There are about 90 oak species found in North America. Below are the most common species found in St. Louis Park.

Bur oak (Quercus macrocarpa)

Bur oak is the most common oak tree in Minnesota. It is a subspecies of white oak. Mature trees have a straight sturdy trunk and broad spreading branches that are gnarly, corked and crooked.

• Leaves: Fiddle-shaped, round-lobed leaves that are dark green. Leaves turn yellow-green to brown in the fall.
• Bark: Bark is greyish with deep ridges.
• Acorns: Large, broad, elliptical acorns at least half enclosed by a deep cap with hairy scales that create a fringe-like edge.

Learn more about the bur oak tree, including pictures and to help identify them. 

White oak (Quercus alba)

White oak is large tree native to Minnesota.

• Leaves: Oblong, round-tipped, deeply lobed leaves that typically don’t fall until mid-winter. In the summer, the leaves are dark green on top and white or pale green underneath. The leaves turn reddish-brown to deep burgundy in the fall.
• Bark: Bark is typically a very light gray and scaly.
• Acorns: Bowl-shaped cap that encloses one-fourth of the light brown acorn.

Learn more about the white oak tree, including pictures and to help identify them. 

Bicolor oak (Quercus bicolor)

Bicolor oak, also called swamp white oak, is a medium height tree with a round top.

• Leaves: Leaves are wide in the middle (pear-shaped) and wedge-shaped at the base, and have wavy, indented margins. Leaves are dark green on top and grayish underneath in the summer and turn yellow-brown in fall.
• Bark: Bark is thick, deeply and irregularly divided into broad ridges, and grayish-brown. 
• Acorns: Thick cap that encloses about one-third of the 1-inch acorn; often found in pairs.

Learn more about the bicolor oak tree, including pictures and to help identify them. 

Northern pin oak (Quercus ellipsoidalis)

Northern pin oak has a trunk that narrows rapidly, with drooping branches that form an open, narrow top.

• Leaves: Leaves typically have seven bristle-pointed lobes separated by round openings. Leaves turn scarlet red in the fall.
• Bark: Grayish-brown bark has shallow grooves and rough edges.
• Acorns: One-half to 1-inch reddish-brown acorn half enclosed in a cup. Acorns are bright green in the summer.

Learn more about the northern pin oak tree, including pictures and to help identify them. 

Northern red oak (Quercus rubra)

Northern red oak is a native, relatively fast-growing oak, with a tall and straight trunk and a narrow top.

• Leaves: Leaves are bristle-pointed and have seven to nine lobes that extend halfway to the midrib, or middle vein, of the leaf. Leaves are dull green, often turning brilliant red in fall.
• Bark: Bark is brown and thick with shallow grooves that create flat, smooth-surfaced vertical plates.
• Acorns: Large acorns that are blunt topped with a flat base enclosed by a very shallow, dark brown cup.

Learn more about the northern red oak tree, including pictures and to help identify them. 

Additional identification resources

• Minnesota's Mighty Oaks | Minnesota Department of Natural Resources
• Woody Plants Catalog: Coniferous Trees, Deciduous Trees, Shrubs, and Woody Vines | University of Minnesota

Short-term weather events and climatic changes have contributed to the recent decline of oak tree health. Consecutive summers of drought have affected tree growth, making them more susceptible to pests and diseases.

Below are some of the common diseases and pests that could be affecting your oak tree.

Note: If you decide to treat an oak on your property, we recommend hiring a licensed tree care professional to apply any chemical treatments. Visit the tree page for a list of licensed tree care providers.

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Two-lined chestnut borer (TLCB)

The two-lined chestnut borer is a native beetle that attacks weakened or stressed oak trees. It lives in and feeds on the inner bark and cambium, a layer of cells between the bark and the wood.

Identification and symptoms of infested trees

• Branches hold onto dead leaves for several months (unlike oaks with other diseases)
• Tree canopy has a dead, red, green pattern – upper portion of tree is leafless and dead, middle leaves are dying (“red”) and lower leaves remain green
• Larvae can be found in dead and dying branches, where they make winding feeding tunnels
• Adult beetles chew out of the bark, leaving D-shaped exit holes

Long-term effects

Trees typically die after two to three years of infestation but can sometimes survive. It is also possible that they may die if they were previously severely stressed.

