Trimming a shrub oak reduces vertical growth. The best time to trim is soon after the chance of frost in the area has passed before spring growth. Trim again during the middle of the spring. Follow with a third trim the first week of August. Utilizing three trimming sessions prevents shrub oaks from excessive growth and makes them manageable. It should take approximately 30 minutes to trim a shrub oak.
Measure the height of the shrub oak with a tape measure. Divide the total height by four. Measure from the top of the oak downward to this amount. Mark with a cloth tie. For example, if the shrub oak is 4 feet tall, a quarter would be 1 foot.
Spray pruning shears with a disinfectant spray. Trim off up to a quarter of the top of the shrub, depending on how tall you want the shrub. Use the cloth tie as a guide to avoid trimming down too far. Spray the shears between each of the cuts, eliminating the potential to spread disease throughout the oak.
Spray each of the cuts with a tree wound dressing. Wound dressing reduces the attraction of bark beetles to oaks, according to the AgriLife Extension.
Remove the tie marker. Measure from soil line upward to 6 inches on the bottom of the oak shrub. Trim away any growth below this level. Seal the cuts with a tree wound dressing.
Trim the tips of the remaining branches to shape the oak shrub. Seal the cuts with a tree wound dressing.
Water the base of the oak shrub with 2 inches of water after trimming to reduce shrub stress.