- Emerald Ash Borer
- Dutch Elm Disease
- Oak Wilt
- Stock Types
- Planting Practices
- Tree Characteristics
- Storm Failure
- Gravel Beds
- Other Pests
- Firewood Regulations
What is Emerald ash borer?
The emerald ash borer insect (EAB), Agrilus planipennis, attacks all North American ash species (Fraxinus, spp.). All Minnesota native ash trees (green, black, white) are susceptible, however Mountain-ash (Sorbus. spp.) are not true ash and are therefore not attacked. Both unhealthy and healthy trees can be attacked by EAB, but stressed trees are a more common target. Ash trees die from EAB due to a large infestation of larvae feeding in the sapwood and cambium.
Recognizing the insect
The emerald ash borer is about ⅓ to ½ inch long. It is a slender insect with its widest part just be hind the head. It is a bright green color, usually with a copper colored area behind the head and a purplish-magenta color under the wings. It is rare to find adults in the land scape. Emerald ash borer larvae can be found in the outer 1 inch of the bark/wood. EAB larvae form s-shaped galleries and can be distinguished from other insect larvae by the two small spine-like projections on their last body segment.
How does it spread?
Logs or firewood from ash trees can be a safe haven for many months for vectoring insects if the wood is not kiln dried to a minimum core temperature of 140˚F for 60 minutes. Other methods for rendering ash wood pest free include debarking, chipping, or burning. Adult beetles will fly short distances, but it is the transportation of firewood from ash killed by and harboring the borer which is significantly increasing the spread across Minnesota. In order to successfully manage EAB, it is critical to monitor the movement and storage of firewood. To learn more about firewood regulations in MN, please visit the Minnesota Department of Agriculture website on the topic here: https://www.mda.state.mn.us/plants-insects/ firewood-information.
Signs and symptoms
Emerald ash borer may cause canopy dieback, epicormic shoots, bark splitting, increased woodpecker activity, and serpentine galleries. The serpentine galleries are visible underneath the bark and disturb the flow of water and nutrients throughout the tree. Identifying trees which are currently infested with EAB can be challenging as visible canopy decline may not appear after several years of infestation. Please watch the short video entitled EAB Visual Inspection Guide produced by the MN Department of Agriculture to learn more about identifying trees infested with EAB in the landscape.
How do you treat an ash for EAB?
If an ash tree still has most or all of its canopy, chemical treatment has a high chance of being effective in protecting the tree. However, treatment is unlikely to work if the tree is in poor health and has lost more than half its canopy.
Effective chemical treatments can reduce larval and beetle activity and prevent the tree’s death. Larger ash trees should be treated by a professional arborist, however, there are some products available to residents. The insecticide applications do not build up the tree's resistance to EAB, so treatment needs to be done on a regular basis. For a much more in depth discussion of insecticides and EAB, please consult the document entitled Insecticide Options for Protecting Ash Trees from Emerald Ash Borer.
Sources: Jeff Hahn, Extension entomologist & Matt Russell, Extension forester - EAB in Minnesota
What is Dutch elm disease?
Dutch elm disease (DED) is a fungus (Ophiostoma novo-ulmi) that infects a tree’s vascular (water conducting) system. The fungus infects a tree and causes clogging in the vascular tissue which prevents water movement to the crown. This results in wilting and leads to the tree’s death. All elm species are susceptible, though introduced Asiatic elms and resistant varieties are less susceptible.
What are the signs and symptoms?
The symptoms caused by DED include leaf wilting and premature drop, brown streaking in sapwood, yellowing and browning of foliage, beginning at the canopy or base depending on the site of infection.
How does it spread?
The life cycles of the native elm bark beetle (Hylurgopinus rufipes Eich.) and the European elm bark beetle (Scolytus multistriatus Marsh.) play a large role in the overland spread of DED. These beetles use the inner bark in unhealthy and dead elm trees to breed by laying their eggs in tunnels called galleries. Once the eggs have hatched and the larvae have matured into adults, they emerge from the elm. If the tree is infected with DED, the fungus will produce sticky spores in the galleries. These spores are picked up by the beetles either by being ingested or sticking to them. Adult beetles then introduce the fungus as they feed on healthy trees. The fungus can also be spread underground through grafted roots when the same or similar tree species are growing next to each other.
How do you treat an elm for DED?
