Horticultural Garden Notes: Spring Updates

Magnolias blooming in Longenecker Horticultural Gardens

Magnolias blooming in Longenecker Horticultural Gardens

Magnolias

With their dazzling display of showy flowers, magnolias are a spring favorite in Longenecker Horticultural Gardens (LHG). Although this year’s magnolia bloom is nearly done, they remain fascinating flowering trees. As a group, they are one of the oldest flowering plants (angiosperms) in the world, with closely related fossil relatives dating back to the upper Cretaceous period 130 million years ago.

Magnolia blossoms have changed little since that time. The large flowers comprise nearly identical petals and sepals (often referred to as tepals) and spirally arranged stigmas and stamens in a central cone-like structure. While usually quite fragrant, they lack a nectary and thus do not produce a sweet treat to entice pollinators like most other flowers. This may be because they pre-date the evolution of bees, wasps, moths, and butterflies. Instead, the primary pollinators are beetles, an evolutionarily older insect, which feed on the abundant pollen the flowers produce, inadvertently coating themselves and transferring the pollen to the next receptive flower they visit.

A quick update on other popular blooming trees: we expect crabapples to peak in the next week, and lilacs within 7–10 days after that.

Reclaimed Area

Section of LHG cleared of invasive shrubs and low quality trees, decorative fence designed and installed by Stephen Nystrand.

Garden staff are reclaiming a strip of land along the southeast corner of the collection, between the maintenance garages to Gallistel Woods, from woody invasive species such as buckthorn and honeysuckle as well as some lower quality trees. Wood from these removals was shared with Allen Centennial Gardens on the UW–Madison campus for an undergraduate shitake mushroom production project. New specimens, including paw paws and viburnums, will be planted in the newly opened area and incorporated into the collection.

Over the winter, garden horticulturist Stephen Nystrand designed and installed a decorative fence to help screen the maintenance area from the gardens. He made the fence from dogwood and black locust removed from Arboretum prairies.

Winter Pruning

If you visited the gardens this past winter you may have noticed brush piles scattered throughout the collection—limbs carefully pruned from garden specimens to enhance their form and vigor and provide better visitor and equipment access. In general, pruning is best undertaken during winter when plants are dormant and insects and diseases are inactive. This also allows the pruning cuts to close rapidly once plants resume growth in the spring. Crabapples, oaks, and lindens received extra pruning attention this past winter.

A pruned crabapple in LHG.

Crabapples are prone to fungal diseases, such as apple scab, that can prematurely defoliate trees and deform fruit. Because these pathogens flourish in damp, shady conditions, tangled and overlapping limbs that cast deep shade and blocked airflow were removed. This trimming should enhance flowering, foliage, and fruit display for years to come.

The oak and linden pruning focused on improving their structure and raising their canopies for better access. This is a multi-year process as we try not to remove more than 25 percent of the branches, which can weaken the tree. We also “tip” or “subordinate” limbs, cutting off only part of the branch to slow their growth. These limbs can then be removed at a later time with less stress to the tree.

Dormant pruning is particularly important for oaks. Beetles transfer oak wilt (Ceratocystis fagacearum), a fatal fungal disease that clogs the vascular tissue, so to reduce its spread oak trees should not be pruned between April and October when the beetles are active and feeding on sap.

Renewal pruning, a technique of removing about a third of the oldest, least vigorous canes to the ground, was done on some larger shrubs in the collection as well, including winterberry holly, mock orange, hazelnut, dogwood, deutzia, and ninebark. This encourages fresh replacement growth, which reinvigorates the plant.

With the return of spring and an early reawakening of plant growth, garden staff have stopped pruning and turned their attention to other projects.

—David Stevens and Stephen Nystrand, LHG curator and technician

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