Yard & Garden Line News
Volume 6 Number 9                                                               June 15, 2004
Features this issue:
 Fast, Faster, Fastest--Tree Growth Rates Defined
Oak Anthracnose
June Garden Tips
Editorial Notes


Fast, Faster, Fastest--Tree Growth Rates Defined
Nancy Rose, Regional Extension Educator, Horticulture

Linden flowers Photo credit:
Beth Jarvis
A Master Gardener recently posed the question "What does it really mean when we say that a particular tree species is fast-growing? How fast is fast?". Well, like a number of other common terms in horticulture (e.g. cold hardiness, mature size, drought tolerance) it's difficult to come up with a precise guideline for growth rates. But let's at least start with a broad generalization: fast-growing trees put on 2 feet or more of growth yearly, medium-growing trees average 1 to 2 feet of growth per year, and slow-growing trees add less than 1 foot of height each year.

But (and you knew that was coming), there are a whole bunch of variables that can and do affect tree growth rates. Certainly the most basic one is genetics - some tree species are just built for speedy growth. Tree species native to floodplains, where soil is usually moist and rich, generally have faster growth rates than upland tree species that usually grow in drier, less fertile soils. Examples of floodplain species include white willow (Salix alba; can grow 3 to 4 feet/year), cottonwood (Populus deltoides; can grow 4 to 5 feet/year), and silver maple (Acer saccharinum; averages 2 ½ feet per year).

Of course there can also be big differences in growth rate among cultivars of a species. For example, Norway spruce (Picea abies) is a medium- to fast-growing tree species (average 1 ½ feet/year) that can reach heights of 60 or more feet. In stark contrast, the dwarf Norway spruce cultivar 'Little Gem' (P. abies 'Little Gem') grows less than 1 inch per year on average, taking more than 10 years to reach a foot in height. 'Little Gem' and many other dwarf conifer cultivars were selected from witches'-brooms, branch mutations with extremely slow growth that often develop high up on otherwise normal-growing conifers.

Red maples in fall. Photo credit:
Beth Jarvis
Climate is another variable in growth rate, even among plants of the same species. Annual precipitation and length of growing season (something we know about here in the Frozen North) can affect growth rate. For example, the red maple cultivar 'Tilford' (Acer rubrum 'Tilford') averaged only 9 inches of growth/year (and suffered leaf scorch) over a 10 year period in Wichita, Kansas , while the same cultivar averaged 17 inches/year in a trial in Alabama. In this case, the much drier conditions in Kansas probably hindered the growth of the moisture-loving red maple.

In addition to water availability, some other major factors that can affect tree growth rates include nutrient availability (fertilizers); damage from insects, diseases, herbicides, and mechanical equipment; soil compaction; restrictions in rooting space; and competition with other trees. One forestry study found a strong correlation between reduced growth rate (measured as trunk diameter) and the number of other trees growing within 9 meters (29.5 feet) of the tree.

Faster growth rate is not necessarily an indicator that a tree will ultimately be taller than its slower growing relatives. A Forest Service study conducted in several cities in upstate New York found that urban silver maples did average 15 feet taller than sugar and Norway maples in the same area after 60 years of growth, but that all three species eventually reached a similar maximum height. Many tree species have faster growth rates when young, eventually slowing down to more moderate growth rates as they mature.

Ohio buckeye in flower Photo credit:
Beth Jarvis
Homeowners desperate for shade on their bare suburban lots often ask for suggestions on "the fastest growing tree I can buy". I try to steer them away from fast-growing but problem-prone species such as white poplar (Populus alba; weak wood, invasive root suckers), silver maple (Acer saccharinum; weak wood, weedy seeds), and Siberian elm (Ulmus pumila; insect damage, weak wood, weedy seeds). Even the ubiquitous green ash (Fraxinus pennsylvanica) is becoming a questionable choice because of increasing incidence of ash decline. Better long-term choices include fast-growing Freeman maple (a natural hybrid between red and silver maples) and medium- to fast-growing trees such as sugar maple, red maple, basswood, littleleaf linden, red oak, pin oak, honeylocust, and cucumbertree magnolia. And of course, everyone should be encouraged to plant a slower growing tree such as bur oak, white oak, Kentucky coffeetree, or ginkgo as well. You may never get a chance to lounge in their shade, but future generations will.

