Q: The shrub in the photo is on the north side of a St Paul, Minn., home. It retains its waxy leaves in the winter and has colorful flowers in May. Can you please identify this bush? — Erik H.
A: The shrub in the photo is a Rhododendron, most likely the cultivar P.J.M., which is probably the most winter-hardy Rhododendron for Upper Midwest zone 3 landscapes. Identifying features are the waxy leaves that remain during winter, and the lavender-pink flowers, blooming in May.
Rhododendrons are close cousins of azaleas. Both are in the same botanical genus, but Rhododendrons are broadleaf evergreens, meaning they retain their foliage year-round, while azaleas drop their leaves in autumn.
The P.J.M. Rhododendron was developed in 1939 and the originator, Edwin Mezitt, named the shrub with his father’s initials. One of P.J.M.’s parents is a Siberian species, making the cultivar winter-hardy to minus 40 degrees, unlike most other Rhododendrons adapted to milder climates.
Although P.J.M. is winter-hardy, this 5-foot-high shrub has additional requirements to thrive. Hot sun and hard-baked clay soil will quickly cause decline. For best results, plant in filtered sun, or a morning-sun-only location, and incorporate generous amounts of peat moss into the surrounding soil and mulch with shredded wood products.
Q: I have a question about fertilizing hydrangea shrubs. I read somewhere to use 10-10-10 fertilizer in the early spring. Is this good advice? — Laurie S.
A: Yes, it’s good advice. 10-10-10 is a well-balanced fertilizer for perennial flowers, vegetables, and shrubs, including hydrangeas. Apply a half cup around the base of each shrub about May 1. If possible, work the granular fertilizer into the top few inches of soil, and water well to activate. Repeat again June 1, and then don't fertilize for the remainder of the season.
Fertilizing flowering shrubs early in the season provides strong nutrition for blossom formation on shrub types that flower on new wood, like hydrangeas, and it increases next year’s buds on types that flower on old wood, like lilacs. Discontinuing fertilizer in early July allows shrubs to slow down in late season, hardening growth against winter injury.
Q: Last July, landscapers planted a hydrangea tree that was covered in blooms which have dried up over winter. Do I cut off the dried blossoms this spring? I’m not sure what’s best. — Rose G.
A: The flower clusters of hydrangeas persist for most of winter, adding interest to the snowy landscape, which is a good reason for not removing them during fall cleanup. They should be removed before new growth starts in spring.
On a pleasant day in late March or early April, prune away the pyramidal-shaped flower clusters of paniculata hydrangeas, like Vanilla Strawberry. While you’re at it, it’s best to prune the entire shrub by cutting back branches by 30 to 50%, which automatically removes the old flowers. Heavy pruning of paniculata hydrangeas promotes better flowering on sturdy new stems, with less tendency to flop.
On arborescens hydrangeas, with the large, round white or pink flower clusters, prune the entire shrub back to 12 inches or less above soil level, which automatically removes the old flowers. Arborescens hydrangeas, like Annabelle and Invincibelle, die back to near ground level each year, so removing old, upper branches is a must for healthy growth.
If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at firstname.lastname@example.org. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.