Don Gordon's articles - May 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.

Clematis Culture

(May 31, 2003)

For flowering beauty, few vines can rival Clematis. Worldwide there are literally thousands of species, and despite our harsh climate, an amazing number of these species will do quite well in the Upper Midwest. Selecting which species to plant here is no easy task. Regional nurseries sell more than 100 types.

I have long admired these plants but they have not been been part of our garden simply because we did not have a spot for them. That recently changed when we decided to eliminate a 30 year old row of Beta grapes. Betas are great grapes for jelly, but keeping them along with a 75 ft row of wine, juice and jelly varieties seemed like a bit of overkill. Eliminating those grapes opened up a spot for a trellis on a eastern garage wall that would now get full sun.

There is an old saying that clematis “like their heads in the sun and their feet in the shade.” That makes an eastern exposure ideal for clematis. These plants must have a minimum of 5-6 hours of sunlight, but intense afternoon sun is likely to keep the soil too warm.

A great guide for determining which Clematis to select is “Growing Perennials in Cold Climates” by Mike Heger and John Whitman. These authors cover nearly 100 cultivars and species. They provide information on hardiness, flower size, height and whether the plants bloom on old or new wood. In our area clematis varieties that bloom on new wood are the most reliable. They also use a 5 star rating system to rank the overall performance of the varieties. Examples of excellent 5-star rated species include: Clematis intergrifolia, fragrant tube clematis; Clematis x jackmanii, Jackman’s clematis; Clematis tangutica, golden Chinese clematis; Clematis terniflora, sweet autumn clematis; Clematis texensis, scarlet clematis. Examples of excellent 5-star rated cultivars include: ‘Abundance’ (red), ‘Comtesse de Bouchard’ (rose pink), ‘Etiole Violette’ (deep purple), ‘Henryi’ (white), ‘Niobe’ (Dark ruby red) and ‘Ramona’(lavender blue).

The Minnesota Landscape Arboretum has an extensive collection of clematis varieties, but Donahue's in Faribault has the largest collection in the State with over 100 varieties. Donahue's sells wholesale and retail and they are the largest producer of clematis in the Upper Midwest. One can view many of the varieties they produce on the web at www.donahuesclematis.com, but I recommend a visit to their greenhouses.

When planting trees, shrubs or vines, I have always urged readers “to dig a 5 dollar hole for a 50 cent plant.” No species illustrates that philosophy more than clematis. Prepare the proper planting site and clematis may reward you with decades of enjoyment. The hole for clematis should not be a cheap $5 one; it needs to be Cadillac-Rolls Royce caliber. For example, before planting the Clematis we purchased, soil was excavated from an area 3 x 3 feet and it was equally deep. Topsoil was saved and subsoil was discarded. The hole was refilled with equal parts of compost and top soil. Proper soil preparation is the single most important step in long term success with clematis.

Clematis is sometimes available bareroot, but most commonly it is sold in 4 inch or 1-gallon containers. We opted for the larger container because roots on these plants are very fine and smaller sized plants have tiny fragile stems.

When planting, place the rootball 2-3 inches below the soil surface and water. Mulch with organic material such as shredded bark, grass clippings, etc. sometime in June or after the soil warms up. Keep the mulch away from the stem of the plant to avoid potential fungal diseases. The mulch will keep the roots cool, help control weeds and conserve water. Fertilize established plants each spring with 1/2 cup of 10-10-10 or an equivalent organic fertilizer.

Clematis have few disease problems but yellowing foliage is often a problem when soil pH is elevated above 7.5. This is often caused by lime leaching from masonry walls. This can be corrected by adding acid peat and acid based fertilizers.

How to prune clematis often depends on whether the plants flower on old or new wood. You can’t go wrong if you delay pruning until late spring after growth has begun. Then simply prune back to live wood.

Pruning Trees

(May 24,2003)

Question: Would you please recommend the best time of the year to prune the following: birch clump, sugar maple, Crimson king maple, American black walnut, Redmond linden, and other common tree species. Also, could you provide a guide on how to do the pruning? My present idea of pruning is that the tree should have one main stem and all branches coming off of it should be smaller. It seems to me this will avoid double or split trunks in years to come.

Answer: Anyone can remove branches, but doing it properly to enhance the appearance of a tree requires some knowledge and skill. There is a town in Indiana where I grew up where one company has been the “butcher of branches” for the past several decades. There is hardly a tree in this entire town that doesn’t have the “flattop” look. Beware of any company that advertises “topping” of trees.

At planting time limit pruning to removal of broken or dead branches. Do not cut back the leader. Begin the shaping of the tree during the proper pruning season (listed below) following planting. Remove branches that rub or are too close together. Branches with narrow crotch angles or those which arch back towards the leader should also be removed. Remember, those branches that come off at right angles will be the strongest and the least likely to break or split in a storm. Water sprouts and sprouts from the base along with lower limbs that would prevent rodent, rabbit or sunscald guards should also be removed.