Prevention

Maintaining tree health is most important for preventing TLCB infestation, including making sure your tree gets enough water and preventing damage to the tree. Activities to consider:

• Water your tree during drought periods.
• Avoid damaging tree roots, branches and trunk.
• Fence off the area under the tree canopy when construction is nearby. Construction activities under the canopy such as digging, material storage and vehicle parking can cause damage to tree roots.

Treatment

Systemic pesticides and sprays are only effective if the tree is in initial stages of tree decline.

• Systemic pesticides
  • Apply after flowering in the spring to reduce pesticide exposure to bees, and don’t apply to the soil when bee-attractive flowers are planted next to trees.
  • Look for products with the active ingredients dinotefuran, imidacloprid or emamectin benzoate.
• Canopy sprays (less effective)
  • Contact a licensed professional tree care company if you are interested in canopy sprays to determine if it is appropriate for your situation.

The city also offers reimbursement for preventive treatments against two-lined chestnut borer infestation. Visit the tree treatment page for more information.

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Oak wilt

Oak wilt is a fatal tree disease caused by a non-native, invasive fungus. This vascular disease blocks water and nutrients from flowing from the roots to the top.

Identification and symptoms of infested trees

• Dull green leaves on one or a few branches at a time that wilt and quickly fall from May – September
• Carpet of leaves under tree mid-summer
• Fallen leaves with brown edges and green centers

Long-term effects

• Red oaks can die quickly (within three months) after developing oak wilt symptoms.
• Bur oaks can withstand oak wilt for one to seven years, while other white oaks can survive up to 20 years after being infested.

Prevention

To help prevent oak wilt, avoid pruning April – July. Pruning creates open wounds on a tree, which can serve as entry points for fungi and make them more susceptible to infection. Pruning should be done when the tree is dormant, typically November through February.

Cover any wounds that occur in the spring and summer with pruning paint, water-based paint or shellac within 15 minutes.

Injections of propiconazole, a fungicide, can help prevent oak wilt and protect an uninfected tree for two years.

Treatment

Propiconazole can be systemically injected into white oaks showing early symptoms of oak wilt (less than 30% of canopy affected). This will prevent further disease development for at least two years. If a red oak is showing symptoms, it is not recommended to try to treat it.

The city also offers reimbursement for preventive treatments against oak wilt infestation. Visit the tree treatment page for more information.

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Bur oak blight (BOB)

Bur oak blight is a fungal leaf disease that affects only bur oak trees.

Identification and symptoms of infested trees

• Rapid leaf loss typically seen mid-July through early fall
• Symptoms start in the lower part of the canopy
• Dark veins on undersides of leaves and brown, wedge-shaped segments between veins

Long-term effects

It is rare that bur oak blight would kill an infested tree.

If more than 50% of the canopy is lost several years in a row, the tree may become more susceptible to other, more fatal problems such as oak wilt and TLCB.

Treatment

If your oak tree has bur oak blight, you can treat it with propiconazole fungicide injections in late spring. Injections should be effective for three years and should be done only after two consecutive years of 40% leaf loss and no dieback or epicormic sprouts.

Watering your oak tree during dry periods is one of the best things you can do to help prevent disease and attacks from insects. Watering will help it fight off infection and make it less appealing to insects. You can also consider mulching or providing a natural landscape underneath the tree to help moderate soil temperature and moisture levels (see natural landscapes tab below).

How to determine if your tree needs water

To determine if your tree needs water, you will need to check the soil. Use a spade or hand trowel to dig a small hole in the ground about 6 to 9 inches deep near the base of the tree. Feel the soil in the hole, and if it is dry, it’s time to water your tree. If it is wet, you can wait to water your tree.