FUN READING: Limits to the Heights of Trees (explains theories for why trees can't grow beyond a certain height) http://www.fathom.com/course/21701736/session5.html

Stradivari, violins, tree rings, and the Maunder Minimum: a hypothesis (how climatic conditions may have affected the growth of trees subsequently used to build famed violins) http://www.urbanfischer.de/journals/dendro/content/sample/1110033a.pdf


Oak Anthracnose
Janna Beckerman, Extension Plant Pathologist

Characteristic symptom of oak anthracnose on a white oak. Photo Credit: Janna Beckerman
Oak wilt symptoms on white oak. Photo Credit: Plant Disease Clinic
Our recent cool, wet weather has contributed to an outbreak of oak anthracnose. Oak anthracnose primarily infects white and bur oaks (oaks with rounded leaves). The later development of oak leaves (compared to ash, lilac, and other early spring flowering shrubs) coincided perfectly with the cool, wet weather system we were locked into for several weeks. While these abovementioned plants (particularly those in the Southern part of our state) escaped severe anthracnose, oaks were hit particularly hard this season. Symptoms first appear on newly emerging leaves the lowest branches. These infections produce irregularly shaped spots and blotches that can deform the leaf. Severe infections can result in leaf drop, usually followed by new leaf growth. The fungus is capable of spreading into the twig where it overwinters. Twigs infected during the growing season may often die before their buds are able to open the following spring, causing branch dieback.

Releafing of an infected tree. Photo Credit: Chad Behrendt
This disease is often mistaken for a much more serious disease, oak wilt (Fig. 3). Unlike oak wilt, oak anthracnose is most severe on white oak, whereas oak wilt is most severe on oaks in the red oak family (pointy leafed oaks), and includes Northern red oak, pin oak and black oaks. Leaves of trees infected with oak wilt develop browning of the leaf margin, and are wilted. Symptoms of oak wilt begin in the upper portions of the tree, unlike oak anthracnose which commonly begins in the lower canopy.

Although ugly, oak anthracnose does not cause permanent damage to otherwise healthy trees. Cultural measures to manage this disease include proper watering, mulching, and removal of infected material. Raking leaves in the fall and pruning dead or dying branches reduces the number of new infections the following year. Trees should be pruned only during dry weather. Do not prune oaks in April, May, or June! This minimizes the likelihood of attracting the beetles that carry the fungus that causes oak wilt.

Trees that have been defoliated several years in a row, or are newly transplanted, may require chemical intervention next year. Fungicide application should begin at bud break to protect new growth. Repeat applications according to the fungicide label during cool wet weather. For smaller trees, read the label carefully and apply only as directed. Large trees will require professional assistance to access the special equipment to ensure adequate coverage. Currently labeled fungicides include thiophanate-methyl (Cleary's 3336), Mancozeb, and copper containing fungicides.

Please check out the new diagnostics web pages at
http://www.extension.umn.edu/projects/yardandgarden/diagnostics/


Get the low down on this month's insect pests at Insects http://www.extension.umn.edu/projects/yardandgarden/EntWeb/Ent.htm


June Garden Tips
Beth Jarvis, Yard & Garden Line

Prune spring-blooming lilacs now. Photo credit:
Beth Jarvis
Compiled from conversations with Patrick Weicherding, Bob Mugaas and Bob Olson, Regional Extension Educators

(These recommendation are based on Twin Cities temperatures. Adjust for northern Minnesota..)

Trees and Shrubs:
It's still too early to prune those oaks. To prevent the spread of oak wilt, don't prune oaks through the month of June.