The DED disease cycle needs to be interrupted by starting thorough sanitation early in the cycle to limit both the amount of pathogen and the population of the insects that transmit the fungus from tree to tree in the landscape. In order for wood from infected trees to be considered pest-risk free it should be destroyed by chipping, burning, or burying. It is also important to disrupt the root grafts between larger trees to prevent the fungus from spreading to healthy trees. In a newly infected tree, pruning is an eradicative treatment that can only be done on trees infected through overland spread. Pruning will only be effective if the impacted area is less than 5% of the crown. Effectiveness will be increased if pruning is conducted on resistant varieties with the help of systemic injection of fungicides.
Sources: Linda Haugen — United States Department of Agriculture Forest Service - Northeastern Area 1998
What is oak wilt?
Oak wilt is caused by the non-native fungus called Bretziella fagacearum. This fungus kills a large number of oaks in Minnesota every year. Oak wilt is of great concern because of the abundance of oak trees and the value they provide as both shade and forest trees. The red oak group, including northern red oak and northern pin oak, are most affected by oak wilt.
What are the signs and symptoms?
In the red oak group, the fungus spreads very quickly and can cause complete wilting and leaf loss in only four weeks. The wilting starts at the top and outer parts of the crown and moves downward. If a tree is infected through root grafts, this can occur soon after leaf-out the following spring. Red oaks are more susceptible to forming spore mats which rupture the bark. In the white oak group, after the first symptoms arise the disease may progress each year with tree death occurring within 2 - 5+ years. Symptoms include browning of the leaves generally from the tip and leaf margin toward the midrib and dark brown/black discoloration underneath the bark on branches with wilting leaves. However, these symptoms may be irregular.
How does it spread?
There are two ways in which the oak wilt fungus can spread to healthy trees: above ground and below ground. Sap beetles are responsible for above ground transmission by feeding on spore mats formed on an infected tree (primarily on dead red oaks) and then visiting a fresh wound on a healthy tree. The volatiles produced by the fungal mats in the cambium of trees killed by oak wilt attract the sap beetles.
Sap beetles are also attracted to fresh wounds on healthy oaks. Spore mats are commonly produced during spring and fall, however, the mats that play the largest role in disease spread are those produced during April through mid-July on red oaks that wilted the previous year. Below ground spread occurs through grafted roots and is the cause of most new infections.
How do you treat for oak wilt?
The most effective way to prevent the spread of oak wilt centers is by cutting root connections between diseased and healthy oaks. This is commonly done through vibratory plowing or trenching with a 5’ blade. Up until the end of September during the year following tree wilt, infected red oaks should be eliminated by debarking, burning, burying, or wrapping/ sealing. Sap beetles are attracted to fresh wounds, so pruning/wounding of oak trees should be avoided during the growing season.
Immediate treatment with a water-based paint, sealer, or shellac should be used on the wound if pruning or felling must occur. April through mid July pose the highest risk of insect spread, mid July through late October pose a lower threat, and November through March are safe months to prune or cut oaks.
Sources: Jennifer Juzwik, Brian Schwingle, Matthew Russell - University of Minnesota - Extension
This section offers a brief introduction to three of the most common nursery stock types. While this is a general overview, there are very specific standards for nursery stock. The American Standard for Nursery Stock, Z-60.1 is a document meant to provide a common terminology amongst both buyers and sellers. The description of the standard states, “...the standards establish common techniques for (a) measuring plants, (b) specifying and stating the size of plants, (c) determining the proper relationship between height and caliper, or height and width, and (d) determining whether a root ball or container is large enough for a particular size plant.” You can download the American Standard for Nursery Stock, Z-60.1 for free.
Bare root trees are a common nursery stock type utilized by municipalities and other organizations conducting larger plantings. This is the cheapest stock type covered here. These trees are typically field dug by nurseries in the autumn and overwintered in large coolers before being shipped in the late winter or spring. ‘Bare root’ is a descriptive term for how the trees are stored and shipped - no soil. Because of this, extra attention must be paid to maintaining good moisture around the roots to prevent desiccation. Bare root trees are a great option for volunteer planting because they are very light and easy to handle. The visible root system makes it easier to find the right planting depth and easily correct any dysfunctional roots. Bare root stock is often spring planted, however, can be planted later in the season if you store them properly in a gravel bed or wood chips.
There are different types of container trees that can be purchased including the conventional smooth sided pot, air root pruning container, and synthetic bags. Container trees often develop encircling roots which need to be corrected before planting.
