Red Mulberry: Propagating an Endangered Canadian Tree

Issue: 
Sept-Oct 2012

Red mulberry is an endangered tree in Canada and according to the Red Mulberry Recovery Strategy, only 10 sites in southern Ontario have five or more individual red mulberry trees. The species range is contracting and its numbers are declining.

One of the largest threats to red mulberry is that it freely hybridizes with white mulberry, an introduced tree common in the landscape trade, which often escapes from cultivation and readily colonizes in natural areas. Due to the high density of white mulberry trees in the landscape, the wind-pollinated female red mulberry flowers are more likely to come into contact with and get fertilized by white mulberry pollen than the pollen from other red mulberry trees, thereby producing hybrid plants rather than pure red mulberry seedlings.

The largest and most northern population of red mulberry trees in Canada exists in urban forests owned by RBG along the Niagara Escarpment in Hamilton. Due to the difficulty of identifying hybrid mulberry trees on physical characteristics alone, in 2010 RBG had the trees from their nature sanctuaries genetically tested. At least 105 trees have been confirmed as pure red mulberry, while the remaining trees were either identified as hybrid or white mulberry trees. It is these pure trees that we set out to propagate.

Propagation Strategies & Challenges
Propagating native trees isn’t always an easy task; however, our challenges were even greater than those you’d expect in typical propagation projects. In order to ensure that RBG’s red mulberry trees produced fruit with seeds that were pure, we had to devise a controlled pollination plan to ensure that only red mulberry pollen reached the female flowers.

Controlled pollination is a common horticultural and agricultural practice in which pollen is collected from one plant with desirable traits and is used to pollinate the mother (seed producing) plant. Pollination bags are generally used to cover the flowers of the mother plant to ensure that pollen from undesirable plants does not come into contact with them.

In theory, it seems fairly simple – unless you’re working with trees that have flowers 4-10 metres up in the forest canopy – and are growing on steep, rocky slopes of the Niagara Escarpment!

Methods & Madness on the Escarpment
So how did we do it? Through trial and error and dedicated interns and students! Hand-made pollination bags were acquired and we set out in the very early spring to cover the buds on as many pure red mulberry female trees as we could locate. Although the locations of the mulberry trees have been well documented and mapped, interns and students can attest that re-locating the trees in April before any of the buds had burst proved to be a challenge in itself.

We learned that the leaves and flowers emerge from the buds at the same time, and it was impossible to predict which buds would produce leaves, which would contain flowers, or which would contain both! The solution was to bag as many buds as we could. Pollination bags were placed on the branch tips, covering between two to four buds with a single bag. The lowest branches of most of our female trees were unattainable, particularly on the trees that are located on steep, rocky slopes where specialized equipment couldn’t be brought in. We used ladders when possible to reach the lower branches and cover the shoots, and in a few cases the lowest branches could be reached by hand.

The next step was to collect male flowers from pure red mulberry trees and experiment with “ripening” them so that pollen could be collected. The height of the branches wasn’t an issue when collecting male flowers, as we were able to use pole pruners to reach flowering branches up to 8 m up in the canopy. Immature flowers from several male trees were collected and were taken indoors to ripen within enclosed containers. 

Mulberry species are known to have explosive anthers that release pollen at extremely high velocities! As soon as pollen was released from the tiny flowers, it was collected and transferred to glass jars and stored under refrigeration.

Shortly after the males released their pollen, the female flowers began to open in the forest. The pollination bags were removed and each female flower was hand pollinated using a soft paintbrush. Flowers were immediately bagged again to prevent any undesirable pollen from contaminating our pure fruit. The bags also ensured that wildlife did not get away with our experimental fruit. After hand pollination, all we had to do was wait and hope that our attempt to fertilize the female flowers would be a success, and it was! By the end of July, red mulberry fruit and viable seeds were produced within our pollination bags!

With the guidance of experts from the University of Guelph Arboretum, interns and students cleaned and processed the seeds and set up germination trials. We are happy to report that close to 70% of the seeds that were collected germinated within 11 days. After being transplanted in a well-drained soil mixture, 45 pure red mulberry seedlings were overwintered at the Henry Koch Propagation Centre at the University of Guelph Arboretum.

This project has provided a unique experience for interns and students to learn specialized propagation techniques, apply a scientific approach to plant propagation, overcome the issue of hybridization, and contribute to the existing horticultural knowledge of a rare and endangered Canadian tree. It is our hope that newly propagated trees will be maintained in botanical collections at RBG, the University of Guelph Arboretum and Fanshawe College where they can be put on display and used for educational purposes. Our seedlings can also be used to replace white mulberry, and where possible can be distributed to appropriate sites within the red mulberry’s Canadian range to diversify the native trees used in our landscape.  

— Natalie Iwanycki & Laura Caddy, Royal Botanical Gardens, 2011 Jack Kimmel Research Grant Recipients

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.