I came across this quote in The Valley of Fear, one of The Greatest Adventures of Sherlock Holmes. As an amateur gardener, I wondered if “pollarded” might mean how the elms were pruned or cut back. That was exactly what I found. Here is a great article about Pollarding from The Daily Garden.
According to the article, pollarding is a type of pruning where all new growth is cut back, leaving the trees with a short, post-like trunk with many smaller branches that grow upwards. Some of the benefits of pollarding are short trees that won’t blow over in wind storms and less large limbs that may fall on power lines and roofs.
Here are some pictures from The Daily Garden: