Don Gordon's articles - July 2003

Don Gordon is professor emeritus of botany at Minnesota State University. Send questions concerning horticulture or the environment with a stamped self-addressed envelope to him at 52794 Deerwood Trail, North Mankato, MN 56003 or e-mail to osokato@aol.com. Professor Gordon is author of Growing Fruit in the Upper Midwest.

Endless Summer Hydrangea

(July 26, 2003)

"Endless Summer", the new winter hardy cultivar of Hydrangea macrophylla developed by Bailey Nursery that I profiled in early July, was supposed to be released in the spring of 2004, but I have learned that a few nurseries throughout the state have a limited number of the plants for sale now.

Thanks to a relative, I was able to obtain one of these plants from a Twin Cities Nursery and I know others have also been successful because I am receiving requests on how to care for this plant. I know one nursery who sold all of their allotment in one afternoon. These plants are hard to find and most nurseries probably won't advertise them this year, but you might get lucky if you specifically ask for this cultivar.

"Endless Summer" is a mop-head pink or blue (depending on soil pH) flowered hydrangea discovered in a St. Paul garden by an employee of Bailey Nursery, the largest nursery wholesaler in the Upper Midwest. This is the first Hydrangea macrophylla deemed hardy enough for repeat flowering in Minnesota.

Since this plant has no history in Minnesota, there is no specific guide for its culture. We do know how it is grown in USDA hardiness zones 6-9, but cultural tips for warmer climates may not work here. Here are some basics that I hope will work for those of you lucky enough to find the plant this year.

On Cape Cod and along other Eastern areas, Hydrangea macrophylla is grown in full sun, but here in Minnesota I suspect these plants will perform best in partial shade. On Michael Dirr's plant information website, he reports these plants demand "moist actually consistently moist, well drained preferably acid soils, abundantly enriched with organic matter." He also recommends mulching the plants at a depth of 2 inches.

Plants available this year have been treated with a timed-release fertilizer so little additional fertilization is necessary. I also suspect that these plant should be winter mulched at least for the first year.

Q: While living in Madelia in the early 1950s, there was a mature Kentucky coffee tree on our property. I was only about 10 years old at the time and was completely fascinated with the pods and the seeds. Are you aware of any of these trees in the Mankato area? Could you tell me how to get these seeds to germinate?

A: Gymnocladus dioica, Kentucky coffee tree, is not overly abundant, but I have seen it in a number of yards and parks in Mankato and surrounding areas. I don't know if it still exists, but years ago I discovered a small woodlot south of Mankato made up almost exclusively of Kentucky coffee trees.

This species is perfectly hardy here, but due to its size, it is not recommended for small postage stamp yards. At maturity this tree may be 50-60 feet tall with a 40-50 ft. spread. Seeds on this species are rock hard and without some "help" they may take years to germinate. Germination can be enhanced by scaring the seed coat with a file or soaking the seeds in concentrated sulfuric acid for 1-2 hours.

It is true that the early settlers used the seeds as a coffee substitute, but I can tell you from first hand experience that these beans make a terrible substitute that resembles coffee only in color.

Q: Most of the leaves on my flowering crab have turned yellow and many have fallen off. What causes this? Is there anyway to prevent this in the future? Will this kill the tree?

A. Apple scab, Venturia inaequalis, is the likely cause. This fungus is prevalent on both apples and flowering crabs. The fungus can be controlled by chemical fungicides applied in the spring, but it is not something I would recommend. This is not a life threatening disease.

A good sanitation program (removing fallen fruit and leaves) will help somewhat, but the best approach is to plant resistant cultivars. In my book, Growing Fruit in the Upper Midwest there is a list of resistant apple varieties and Dirr, Manual of Woody Landscape Plants has a similar list for flowering crabapples.

Bronze birch borer

(July 14, 2003)

Q: One of my neighbor’s birch trees has birch borer. I have been told that a local lawn services is treating trees to prevent the disease. What is birch borer? What are the signs of it? How is it spread from tree to tree. Do you recommend that I have my birch treated? If so, can I treat it myself or is it something that needs to be done professionally? Is there more than one method to prevent a tree from getting birch borer? If so, which treatment is most effective? How often must trees be treated and can a tree once infected be saved?

A. Bronze birch borer, Agrilus aniux, is a destructive pest that may be found on all types of birch, but susceptibility varies from species to species. Of the most common species grown here, Betula pendula, European birch is probably the most susceptible and Betula nigra, river birch, is the most resistant.

The adult is a small, slender dark olive-bronze beetle that is less than 1/2 inch long. Underneath the wings, the body has a dark green iridescent color. These adults emerge from leaving a characteristic D-shaped hole in the bark in May and early June in Southern Minnesota. After mating, the females lay eggs in crevices of the bark. The oval eggs are white, but eventually become yellowish in color.

When the eggs hatch, emerging larvae bore into the bark effectively girdling the branch causing it to wilt. Typical symptoms include yellowing leaves in the upper part of the tree by midsummer, followed by eventual browning of leaves and death of small branches. Birch borer damage appears first at the crown of the tree and then progressively moves downward.

