Short answer
Avoid pruning oaks from February through June, when the sap beetles that carry the oak wilt fungus are most active. November through January is the safest window. Whatever the month, San Antonio city ordinance requires every oak wound that exposes sapwood to be painted within thirty minutes of the cut.
The oak pruning calendar for San Antonio
Elective work is what waits: shaping, thinning, raising a canopy, clearing a roofline or a driveway. If it can be scheduled, schedule it for winter.
| Months | Risk | What to do |
|---|---|---|
| February to June | Highest | Do not prune. Texas A&M Forest Service advises that pruning or wounding trees should be avoided from February through June. |
| July to October | Lower | Pruning is possible. Paint every wound that exposes sapwood within thirty minutes. |
| November to January | Lowest | The preferred window. Trees are dormant and, in the Forest Service's words, fewer insects and diseases are present. |
Painting the wound is not a suggestion here, it is city law
The ordinance applies to any person, firm, corporation, business entity, city department, public or private utility. There is no season in that sentence. A cut made in December gets painted exactly like a cut made in April.
Texas A&M Forest Service reaches the same conclusion from the biology: no matter the time of year, all oak tree wounds should be painted immediately. On what to use, the Forest Service is blunt. Any kind of paint will accomplish this task.
The thirty minute clock is the part crews get wrong. Painting at the end of the job is not the same as painting the cut.
All wounds to the trunk, limbs and root system of oak trees in the city that expose sapwood shall be painted within thirty minutes of the wound with asphaltic or exterior oil or latex based paint.
City of San Antonio Code of Ordinances, sec. 21-170
Why February through June is the dangerous window
Oak wilt moves two ways, and only one of them cares about your pruning schedule.
Above ground, sap beetles carry fungal spores from infected trees to fresh wounds. The Forest Service notes the disease spreads above ground by sap beetles more rapidly in late winter and spring because of high fungal mat production and high insect populations. A fresh cut on an oak in April is an open invitation.
Below ground, it travels through interconnected root systems from tree to tree, and it needs no wound at all. The Forest Service puts that spread at an average of 75 feet per year through the root system. That is why an oak wilt problem is almost never one tree.
Which oaks are most at risk
| Oak group | Susceptibility | What that means here |
|---|---|---|
| Red oaks | Most susceptible | Can die in as little as one month after being infected. Often the first tree on a street to show a problem. |
| Live oaks | Intermediate | The dominant shade tree across much of San Antonio, and the reason root spread matters so much locally. |
| White oaks | Least susceptible | Lower risk, but the Forest Service notes they are not immune to infection. |
What an infected oak looks like
In live oaks, look for veinal necrosis: yellow and brown veins showing in the leaves. In red oaks, early symptoms show as leaves turning red or brown.
If that pattern is moving through several trees in a line rather than sitting in one crown, you are likely looking at root spread, and the response is different from treating a single sick tree.
The firewood rule most people have never heard of
Infected oak firewood can carry fungal mats, which is how the disease hitchhikes past every root barrier and beetle flight range you were worried about.
It is the reason where your firewood came from is a legitimate question and not a sales pitch.
Firewood from oak wilt infected trees shall not be brought into the city at anytime.
City of San Antonio Code of Ordinances, sec. 21-170
What about storm damage in the wrong month?
Storms do not check the calendar. If a limb tears out in May you deal with it, and the thirty minute paint rule matters more, not less, because that wound is fresh in peak beetle season.
What the February to June window rules out is elective pruning, not emergency response to a broken or hanging limb. Make the cut clean, paint it immediately, and get the shaping work back on the winter schedule.
Sources
- Texas A&M Forest Service, Prevent the spread of oak wilt in Texas
- City of San Antonio Code of Ordinances, sec. 21-170, Prevention and Spread of Oak Wilt
General guidance for San Antonio and the surrounding Hill Country, not legal advice. Rules and utility procedures change. Check with the city or CPS Energy for your specific property.