It's safe to prune spring flowering shrubs. Pruning after they flower will encourage maximum blossoms for the next year.

Watch for early signs of oak wilt and Dutch elm disease. In the case of oak wilt, look for dead and dying branches. Leaves on these branches often bronze, or turn tan or dull green, starting at the tips or outer margins. Leaves may also droop, curl, or fall from the tree. Infected trees usually die. With Dutch elm disease, wilting leaves, often on a single branch are the first symptoms. Yellowing of leaves and leaf drop follow. Infected branches often have brown streaks under the bark that follow the wood grain. Report suspicious trees promptly to your city forestry office.

If you haven't already done so, it's time to renew the mulch around your trees and shrubs. Apply a new layer of mulch so that the total depth (old and new) does not exceed three to four inches. Be sure to keep the mulch several inches away from the trunks of trees and stems of shrubs. Mulching is an effective technique to keep soil temperatures even, to retain soil moisture and to prevent weed seeds from germinating.

Be careful when doing yard work not to wound trees with lawn mowers and trimmers. Use mulch around trees and shrubs for weed control instead.

To remove tree sap or pine pitch from your vehicles surface, use mineral spirits (it will also remove tar). Use a soft towel, or wash cloth dampened with mineral spirits. After removal, wash the vehicle and apply wax to the affected area. Stubborn spots can be treated with finger nail polish remover on a cotton ball. Be sure to cleanse the area with a paste made of water and baking soda, and then apply wax.

Lawns:
If your lawn is looking a bit pale, especially if you have sandy soil, it is ok to put down some nitrogen fertilizer at a rate of 1/2# per 1,000 square feet, which is half the normal rate. Nitrogen leaches readily from the soil.

Do not fertilize you lawn heavily now. The growth rate of temperate-season grasses slows in summer's heat.

Crabgrass is at a 2 to 3 leaf stage now so put away the pre-emergence herbicides, as they're useless. Post emergence crabgrass herbicides may be used. They contain DSMA and MSMA which often yellow bluegrasses if applied under high temperatures and dry conditions. (And we've had neither.)

You can still spot treat broad leafed weeds, such as dandelions. However, check the label as they often stipulate a maximum air temperature--generally 80 or so. Follow the label directions.

Lawns should be mowed several times a week if you're maintaining a shorter height of cut, 2" or less. Lawns growing to 2 1/2 to 3" need to mowed at least weekly.

It is much too late to core aerate, dethatch or seed. Leave those projects until August and September. Grass willl recover from the damage of ore aeration and dethatching more quickly in cooler temperatures.

Flowers
Coneflowers,
impatiens,
forget-me-nots
brighten the shade. Photo credit:
Beth Jarvis
Perennial plant gardeners have been reporting seeing lots of small black and red bugs in their gardens and holes or spots on a wide range of plants. This could well be caused by the four-lined plant bug. The nymphs are red and black and the adults are black and yellow striped. Read this brief for more information:
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/e121plantbugs-fourlined.html
Pictures of both the nymph and adult stages are at: http://www.extension.umn.edu/projects/yardandgarden/EntWeb/galleries/gardpest-4-lined-pbug.html

If you want to relocate tulips and other spring bulbs, you can start digging them as soon as the foliage has yellowed. Tulips in the Twin Cities are turning yellow but daffodils are still quite green and should be left alone.

Houseplants can be moved outside to enjoy summer. Acclimate them gradually by leaving them outside for longer periods of time each day. Start with a shady, protected site at first.

Remove faded flowers from peonies, columbine and other perennials so all photosynthates will be preserved for future growth. Otherwise, some will be frittered away on developing unwanted seeds.

Deadhead roses and annuals, such as geraniums, to encourage new blooms.

If purchased plants have roots circling the pot, tear apart some of the circling roots when planting. Circling roots keep circling.