Pruning large trees is a dangerous business best left to professionals who have the right equipment. The only pruning I attempt is what can be reached from the ground with a hand or power pole pruner. I am fan of the power pole pruners that are readily available from area rental stores. These are great for removing lower branches on more mature trees to provide more space under sidewalks, lawns, flower beds, drives and streets.

All the “old” pruning guides advise making flush cuts, leaving no stubs. We now know that pruning cuts will heal quicker if the cut is made at the branch collar. This collar is easy to spot. It is just a small swelling area where the branch originates. Also, forget about wound dressings or paint (except in very rare cases) because they are unnecessary and actually impede the healing process.

When shortening any branch, always make the cut at an angle just above a bud. Also, look at the direction of the remaining bud. The way it is pointed will determine the direction of new growth.

When to prune

• Bleeder trees, such as walnut, butternut, birch, ironwood, blue beech, maples and box elder are best pruned in the spring after the leaves have appeared. Now is an excellent time to prune these species. Pruning during the bleeding stage (late winter or early spring) will not harm the trees, but it is unsightly.
• To minimize fireblight problems caused by the bacteria, Erwinia amylova, prune rosaceous plants like apples, crabapples, pears, hawthorns, mountain ash, and cotoneaster in late winter (February is an excellent time.
• To minimize oak wilt problems caused by the fungus, Ceratocytsis fagacearum, do not prune oaks during April, May or June. Emergency wound cuts after storm damage during this period should be sealed with latex paint. Late winter is probably the best time to prune oaks.
• To minimize stem canker problems caused by the fungus genus Thyronectria, do not prune honey locust during the summer months. Late winter is the best time to prune this species.
• The following trees should be pruned immediately after they finish blooming: apricot, chokecherry, cherry, flowering plum, lilac, and magnolia.
• Most other deciduous trees not mentioned above should be pruned in late winter before growth begins in the spring.
• Spruce, fir (Abies), and douglas fir are probably best pruned in late winter, although some spring and summer pruning is not harmful.
• Pines, should only be pruned in the spring before the new flush of growth (candles) reaches maturity. Prune up to 2/3 of the new candles if you want more dense growth. Do not prune older wood.
• Arborvitae, junipers, yews and hemlocks are best pruned from early spring through the middle of the summer.

For additional information on pruning both trees and shrubs, the Extension service has an excellent, inexpensive publication by Mike Zins and Deborah Brown.

Selecting Hedges

(May 17, 2003)

Question: Could you advise me on a 50 feet hedge we want to plant in full sun on edge of a vacant lot? The hedge would create a screen for the house next door but we don’t want to block the neighbor’s windows too much. This means we need to keep the hedge, without pruning, between 5-8 feet tall. Low maintenance and hardiness are mandatory, but cost is also a factor. We have two different suggestions. One is Holmstrup arborvitae that stays at 5-7 feet, the other is Chinese lilac. What do you think? Any other suggestions?

Answer: Thuja occidentalis ‘Holmstrup’ was introduced from Holmstrup, Denmark, by A. M. Jenson. This cultivar is very compact with an upright habit. In the right location it might be okay, but it would not be my choice, and here is why. Holmstrup arborvitae is not that resistant to winterburn.

Dirr, in Manual of Woody Landscape Plants, recommends Thuja occidentalis ‘Techny’ as “probably the best form for northern gardens.” I couldn’t agree more. Techny is highly resistant to winterburn and stays dark green all year long. This is an excellent variety but I would not recommend it for a hedge because deer absolutely love all varieties of arborvitae. I would recommend Chinese lilac, Syringa x chinensis. This is an excellent plant for hedges. Some pruning will be necessary to keep the plants under 8 feet. I consider its susceptibility to powdery mildew to be a minor problem. . Few lilacs can match this species in terms of number of flowers produced.

I have said this many times, but if you are thinking of planting a hedge, visit the Minnesota Landscape Arboretum at Chaska. They have one of the largest demonstration hedge gardens in the United States.

Here is another tip. If you live in an urban area, be sure and check with city officials regarding property line set back requirements before planting. I know one gardener who ended up replanting.

Question: I have access to fresh rabbit manure and would like to use it in a trench before planting asparagus. Would this be okay to use?
Answer: No, animal manure of any type should be “aged’ or composted before use on vegetables or flowers. Fresh rabbit manure is especially “hot”. This means it has a higher percent of nitrogen than cow, horse, pig or chicken manure. It is a great source of organic matter, but don’t use it fresh.

Question: Which variety of asparagus do you think is best, Jersey Giant or Jersey Knight? How far apart should asparagus be planted?