Methods of Control

Bronze birch borers seem to prefer trees that are planted in stressful habitats or those that fail to get proper care during stress periods. If your tree is in the right habitat and it is vigorously growing, it is highly unlikely that bronze birch borer will be a problem. Birch species perform best in locations shielded from intense sunlight; thus, east and north sides of the home are best.

Birch are like Clematis in that they perform best with cool roots and a warm top. Birch also have very shallow roots. This should be the first tree to water during drought periods and, of course, mulching will also aid in water conservation.

Planting resistant species such as Betula nigra is also a good way to avoid this pest. I prefer the cultivar “Heritage” since it is quite resistant to the borer and it has very attractive bark.

Birch trees are not heavy feeders. Fertilization will not help in prevention of borer infestation unless nutrients are lacking.

Bronze birch borer adults are only active from May to July. Do not prune during this period because adults flying from tree to tree will seek out cut surfaces.

Preventive chemical treatments of birch borers are not recommended. Chemical treatment of infected trees is recommended, but this is best left to the professionals. Some of these chemicals are restricted use and can only be used by licensed professionals. These chemicals will be most effective if used in conjunction with proper cultural techniques.

There are two Extension publications that I can recommend to anyone having birch problems. Birch Leafminers by Jeffrey Hahn and Mark Ascerno is essential for clearly distinguishing birch borer from leaf miner damage. Leaf miners rarely warrant treatment and damage usually does not exceed 40% of the leaves. Treatment is more for appearance.

The Bronze Birch Borer and Its Management by Wawrzynski et al is one of the main references used for today’s column and the best Minnesota publication for preventing and managing this pest.

Q: While traveling out East I saw a tree with pink leaves and very green veins that was identified as some sort of beech. Is there any chance these plants will grow here?

A: This sound very much like one of the cultivars of the the European beech, Fagus sylvatica. Some books list this species as being hardy in zone 4, but I have seen only a handful of survivors in the whole State. I tried 2 specimens in the arboretum at Minnesota State University but both failed. The Landscape Arboretum has at least one survivor, but it is located in a very protected spot.

New hardy Hydrangea

(July 7, 2003)

Come next spring I believe the most sought after new landscape plant will be Hydrangea macrophylla Endless Summer TM. Those of you who are up on scientific names may wonder about this prediction because this species is recommended for USDA hardiness zones 6 and 9. Endless Summer is the exception to this recommendation. This cultivar of bigleaf hydrangea is fully hardy here and it will bloom all summer.

In the past attempts to grow this species in Minnesota have been a big disappointment for most gardeners. It wasn’t that difficult to get the plants to survive a Minnesota winter, but unless the plants were grown next to a heat pipe or a warm foundation, it was next to impossible to get the them to flower. Flower buds are formed on old wood in the fall, but they were just not hardy enough in the winter. In contrast, flowers on Endless Summer form on new growth, thus winter dieback of above ground parts will not affect blooming.

Like many new plant introductions, Endless Summer is not the result of genetic engineering or even selective breeding. It is a natural mutation that just happened to appear in a St. Paul garden. This mutation caught the attention of Bailey Nursery, the largest wholesale nursery in the Upper Midwest, and they have been testing it for hardiness and boom reliability since 1983.

Michael Dirr, author of Manual of Woody Landscape Plants, saw this mutation growing in the Bailey test plots and he encouraged the nursery to propagate the plant; he even suggested the name. In 2004, Endless Summer will be released nationwide.

The flowers on Endless Summer are pink in basic or alkaline soils and blue in acidic soils with sufficient aluminum. Adding aluminum sulfate to make soils acidic may help develop a better blue color in some cases. According to Dirr, a pH range of 5.0 to 5.5 is listed as satisfactory for producing blue flowers and a pH of 6.0 to 6.5 or higher is okay for pink blooms.

In the summer, bigleaf Hydrangea is the eye-popping show stopper on Cape Cod and in other Eastern maritime gardens. I have also seen some of these spectacular hydrangeas in botanical gardens in Portland, Seattle and in Vancouver. Come spring, thanks to Endless Summer, we will also be able to rave about these plants in Minnesota.

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My Scotch father would turn over in his grave if he knew his son just spent $18 for 20 ounces of Coyote urine powder. Yes, folks, for the first time I am being forced to deal with the deer problem. In the past I have been willing to share a portion of our plant smorgasbord with these beautiful creatures, but I am not willing to donate everything. Right now we have a pair of new fawns that eat, sleep and play on our lawn. They seem to have no fear of us and they do love our scores of Hosta. I know there are several other deer repellents on the market, but I decided to try the organic approach first. Tomorrow, when they smell that Coyote urine our problems will be solved, won’t they?

All articles © Don Gordon 2003. For information about reprints, copies, or use of the material, contact Don Gordon, 52794 Deerwood Trail, North Mankato, MN 56003 or e-mail to osokato@aol.com.