If you're finding bits of froth/foam on your plants, concentrated at the point where the leaf and stem meet, youhave spittle bugs. They'll only be around a bit longer and they don't cause serious plant damage. http://www.extension.umn.edu/yardandgarden/YGLNews/YGLN-Jun1599.html#spittle

Vegetables:
Cole Crops
This spring’s miserable weather hasn’t been very accommodating for warm season crops like tomatoes, peppers and vine crops, but our cold-loving vegetables have thrived.

Look for cauliflower to soon initiate flowering and head formation. As soon as you see those bright white heads forming in the center, it’s time to shield them from sunlight. Otherwise those nice white heads will turn a dingy yellow and brown. This process, called blanching, requires that you tie up leaves over the forming head by using twine or heavy rubber bands. As the head enlarges you may have to re-tie the leaves to accommodate the growth.

Harvest broccoli clusters when the immature flowers are still tightly wrapped. Keep a watch-out for really warm weather, as this can rapidly speed up the maturation process and cause the flowers to open, rendering your clusters unacceptable. Harvest the main head and then look for smaller side-clusters to form in the following weeks.

Nitrogen Losses?
With the excessive moisture this spring, gardeners are wondering whether they need to re-apply nitrogen to their gardens because of the loss from leaching. Actually, most of the nitrogen-containing fertilizers are ammonia-based (including manures and other organic sources), and when nitrogen is in the ammonium form in soils it is quite resistant to leaching below the rootzone. Nitrogen losses occur when soil bacteria convert this stable nitrogen into the unstable nitrate form. Because our temperatures have been below normal, the activity of soil bacteria has been minimal. Thus, most of your nitrogen that was applied earlier in the season is still hanging around, despite the excessive rainfall.

Fruit:
Strawberry Renovation
It seems like we just fast-forwarded through the strawberry season, and now we’re already talking about preparation for next year. As soon as your June-bearing harvest is over, it’s time to renovate your beds.

With all of this spring’s rainfall the foliage of your strawberries is quite lush. Not only are your strawberries lush- so too are a variety of perennial weeds. Now is a great time to eliminate these perennial weeds by pulling or hoeing them from your beds. The excessive moisture should allow for easier pulling this year as compared to most.

After removing weeds, take your lawnmower and mow off the tops of your beds. Your neighbors may think you’re crazy, but this is an important step in the process of ensuring a great crop of berries for next season. Your lawnmower can be set so that it removes all the foliage but doesn’t damage the tender crowns that are just at the soil surface. Rake off all of the tops and add them to your compost pile.

After the tops have been removed, narrow your rows to about 15 inches by using a roto-tiller or shovel. If your planting is a few years old, you might want to roto-till right over the top of your initial plantings and use the runners as the “new” plants.

Apply a general purpose fertilizer over the rows and water in. I favor low analysis fertilizers like 10-10-10 for homeowner situations because it’s a lot more forgiving if you accidentally over-apply. Put down about 5 pounds per 100 feet of row.

Keep your strawberry plants actively growing throughout the summer. Next year’s flower buds are formed this season, so you want to pamper your plantings now for a good harvest next June. Watering and weed control will go a long way to stimulate a healthy planting for over-wintering and the subsequent harvest.

Editorial Notes

Pineapple
growing in Mn. A traditional symbol of
hospitality. Photo credit:
Beth Jarvis
Summer has finally arrived, it seems. And so have the mosquitos! Don't forget to apply mosquito repellant when you go outside, especially with West Nile virus a real threat. Last year, some Master Gardeners noticed damage to plants beside their home. They realized it was where they sprayed themselves with repellent and the damage was overspray.

This is a short issue for a number of reasons. We should be back to "normal" for July . Speaking of which, next issue, Bob Olson, will talk about home irrigation systems.

Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.

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Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Thursday of every month at 10 a.m. The program is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations.

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Happy gardening!

Beth Jarvis
Yard & Garden Line Project Coordinator


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