Don Gordon's articles - June 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.

Non-chemical methods of weed control

(June 28, 2003)

Q: Can you suggest any “natural” dandelion killers or alternatives to the synthetic herbicides? I do dig mine out by hand, but have an area that’s overrun.

A: Greg Boland, biologist at the University of Guelph in Canada, is currently researching environmentally friendly ways to control dandelions. He has found that certain naturally occurring fungi can weaken the weed’s growth and decrease pesticide use. Unfortunately, commercial products containing these fungi are still in the testing stage and are not available to the general public.

There are several herbicidal soaps on the market that have been billed for dandelion control. These soaps seem to work best on annuals and biennials. For example, they seem to be quite effective on lawn weeds such as chickweed, crabgrass, oxalis and plantain. Their effectiveness on perennial older weeds and grasses is another story. Herbicidal soaps work to kill exposed portions of plants, but those with taproots will simply regenerate new growth. Repeated applications to weaken the plant will be necessary to kill dandelions with herbicidal soaps. These soaps do not contain any super chemicals. They are much like regular soap except that they are highly refined to penetrate the cuticle (protective coating) and cause the plant to dehydrate and die.

A few years ago a Minnesota company that marketed herbicidal soap invited one of my classes to tour their test plots. Why they allowed this visit still amazes me because it was difficult to distinguish the control from the treated plot. The class was not impressed by the product.

Some vinegar (acetic acid) based products are also billed as “natural” weed killers. They work much like the herbicidal soaps, ultimately resulting in dehydration and death. It is important to realize that both herbicidal soap and vinegar based products are non-selective. Spray only on the plants you wish to kill.

Propane weeding torches are another option for lawn weeds, but for tough weeds like dandelions repeated application are necessary.

I think the safest and most environmentally friendly way to combat dandelions and 20 plus other lawn weeds is with corn gluten meal (CGM). This natural product will not kill established plants but it will inhibit germination of new ones. Use CGM for 5 years and your dandelion and other weed population will be greatly reduced.

Mowing at the proper height is another way to minimize weed problems. Mow low and scalp the lawn and you have created a perfect bare spot for a weedy species like dandelion to germinate. Set your mower at a height of 3 inches or more and you will have fewer weed problems.

With respect to care of the lawn, many (perhaps most gardeners) do everything backwards. For example, the best time to fertilize the lawn is in the fall. Yet, statistics demonstrate that more fertilizer is sold in the spring—a time period when nutrients are not needed in abundance. The same thing is true of weed control. Many gardeners panic in the spring when the dandelions start to bloom, but study after study has shown that if you are going to use herbicides to control lawn weeds, fall treatment gives you the most effective control.

Since they are getting blamed for everything else, it would be tempting to blame Hilary Clinton and Martha Stewart for America’s desire for the perfect lawn, but the real culprits are the pesticide and the golf industry. Those of us who dispense how-to information on lawn care must also step up to the plate. We apparently have been doing a rather poor job on educating the public on how to care for the lawn and the environmental cost associated with most golf course quality turf.

Japanese maples

(June 21, 2003)

Q: What is the minimum planting depth for a Japanese maple? Here is the problem. We dug a hole about 20 inches deep and then hit what I think is rock. This seems to be the best location so I hate to try another spot. What do you advise?

A: I advise you not to plant the Japanese maple, Acer palmatum, unless you have a lot of free time and a very protected spot. These plants are recommended for USDA Hardiness Zones 5-8 (we are in 4). Bailey Nursery, the largest wholesaler in the Upper Midwest, doesn’t even sell this species. Snyder, in Trees and Shrubs for Northern Gardens, doesn’t even list it. Rose, Selinger and Whitman in Growing Shrubs and Small Trees in Cold Climates covers this species but cautions that all members of this species “are borderline hardy in cold climates” and “they require extreme winter protection.” They advise would be growers to “mound soil around the base of the stems, cover the entire plant with leaves, then add a tarp just as the ground freezes.” Another option which I prefer is to keep the plant in a container and store in a cool area where there is no chance of freezing. Gardeners at the Minnesota Landscape Arboretum use this method and it seems to work fine.

