until the middle of February.
This past Wednesday, the Arnold Arboretum’s Nancy Rose shared the story of the plant’s origin. In 1928, William Judd collected seeds from a Chinese witch-hazel (Hamamelis mollis) growing in the Arboretum. Seven plants survived the germination process but none were like its parent. Judd deduced that the plants were a cross with the Japanese witch-hazel (Hamamelis japonica) planted nearby. The best of these plants, one that did not hold onto its dead leaves and whose flowers had long, bright yellow petals, was named ‘Arnold Promise.’