Complete Guide to Tree Health in Pennsylvania

Pennsylvania's 16.6 million acres of forestland cover nearly 59% of the state, making it one of the most heavily forested states in the nation. Known as "Penn's Woods" — the literal translation of its name — the Commonwealth spans USDA Hardiness Zones 5b through 7a, from the cold plateaus of the Alleghenies to the temperate Delaware Valley.

Pennsylvania's forests produce more hardwood lumber than any other state, with red oak, black cherry, hard maple, and white ash historically driving a multi-billion dollar forest products industry. The state's diverse topography creates a patchwork of forest types: northern hardwoods and hemlock in the Pocono Plateau, oak-hickory forests in the Ridge and Valley province, and mixed mesophytic forests in the southwestern hills.

Pennsylvania has been the epicenter for several invasive pest emergencies, from the early detection of spotted lanternfly in Berks County in 2014 to ongoing battles with hemlock woolly adelgid and emerald ash borer. This guide provides comprehensive information for managing tree health across the Keystone State.

Climate and Growing Conditions

Pennsylvania's climate varies dramatically with elevation and proximity to the Atlantic coast.

Temperature Ranges — The state spans from Zone 5b in the northern Alleghenies (winter lows -15°F to -10°F) to Zone 7a in the southeastern coastal plain (lows 0°F to 5°F). The growing season ranges from 120 days in the highest elevations to 200+ days in Philadelphia.

Precipitation — Annual rainfall ranges from 36 inches in the north-central mountains to 48 inches in the southeast. Precipitation is well-distributed throughout the year. Tropical storm remnants can bring excessive rain in late summer and fall.

Elevation Effects — The Allegheny Plateau (2,000-3,000 feet) creates cooler, wetter conditions supporting northern hardwood and spruce-fir communities. Valley floors and the Piedmont region are warmer and drier.

Soil Diversity — Southeastern Pennsylvania has deep, fertile Piedmont soils. The Ridge and Valley province has alternating shale and limestone soils with variable pH. The Allegheny Plateau features acidic, sandstone-derived soils. The Pocono region has glacially derived soils.

Urban Influences — The Philadelphia and Pittsburgh metropolitan areas create heat islands that shift effective hardiness zones warmer and create unique challenges for urban forest management.

Common Trees of Pennsylvania

• Eastern Hemlock (Tsuga canadensis) — Pennsylvania's state tree. Found in cool ravines and northern slopes. Severely threatened by hemlock woolly adelgid.
• Red Oak (Quercus rubra) — The most commercially important hardwood in PA forests. Found statewide.
• White Oak (Quercus alba) — Dominant on upland sites. Slower-growing but more resistant to oak wilt than red oaks.
• Black Cherry (Prunus serotina) — Pennsylvania produces more black cherry lumber than any other state. The Allegheny Plateau is the center of production.
• Sugar Maple (Acer saccharum) — Co-dominant in northern hardwood forests. Supports PA's maple syrup industry.
• Red Maple (Acer rubrum) — The most abundant tree in Pennsylvania by stem count. Adaptable to many conditions.
• American Beech (Fagus grandifolia) — Widespread but threatened by beech bark disease and beech leaf disease.
• Eastern White Pine (Pinus strobus) — Common in reforested areas and as an ornamental.
• Tulip Tree (Liriodendron tulipifera) — Fast-growing, reaching great heights. Common in southern and central PA.
• White Ash (Fraxinus americana) — Formerly a major timber species. Populations decimated by EAB.
• Chestnut Oak (Quercus montana) — Dominant on dry, rocky ridgetops throughout the Ridge and Valley.
• Pitch Pine (Pinus rigida) — Found on poor, sandy soils and rocky ridgetops, particularly in the Poconos.
• Hickories (Carya spp.) — Several species including shagbark, pignut, and mockernut found in oak-hickory forests.

Top Tree Health Threats in Pennsylvania

Spotted Lanternfly (SLF) First detected in Berks County in 2014, this invasive planthopper has spread to most Pennsylvania counties. Feeds on over 70 species including maples, oaks, walnuts, and fruit trees. Tree of heaven is the primary host. SLF weakens trees, promotes sooty mold growth, and is a significant agricultural pest.

Emerald Ash Borer (EAB) Found in all 67 Pennsylvania counties. Has eliminated the majority of untreated ash trees. Continues to impact rural forests and remaining urban ash.

Hemlock Woolly Adelgid (HWA) A devastating pest of eastern hemlock, now found across most of Pennsylvania. Has killed thousands of acres of hemlock. Biological control agents (predatory beetles) are being released as part of long-term management strategies.

Beech Leaf Disease Spreading across northeastern Pennsylvania. Caused by a nematode. Leads to dark leaf banding, canopy decline, and eventual tree death. Combined with existing beech bark disease, threatens the long-term viability of beech in PA forests.

Oak Wilt Confirmed in several western Pennsylvania counties and spreading. A significant threat to the state's commercially valuable oak forests.

Elongate Hemlock Scale An armored scale insect affecting hemlocks and other conifers. Works in conjunction with HWA to stress hemlock trees. Increasingly common in southeastern Pennsylvania.

Gypsy Moth (Spongy Moth) Established across Pennsylvania since the 1970s. Periodic outbreak years cause extensive defoliation of oaks and other hardwoods. State-managed suppression programs are active.

Seasonal Tree Care Calendar for Pennsylvania

Spring (March - May) - Inspect trees for winter damage from ice and snow - Plant trees from mid-March (southeast PA) through May (northern PA) - Apply mulch 2-4 inches deep, keeping clear of trunk - Avoid pruning oaks April through July - Begin EAB treatments for valued ash (May) - Check for spotted lanternfly egg masses on trees, fences, and vehicles — scrape and destroy

Summer (June - August) - Water newly planted trees deeply during dry periods - Monitor for spotted lanternfly nymphs and adults — use circle traps on tree of heaven - Watch for oak wilt symptoms - Scout for spongy moth caterpillars and Japanese beetles - Check hemlocks for woolly adelgid (white cottony masses at needle bases)

Fall (September - November) - Excellent planting season: September through October - Begin structural pruning after leaf drop - Deep water all trees before ground freezes - Destroy spotted lanternfly egg masses (gray, mud-like patches on flat surfaces) - Remove leaves from diseased trees to reduce overwintering pathogens

Winter (December - February) - Prune dormant trees on dry days (avoid oaks and elms) - Continue checking for and destroying SLF egg masses - Plan diverse spring plantings - Schedule hazard tree assessments for large property trees - Review spotted lanternfly quarantine compliance for businesses

Pennsylvania Tree Health Resources

• PA DCNR Bureau of Forestry — dcnr.pa.gov/Conservation/ForestsAndTrees — Forest health monitoring, pest alerts, and community forestry
• Penn State Extension — extension.psu.edu/trees-and-shrubs — Tree care guides, pest identification, and research
• Pennsylvania Arborist Association — paarbor.org — Find ISA-certified arborists in Pennsylvania
• Penn State Plant Disease Clinic — plantpath.psu.edu/facilities/plant-disease-clinic — Submit disease samples
• PA Department of Agriculture — Spotted Lanternfly — agriculture.pa.gov/spottedlanternfly — SLF reporting, quarantine info, and management
• Allegheny National Forest — fs.usda.gov/allegheny — Federal forest management in northwestern PA
• TreeVitalize — Pennsylvania's urban tree restoration partnership