Despite claims to the contrary, the oaks split the tree shelters without girdling. We have now watched this happen hundreds of times - we started planting California native oaks with Tubex tree shelters in 1991. Tree shelters appear to work particularly well in Mediterranean climates, especially with trees that have relatively small, tough leaves. In wet summer climates trees often have larger leaves, and, combined with summer moisture, the tubes can become crowded with big leaves, fungus and borers. Also, our oaks appreciate shade on their leaves when young, to protect from the hot sun. Tree shelters typically accelerate young tree growth, often remarkably.