Balled and burlapped
Balled and burlapped trees usually require equipment to move and plant. These trees can be much larger in size than container and bare root trees.
Sources: Julie Weisenhorn, Extension horticulture educator; Jeffrey H. Gillman and Gary R. Johnson - UMN Extension,
Gary Johnson - Nursery Stock Types
Planting a tree or shrub
The planting hole should be wide enough for all the roots to fit and deep enough so the first main lateral root is within 1 inch of the soil surface (figure 1). Digging the hole too deeply is a very common planting mistake that can cause problems down the road such as stem girdling roots.
Planting B&B trees
Check for excess soil covering the first main lateral root by removing the wire and burlap on top of the soil ball (figure 2). Remove excess soil so you can identify the first main lateral root during planting. This is an important step to ensure the tree is planted at the right depth.
Planting container trees
Remove the tree from the container and check for encircling roots. This is very common and happens when the roots grow and hit the side of the container and start to circle around the container. This needs to be corrected before planting to allow the root system to grow correctly. This can be done by using the box cutting method (figure 3).
Caring for a new tree
Create a mulch in a ring around the tree, keeping the mulch off the trunk (figure 4). Thoroughly water the tree at planting and then continue to water your tree about twice a week for a total of 15 - 25 gallons per week. Continue this process until the ground freezes.
Using the right soil
The Percolation Test is used to determine a soil’s ability to deal with drought or excessive water. Soil that drains a 24” deep column of water within 24 hours is considered a well-drained soil for healthy landscapes. If soil is very compacted and clay, make the hole as wide as possible for planting. However, if the soil is loose, sandy, or loamy, dig the hole wide enough to fit the roots in.
To learn more, watch this video: Soil Drainage and Percolation Test
Sources: Julie Weisenhorn, Extension horticulture educator; Jeffrey H. Gillman and Gary R. Johnson - UMN Extension
End grain types
When sorting through firewood piles, there are three end grain types that are important to know:
1. Ring porous
2. Diffuse porous
3. Semi-ring porous
Ring porous wood is the end grain type that may house insect pests and diseases in Minnesota, so it is important to be able to identify it. Ring porous wood is found in all species of oaks, elms, and ash. Trees with diffuse porous wood include maples (including boxelder), birches, sometimes poplars, basswood (linden), ironwood, buckeye, & black cherry.
Elm (Ulmus species), oak (Quercus species) and ash (Fraxinus species) have unique wood grain and bark types that help to distinguish species from one another. The bark on American elm (Ulmus americana) and Rock elm (Ulmus thomasii) has alternating dark and light bands that often look like strips of bacon. The bark on oak trees can widely range from gray and platy (Q. alba) to deeply ridged and furrowed (Q. macrocarpa). The red oak group has smaller diameter pieces of bark that are flat, gray, and smooth while the larger diameter pieces are ridged and furrowed.
Sources: Gary Johnson, Rebecca Koetter, Peter Gillitzer and Dave Hanson - What’s in that Woodpile?
Revised: January, 2016. Gary Johnson ; May, 2020. Ryan Murphy
Components of the triangle
The Storm Failure Triangle looks at the main factors of a damaging weather event that lead to failure potential of a tree. Failure potential is defined as the likelihood that a tree will fail or suffer some degree of damage:
1. The loading event
2. Site characteristics
3. Tree condition and any defects
A loading event is any weather event that puts abnormal pressure on a tree's architecture/stability. This can range from a mild thunderstorm to a tornado. Events may also include the weight of rain, ice or snow. The potential for damage rises as the amount of loading on a tree rises.
Site characteristics include factors that can be somewhat controlled such as soil types, wind exposure or protection (friction), soil profile and saturation level, root plate space, and plant competition. Lowing the failure potential of a tree can be done by avoiding planting high density trees in open areas and saturated soils and by avoiding planting trees that reach 60 feet or higher in four foot wide boulevards.
Tree condition and defects
Tree condition defects include size, extent of decay, unusual lean, bark branch attachments, codominant leaders, canopy density, presence of cabling systems, live crown ratio, stem girdling roots, restricted space for roots, construction damage, and die-back.