Answer: I don’t know which variety is best. Both of these are excellent all-male varieties that will produce about 1/3 more than older selections such as Mary Washington. Asparagus crowns should be planted 18 inches apart.

Question: I have been reading about the dangers of pressure treated wood. I have a swing set for the kids made out of this stuff and I am wondering if I should get rid of it? What would you do for your kids?

Answer: Treated wood containing CCA (copper,chromium, arsenic) is being phased out due to cancer risks, but there are ways to minimize the risk for existing lumber containing these chemicals. I would advise you to keep the swing set and seal it once a year with polyurethane.

Question: One of the discount stores is advertising “environmentally safe” lumber that is treated with ACQ? What is this and is it really safe?

Answer: ACQ stands for Alkaline Copper Quat. Quat is an organic copper based fungicide. Alkaline is a natural bases. This preservative can be safely used in wetlands, and all the reports I have read indicate that ACQ treated lumber is kid safe.

Reducing Phosphorus Pollution

(May 10, 2003)

The Minnesota Pollution Control Agency (MPCA) and the Minnesota River Board wants advice from local citizens on how to clean up the Minnesota River. They will hold a public meeting at 1:00 June 2 at the Country Inn & Suites in Mankato to specifically look at ways to reduce phosphorus discharges. Sources of phosphorus include municipal wastewater treatment facilities that enter into the river, runoff from feedlot sites, runoff from farm fields, urban runoff, storm water discharges from industries and cities, and failing sewage treatment facilities. Here are some obvious solutions and I am sure readers will have others.

For farmers, investigate the article on organic farming published by University of Minnesota researchers in the March-April 2003 issue of Agronomy Journal, a publication of the American Society of Agronomy. The study conducted here in Minnesota compared organic and conventional methods and found nearly equal net returns on corn and soybeans.

Another must read for farmers and all who care about agriculture is “Intensive Corn-Soybean Agriculture not Sustainable” by University of Minnesota soil scientist, Gyles Randall. Mankato Area Environmentalists have posted this article on the web at mankato-enviros.org.

For home gardeners, consider switching to a phosphorus free lawn fertilizer. Last year, the legislature enacted a Phosphorus Lawn Fertilizer bill which goes into effect in 2004. Basically, the bill sets up phosphorus-free fertilizer zones in the Twin Cities metropolitan area and Crow Wing County. Outside of these areas lawn fertilizer is restricted to three percent phosphate content. In lawn soil tests conducted in this area, phosphorus is rarely needed. Phosphorus free lawn fertilizer is now readily available throughout the State. Unless there is a proven need for additional phosphorus or potassium, corn gluten meal is the obvious choice for a lawn fertilizer. It contains no phosphorus, inhibits weed germination and has enough nitrogen (10%) to make other other types of fertilizers unnecessary.

Some urban and rural lawn mowers need to change the way they mow the lawn. Blowing grass onto roadways is against the law. When it rains that grass may up in a storm sewer or roadside ditch that goes directly to the river.

Question: Last week I was in a friend’s garden and admired her Virginia bluebells. I asked how they were propagated, but she had no idea since her family recently inherited the garden. I never see these plants in local nurseries. Can you tell me how to propagate these plants or suggest a nursery where they are sold?

Answer: Virginia bluebells, also known as cowslip, Roanoke bells and scientifically as Mertensia virginiana, are rather easy to propagate by two methods. Established clumps can be divided in the early spring (now), but the roots are very fine, so try to move the plants with a root ball of soil. After transplanting, pinch off all flowers and water for 2-3 days. These plants are also easy to grow from seed. Collect fresh seed this spring and sow where the plants are to grow. I have scores of these plants and they all came from a handful of seed collected from a friend’s garden.

Question: I would to like to plant some yews around the foundation of our new home, but we don’t have any trees to provide shade. All my books say to prevent winter browning they must be planted where they will be protected from winter sun. Can you suggest any varieties that will grow where they don’t get protection from winter sun?

Answer: It is a myth that all yews must have protection from winter sun. The big culprit is not sun but drying winter winds and low temperatures. Here in Minnesota there are few varieties that resist winter browning. Here are the best species and varieties I have found for this area. Taxus cuspidata ‘Dark Green Spreader’ is a Japanese yew that Jim Koberoski of Edenvale Nursery suggested to me about 20 years ago. It is highly resistant to winter burn and it has a wonderful deep green color all year long. ‘Cross Spreading’ is another excellent Japanese yew developed by Cross Nursery of Lakeville, Minnesota, but it is difficult to locate.

Taxus x media ‘Taunton’ is probably the best Angojap yew for Minnesota. Three of these plants have graced the north foundation of our home for the past thirty years and they just keep getting more attractive every year.

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.