Q: What do you think of cocoa beans for a mulch?

A: The good new is that cocoa beans have a great aroma and they look great as long as it is fairly dry. The bad news is that are spendy and if we get a of rain they will start to mold, especially if there is poor air circulation or shade. Our garden has too mulch shade to use this mulch and my Scotch blood keeps telling me there are better, cheaper alternatives.

Q: I purchased a Firecracker fuchsia, Fuchsia triphylla ‘firecracker’, at Wal-Mart. The tag gives some information including perennial shrubs, tubular flowers, but fails to give information related to zone. Do you have any information related to the hardiness of this shrub?

A. There are no fuchsias hardy enough for any areas of the Upper Midwest. This particular fuchsia is recommended for USDA hardiness zones 9 to 11.

Q: I built a small pond and waterfall last year and would like to add some scouring rush around the perimeter. We have a spot on our property where this plant grows, but I am wondering how difficult it would be to transplant. Have you had any experience with this plant?

A: Equisetum or scouring rush (used by the early settlers to scour pots and pans) is fairly easy to transplant in the spring. Plants growing in wet rocky areas are the easiest to move because they usually have numerous roots that are easy to dig or hand pull.

Q: I have a plant that just finished blooming that looks just like Phlox except it has 4 petals. Can you identify this and tell me how to propagate it.

A. Dames rocket, Hesperis matronalis, has 4 cross-shaped petals that look very much like our 5-petaled wild blue phlox. Dames rocket is an Eurasia introduction that has escaped from gardens and now dots the landscape throughout the State. This biennial, sometimes perennial, is easy to propagate from seed. Simply collect the mature seed and scatter where you want it to grow.

Transplanting trees

(June 14, 2003)

Q: Recently we planted a maple tree that was obtained from our woods. It is a fairly tall tree probably between 12 and 15 feet tall. Prior to planting, we dug a good sized hole and then added 5 gallon of water with a root starter. After a couple of days the leaves started to dry up. We have been watering every day. Will it survive or do you think the tree will die? How long can a person plant trees before its too late?

A: Trying to get a bareroot 12-15 ft tree to survive transplanting after it has leafed out is extremely difficult. Even with a tree spade a “wild” tree from the woods may struggle after transplanting if it has already leafed out.

There is a good reason we have to pay the big bucks for nursery grown trees. These trees will have an extensive root system because not only have they been transplanted, but many have also been root pruned to promote formation of secondary roots. A transplanted, root pruned tree with lots of secondary roots will recover from transplant shock much quicker than a native grown tree.

Whether you purchase a nursery grown tree or dig one of the woods always remember “ the bigger the tree, the more difficult it will be to transplant.” Years ago, one of the administrators at the University decided the MSU campus needed a $900 basswood. That tree was so large it had to be transported on a semi from a Twin Cities nursery. The tree managed to survive the guarantee period, but it never recovered and after about 5 years, it was cut down.

A friend of mine paid to have a tree spade transplant a 12-15 ft sugar maple from a local woods. The tree did survive, but for the first 5 years after transplanting, growth was minimal. Even today, I would rate this specimen as second class.

The best time to transplant native trees and shrubs is in the early spring before the leaves have formed. Resist the temptation to plant large specimens. A 3-4 ft maple will have a much better chance of survival and will ultimately grow faster than a 12-15 ft. specimen. Also remember that even small trees with a taproot or poorly developed secondary roots like oaks, walnuts and butternuts are extremely difficult to transplant. A 3 ft tall walnut is likely to have a 3 ft taproot. If you break that taproot, it is all over.

Q. I appreciate your columns on pruning trees and shrubs, but I need some followup advice on pruning lilacs. I have often been told to remove old wood from lilacs, but never learned if that means dead wood or more. Would you explain?

A: When pruning overgrown lilacs or those that no longer produce abundant flowers, rejuvenate by removing 1/3 of the oldest stems each year for a 3 year period. For lilacs in good shape, just prune off the old spent flowers immediately after bloom.

Q: How good a shrub is Potentilla for our area? I would be most interested in your evaluation of Potentillas ability to maintain a full head of leaves and flowers.