Sources: Gary Johnson & Eric North - University of Minnesota Department of Forest Resources 2013 - Storm damage to landscape trees
Use and benefits
Gravel beds provide a temporary holding area for young trees, specifically bare root trees. There are many benefits to storing a tree in a gravel bed including:
• cheaper price for bare root trees
• trees produce more fine roots
• trees recover from poor root systems
• planting schedules are more flexible
• larger species selection
• easy to remove and plant
Construction of a gravel bed
Gravel beds can vary depending on budget, space constraints, desired size, and the capacity to maintain the bed. The materials needed to build a gravel bed include pea gravel, bare root tree stock, and water. The gravel bed should be constructed on an impervious draining surface like an asphalt lot or concrete pad. However, gravel beds can also be built on top of soil with the use of liners to provide a barrier between the gravel and soil and exterior sides made out of different construction material.
Watering a gravel bed
Maintaining a consistent watering schedule for gravel bed trees is extremely important to avoid drying out the root systems. This could be done by setting up a sprinkler system that runs on a timer, installing soaker hoses or a drip system, or having volunteers water throughout the day. The irrigation system that is used should be checked regularly to ensure there are no leaks or malfunctions.
To learn more about community gravel beds, watch this video: Use and Benefits of Gravel Bed Planting
Sources: University of Minnesota Forest Resources - Community Gravel Beds
Spongy Moth (Lymantria dispar)
Spongy moth is an invasive insect that feed on over 300 species of trees and woody plants. They prefer popular species such as aspen and oak. These insects can completely defoliate entire trees which can lead to tree death. The moth lays eggs on almost any protected surface such as under bark, leaves, camping gear, vehicles, etc. This causes people to unknowingly spread the insect to new areas.
As caterpillars, spongy moths grow up to two inches long. They can be recognized by the five pairs of blue spots and six pairs of red spots down their back. After the pupa stage, adult moths emerge in mid-summer. The females are cream colored with darker zig-zag markings and do not fly. The males have similar markings but have feathered antennae. The larvae and caterpillar life stage of the spongy moth is the only damaging stage to a tree’s health.
Spongy moths are rare to find in Minnesota. However, management strategies are still in place if populations need to be controlled. A program called Slow the Spread works to control the spread of spongy moth by trapping and treatment. Through this national program, the spread of spongy moth has been reduced by 70%.
Forest Tent Caterpillar (Malacosoma disstria)
Forest tent caterpillars feed on many different species including quaking aspens, balsam poplar, basswood, oaks, ashes, birches, alder and fruit trees. These insects cause damage on deciduous trees which can slow down the tree’s growth rate overtime. Conifers (pine, spruce, firs) and red maple are rarely fed on by forest tent caterpillars.
As caterpillars, they can be identified by their mostly blue and black color, row of white keyhole shaped markings on the back, and hairs along the side of the body.
Forest tent caterpillars lay eggs in dark masses of up to 350 eggs around small twigs. Eggs hatch in the spring and the larvae start to feed on their host tree’s foliage. Trees usually survive this defoliation, so management practices can be minimal. Natural predators help to regulate populations, but if management is needed, mechanical and insecticidal options are available.
Sources: University of Minnesota Extension,
United States Department of Agriculture Forest Service
Regulations in Minnesota
Firewood movement in Minnesota is regulated through state and federal pest quarantines by the Minnesota Department of Agriculture (MDA). Minnesota has an exterior Quarantine for firewood. No firewood is allowed to enter the state of Minnesota without compliance agreements and written permission from the MDA. Firewood cannot leave a pest quarantine without certification, a compliance agreement, or written permission from the MDA. Any person violating these quarantine regulations is subject to civil penalties up to $7,500.00 per day of violation or misdemeanor penalties set forth in Minnesota Statutes Sections 18J.10 (2011) and may be subject to criminal penalties set forth in Minnesota Statutes Sections 18J.11 (2011). Since May 1, 2008, the illegal transport of firewood into a Minnesota DNR-administered land will be punishable by confiscation of firewood and a $100 penalty.
Firewood on DNR Land
The Minnesota DNR makes their own rules for what wood is allowed on state lands. The following is allowed on state lands.
• Firewood purchases at a state park can be used only in the park it is purchased in.
• Firewood that has been certified by the Minnesota Department of Agriculture. Certified firewood may be used on any DNR lands in the state. Look for the certified safe-to-move logo on the firewood bundles when purchasing.
• If the firewood bundle is not certified, the wood must be harvested in the same county the state land is in. The county of harvest must be on the label.
• Dimensional lumber scraps that are kiln-dried, unpainted, unstained, and free of any metal or foreign substances.
Sources: MN Department of Agriculture