Cucumber Tree

Issue: 
September-October 2002

QUEEN OF THE CANADIAN CAROLINIAN TREES and our only native species of magnolia, the cucumber tree is one of the rarest native trees in Canada. It grows in a few remnants of our Carolinian forest in southern Ontario. There, it is represented by some fairly large trees, less than 1 metre in diameter with crowns reaching to the canopy about 25 metres above the ground. It thrives best on rich, moist soils, but most such areas have been converted to agriculture and drained.

As a tree for landscaping, growing it is not for the faint of heart. It’s not that it is particularly slow growing, but in an open and park-like setting, it becomes magnificently immense. The forest growth form becomes emphasized, even exaggerated. The lower branches swoop downwards from the trunk, then curve gracefully upwards. When these branches are close to the ground, the embrace of the crown with its large simple leaves is shady, cool and quiet. Most trees that grow in such places have had the lower branches pruned so that the distinctive form is not realized. To grow cucumber trees to such magnificence must take many, many years and require commitment to the space. It strikes me that a single tree would require about 1/4 acre to display its full potential.

Although it is naturally restricted in Canada to southern Ontario, its range extends throughout the Carolinian zone of North America. I am not aware of how far north it may be grown, but an old tree thrived at the University of Guelph, presumably since establishment of the campus. When it was finally removed, its trunk and limbs, held in place for years with iron bolts, had succumbed to the ravages of time. The usual horticultural magnolias or “saucer magnolias” (magnolia x soulangiana), so well known for their flamboyant early spring blossoms, can be grown in zones C5 and NA4.Why is it called the “cucumber tree”? The answer to that lies in the fruit. But first, let’s look at the flowers, how they are pollinated, move on to how the fruit develop and ripen, and finally at the way in which the seeds may be dispersed. Understanding the reproductive ecology of cucumber trees is crucial to its conservation and survival.The flowers (see picture above) open as the quite large leaves expand. The flowers are rather inconspicuous, despite their large size. Their greenish-yellow petals may be up to 8 cm long. When the buds first open the petals stand upright and the bulky and receptive pistillate assembly resembles a complex squid-jigger with its set of many backward hooking, pale coloured, stigmas. Unlike most flowers, they are first female in function (a condition called protogyny). As the flowers age over a few days, the stigmas become brown and non-receptive while the anthers split and shed pollen as the petals reflex.

It strikes me that a single tree would require about 1/4 acre to display its full potential.

Although little is known about the pollinators, we have found beetles in the flowers. The flowers act as temporary traps for the pollinators. The pollinating insects can enter the flowers through the gaps between the petal bases, or from the top. However, once in the female-stage flowers, the pollinators can not escape. The inside surfaces of the petals are covered with microscopic wax rollers that cause the insects to fall back against the stigmas every time they try to climb out. Once the pollen is released and the petals reflex, the pollen-dusted insects are released and can visit another flower. Thus, pollination takes place, but studies in Ontario indicate that pollination must be between trees for fruit-set to occur.The petals fall, and if pollination and fertilization are successful, the fruit starts to form. It is a gherkin-like structure that changes from green to red as it ripens. It is these fruit that give the tree its name. Closer inspection reveals a complex of carpels. When these ripen and open late in summer and early fall, the red-orange coated, oily, scented seeds are exposed. They hang by fine threads from the “cucumber,” ready to be removed by dispersers. It is assumed that birds such as waxwings may be the primary consumers and dispersers, but we have no proof.Without cross-pollination by insects and seed dispersal by birds, the cucumber trees we have will age and eventually die. Many of the easily accessible trees show signs of senescence, with missing tops and broken limbs. In a very few forest fragments, there are established populations with old and young trees. We do not know how old a cucumber tree must be before it can reproduce, but initial observations suggest at least 20 years. With its ecological mutualists and space for seedlings to become established, the cucumber tree could survive naturally. With proper planning in public parks and private lands, artificial populations can be established, nurtured and appreciated.

— Peter Kevan, University of Guelph.

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