Why Winter is the Best Time for Tree Protection

Winter on the Peninsula may feel mild, but it’s one of the most active seasons for serious tree pests and diseases. Two of the biggest threats, Invasive Shot Hole Borer (ISHB) and Sudden Oak Death (SOD), continue spreading through our region when the air is cool, and moisture levels rise.

That’s why winter is the most important window for prevention. At Maguire Tree Care, our ISA-Certified Arborists focus on stopping these pathogens before they gain ground in spring.

The Winter Threats Hiding in Our Trees

Invasive Shot Hole Borer (ISHB)

This tiny beetle burrows into trees and farms a fungus inside the wood, cutting off the tree’s ability to move water and nutrients. Once infested, affected trees quickly decline, showing branch dieback, bleeding sap, and small pinholes in the bark.

• Still active in mild winters: ISHB doesn’t go dormant during our cool coastal months.
• Confirmed locally: The pest was verified in San Jose in 2024 and continues to expand north into Maguire’s core service areas.
• Favorite hosts: Sycamore, Coast Live Oak, Box Elder, and Avocado are high-risk species.

Sudden Oak Death (SOD)

Caused by the pathogen Phytophthora ramorum, SOD thrives in cool, wet weather, exactly what we see from late fall through spring on the Peninsula.

• How it spreads: Spores move from infected Bay Laurels to nearby oaks via rainfall splash.
• Early symptoms: Red-brown bleeding on oak trunks, canopy dieback, and premature leaf drop.
• High-risk hosts: Coast Live Oak and California Bay Laurel are the primary local species affected.

Why Early Detection Saves Trees

By the time major canopy dieback appears, both ISHB and SOD have already done serious internal damage. Early identification by an arborist is critical to saving the tree.

ISHB warning signs:

• Tiny, perfectly round entry holes on the trunk or branches
• Bleeding sap or dark staining around holes
• Frass (fine sawdust) around the tree base

SOD warning signs:

• Bleeding patches on the trunk with reddish or amber fluid
• Wilted or brown canopy sections
• Nearby Bay Laurels within 100 feet acting as potential carriers

If you spot these symptoms, don’t delay inspection — acting in winter is your best chance to intervene before spring activity spikes.

Proven PHC Treatments for Winter Protection

Maguire Tree Care’s Plant Health Care(PHC) services target both pests and pathogens at their most vulnerable stage right now.

ISHB Trunk Injections

Injecting a targeted insecticide during winter helps prevent new ISHB colonies from establishing before the spring reproduction cycle. Treatments penetrate the vascular tissue to block further fungal spread.

Bark Spray or Trunk Injection (for SOD)

During wet weather, this systemic treatment protects Coast Live Oaks from infection by creating a defensive barrier inside the tree. When applied proactively, it significantly reduces SOD infection rates.

Premium Deep Root Fertilization

This fertilization treatment improves soil structure and delivers essential nutrients and beneficial microbes, helping trees recover from stress and build natural resistance against both diseases.

Monitoring & Scouting Services

Our arborists perform detailed inspections to track early symptoms, especially on properties with Bay Laurels or past SOD history. Ongoing monitoring helps detect new activity before it spreads.

The Environmental Benefit of Acting Early

Treating now isn’t just about saving one tree — it’s about protecting the entire local canopy. Preventive treatments reduce the need for removals, preserve habitat for wildlife, and help maintain our Peninsula’s urban forest.

As certified arborists, Maguire Tree Care follows environmentally responsible practices designed to balance protection with ecosystem health.

Frequently Asked Questions

Can ISHB and SOD infect healthy trees?

Yes, even strong trees are at risk if nearby hosts are infected. Preventive treatments dramatically lower susceptibility.

Are winter treatments safe for the environment?

Absolutely. We use ISA- and EPA-approved methods that target pests and pathogens precisely, minimizing environmental impact.

Can I tell the difference between ISHB and SOD myself?

It’s difficult; both can cause similar bleeding symptoms. A professional inspection is the only reliable way to make a correct diagnosis.

How often should I treat my trees?

Preventive treatments are typically repeated every 12–18 months, depending on exposure risk and species. Your arborist can create a custom PHC plan.

Protect Your Trees Before Spring 

Mild Bay Area winters let pests and pathogens stay active, so now is the time to defend your oaks and sycamores.

Contact Maguire Tree Care today at (650) 676-7888 to schedule your ISHB/SOD inspection and customized winter treatment plan.

Early prevention protects your landscape investment and keeps our Peninsula canopy thriving for years to come.