island of trees: Do cucumbers grow on trees?

The Cucumber-tree, Magnolia acuminata,  is the tallest tree in the Jardin des arbustes at the Montreal Botanical Garden. Note the long leaf and the cucumber-shaped fruit, which, at this time of year, are a string raspberry red. lllustrations: Charles L'Heureux

There’s a delightful minor trend in the nomenclature of trees that tends to fantasy. There are tropical trees named strawberry, sausage and cannonball though none actually produce their namesake products. Closer to home we have the cigar tree - also known as the catalpa – so name because of the shape of its seed pod. Rather thin and long for a cigar, I’d say. Still, a compelling image.

Better named is another local tree, the cucumber-tree. At this time of year when the fruit are bright red and 5 – 8 cm long, naming the tree after the popular vegetable is a bit of a stretch. Earlier in the summer, however, when the short, green fruit clusters emerge from the pale yellow, long-petalled flowers, the name seems apt. Gherkins, or baby dills, seem to grow from the tallest of all North American magnolias.

Not only is the cucumber-tree native to North America, it is the only magnolia to make it into Canada, albeit only, in the most southwestern section of Ontario, known as the Carolinian forest, where it is a rare tree. In Montreal, I know of only three cucumber-trees. Easiest to find but the least attractive of the three, is one standing immediately in front of the McGill University School of Architecture. Tall and scrawny, standing next to a thriving Siberian elm, this tree, which loves moist rich soils, has clearly suffered from lack of water and soil compacted by years of students attaching their bikes to the narrow trunk. Still, this battered tree returns each spring.

Better located and sheltered from the madding crowd are two beautiful specimens, one in the west, at the Morgan Arboretum, growing in Blossom Corner, the other standing dead centre in the Jardin des arbustes on the western edge of the Montreal Botanical Gardens. This latter was a real surprise to me, mostly because I’ve generally bypassed that garden, which consists not only of shrubs but of a collection of ornamental trees including magnolias, on my way the Quebec forest section.

At this time of year, the beauty of this garden is subtle, the interesting fruit and yellow flowers of the late-flowering magnolias must be seen from close-up, as opposed to the riot of colour in the herbaceous gardens, just to the south. Luc Thériault, who’s the horticulturist in charge of this section, introduces Charles L’Heureux and I to the cucumber-tree. At a distance, standing tall with large bright green leaves, it would be easy to mistake it for the Northern catalpa, Catalpa speciosa. But up close, the narrowness of the leaf, as compared to the broad heart-shape of our most common catalpa, ends the similarities. In fact, the species name of the cucumber-tree, Magnolia acuminata, means coming to a narrow point.

The fruit are spectacular, like raspberry-coloured cones. Soon, the individual seed pods will split away from this structure and the seeds, most unusually, will hang down on gossamer threads. I have only read of this phenomenon, unique, among magnolias, to the acuminata, and hope to get the timing right this year to see them. Thériault believes the fruit are more interesting than the flower but tells us that the acuminata species has been cultivated into numerous yellow-flowered magnolias, including the popular dwarf magnolia, the “Elizabeth.”

Sadly, however, the cucumber-tree and its cultivars are all vulnerable to the magnolia scale, a recent and nasty bug which, if not treated, will kill magnolias of the acuminata, stellata and liliiflora species, which include the very common, pink-flowered hybrid, Magnolia x soulangiana. During walks in your neighbourhood, check the garden magnolias and you’re likely to  find some with white spots on the pale grey branches. In advanced cases of scale, whole branches will be coated in the waxy substance and the bark will have darkened due to a mildew caused by the sweet sap excreted by the insect.

The only way to get rid of scale is to kill the larvae, or “crawlers,” as they are known because they crawl to an available bit of branch then fix themselves, suck out the sap and make their sweet and waxy sun protection. Thériault says that this is the time of year to look for the tiny bugs and spray them with a plant detergent. The other period when the scale is vulnerable is in early spring, before the magnolia leaves have emerged. At that time, painting or spraying the branches with mineral (dormant) oil – huile de dormance, in French – suffocates any adults, eggs or larvae that have survived winter. If you use the oil once the leaves are out, the leaves too will be killed.

Pruning badly afflicted branches is also a solution but be sure not to compost those branches and to bundle them into garbage bags so the bugs can’t re-emerge from the ground. Of course, the best solution is to avoid buying the magnolia species attacked by this newly arrived scale species, from south of the border, and to check carefully the magnolias you may buy at the nursery.