Your soil’s water-holding capacity

Changes seasonally

In the spring and fall, soil stays moist longer than in the summer, when temperatures rise and water moves more throughout the tree. Trees also use more water in the late spring and early summer when most growth happens.

Depends on soil type and planting site

• Sandy soil holds less water and might need smaller amounts of water, more frequently. If your soil feels sandy, try watering a half inch of water at a time every three to four days.
• Clay soil may need more water but less frequently. If you have clay-like soil, try watering 1 inch of water at a time once per week.
• Water may run off any soil type at a hilly site before being fully absorbed, so water more frequently for less time.

If you’re interested in learning more about the soil in your yard, check out the University of Minnesota Soil Testing Laboratory.

Determining your water schedule

Start planning your water schedule in the early summer when there is no rain predicted for several days. During drought periods, or after a week with no rain, mature trees need approximately 1 inch of water per week.

Follow these steps to help determine how much and how frequently you should water your tree.

1. Place several shallow containers on the ground underneath the tree’s canopy (leafy area). These will help you measure 1 inch of water.
2. Start a timer, then use a sprinkler to water the area underneath the tree canopy until there is 1 inch of water in the containers. Record the time this takes and empty the containers.
   • Wait two hours, then dig into the soil 6 to 9 inches to check the moisture level. If it is cool and moist, the watering time you recorded is appropriate.
   • If is not moist, refill the hole you dug and water again for half of the recorded time or until the containers are filled with a half-inch of water. Check the top 6 to 9 inches of soil again.
   • Repeat this process as needed, watering a half-inch at a time, until the top 6 to 9 inches of soil are cool and moist. Add the times together; the total will be how long you should water your tree.
3. Every other day for the next few days, check the moisture level. If the soil is still damp, the moisture level is adequate.
   • Refill the hole and continue to check the soil every other day until it feels dry.
   • Record how many days it took for the top 6 to 9 inches of soil to dry. This is how many days to wait before watering again.
   • If it rains enough to rewet the top 6 to 9 inches of soil, start the day count between waterings over again.

Note: Make sure you are aware of the city’s sprinkling restrictions.

Another way to improve the health of your tree is to create a natural landscape underneath it. This will help protect the tree’s roots and moderate soil temperatures and moisture levels. You can create a natural landscape by allowing leaves to accumulate under the canopy or by planting native plant species.

Keep these tips in mind when starting your natural landscape.

Avoid compacting the soil around your tree.

Tree roots need oxygen and water to thrive. Compact soil makes it difficult for water to flow to the tree roots and limits the air flow. Avoid mowing near your tree – the weight of the mower will compact the soil more. Extend the natural landscape area from the trunk to the dripline (the outer edge of the tree’s canopy) or beyond.

Don’t use landscape fabric

Landscape fabric creates a barrier that prevents insects from getting to the soil around the tree and inhibits beneficial interactions between soil organisms and tree roots.

Leave fallen leaves under tree

Instead of raking fallen leaves, allow a layer of leaves to accumulate under the tree. This natural mulch provides habitat for caterpillars and other insects, which in turn support tree health by breaking down organic matter that can be absorbed and used by the tree. Some also serve as natural pest control for more destructive insect species.

If planting native plants, select diverse plant species that thrive in partial shade.

Minnesota native plants have evolved alongside oaks in particular and provide critical shelter and habitat for various insects and wildlife. Native plants are adapted to the local climate and soil conditions where they naturally occur, so they can best support soil health by creating habitat for local microorganisms. Plants help tree roots by providing organic matter, aerating soil and attracting pollinators and insects that also benefit trees. Consider options like wild columbine, prairie smoke and wild geranium—all native to Minnesota.

Check out the University of Minnesota’s native plants page for more resources.

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More information on natural landscapes:

• Soft Landings | UNC Charlotte Urban Institute
• Give caterpillars a chance with soft landings under trees | University of Illinois Urbana Campaign
• Soft Landings-Crucial Habitat for Pollinators | Pennsylvania State University

Contact the natural resources division at naturalresources@stlouisparkmn.gov or 952.924.2562 if you want help planning your native landscape.