A: Potentilla is good shrub for this area, but it must be properly pruned to remain attractive for an extended period. I think the yellow varieties are the most attractive. The cultivar “Coronation Triump” has excellent foliage throughout the growing season and it has one of the longest blooming periods of all Potentillas. “Dakota Sunspot” and “Yellow Gem” are also good selections.

Herbicides, asparagus, honeylocust, & green ash

(June 7, 2003)

Q: My wife recently purchased a product called Roundup to kill some dandelions in our yard. It worked, but we also now have several 8-12 inch brown spots where it also killed the grass. My question is, what is the best way to replace the grass this summer? Will it re-grow in that are by itself? Should I use “yard patch” products that include both seed and fertilizer? Or is the ground where she sprayed so contaminated that I need to dig it out and replace with new sod?

A: Unfortunately, you learned the hard way that this product is an equal opportunity herbicide—it kills everything. After about 10-14 days, you can safely reseed the damaged areas. Grass patches work, but what kind of seed they contain is usually a mystery. I would rough up the soil in the damaged areas with a rake and then reseed with a top quality grass mix designed for your sun or shade yard.

Q: I lost most of my asparagus bed over the winter and what did come up is pencil size. A gardener friend told me I would be better off to plant seed instead of the roots. I did plant 6 roots last year that all came up but none survived the winter. What is your experience?

A: I think most gardeners would be disappointed with the seed route. The reports I have gotten over the years indicate that it takes considerable patience and expertise to produce top quality plants.

I would suggest buying 1-year-old roots of one of the new disease resistant hybrids produced from Rutgers University. These hybrids are all-male and will produce 1/3 to 1/2 more than the old fashioned varieties such as Martha Washington and Mary Washington. In the nursery trade these plants are sold under a whole host of names such as Jersey hybrids, Jersey Knight, and Jersey Centennial.

When planting asparagus, excavate the hole to 12-18 inches and add lots of compost . Plant the roots 8 inches deep and 10 inches apart. Cover initially with just 2 inches of soil and continue to add more soil as the plants mature. Never completely bury the green photosynthetic portion. Leave the above ground parts when fall arrives and mulch the first winter.

Q: We have 2 honeylocust in our backyard that are approximately the same age. Both trees have always been healthy, and last fall we raked a ton of leaves. This spring, one of the trees has leafed out , but on the other one there are no leaves and no obvious signs of disease. Should I have someone come out and look at this tree, or should I fertilize it and hope it makes a comeback?

A: I would do nothing except wait. From your description, this sounds like winter damage. Adding water and/or fertilizer might help, but I doubt it. You will know in 2-3 weeks if the patient is going to survive.

Q: We live in upper North Mankato and have 4 large green ash trees that are being attacked by some sort of insect. Some leaves have already dropped. Should I have someone spray these trees? They provide a lot of shade and we don’t want to lose them.

A. I would advise you not to spray. It is an extremely rare situation when insecticide use can be justified for large trees, but second opinions are always welcome. Insect damage on ash and many other landscape trees is something we see every spring. In most cases, the damage is minimal and the pests disappear in a short amount of time.

Q: I appreciated your article on pruning trees and wonder if you could do a follow up on when to prune common shrubs such as alpine currant, bridal wreath, and potentilla?

A: Shrubs that bloom early in the growing season on old wood (last years growth) should be pruned immediately after they finish blooming. Shrubs in this category include Azalea, chokeberry, ornamental flowering currant, forsythia, lilac, and early blooming spirea (bridal wreath).

Shrubs grown primarily for foliage rather than flowers should be pruned in the early spring before growth begins. Plants in this category would include alpine currant, barberry, Euonymus (burning bush), dogwood, honeysuckle, ninebark, smokebush and sumac.

Plants that bloom on new growth should also be pruned in the early spring. Shrubs in this category would include late blooming spireas and certain hydrangeas. Some plants like Hydrangea arborescens and Anthony Waterer spirea should be pruned nearly to the ground.

Pruning potentilla is a little tricky. To keep older plants in top blooming form, it is essential to remove 1/3 of the oldest stems each year. Also, if you have the time (I don’t) you can make the plants look better by deadheading old spent flowers.

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.