Avoid Unlicensed Arborists
It's hard to understand, but DC does not require arborists working in the city to be licensed. I hope you'll contact Mary Cheh about changing that - it's a no-brainer that people working on trees should be licensed, just like the many other licensed professions, in order to ensure they have the requisite training.
I saw Stringfellow Tree Service did some work at a home on Macomb St yesterday - they blocked all traffic on Macomb for several minutes, telling people to "go around" ... (No one could go anywhere until the truck moved.) Stringfellow Tree Service is NOT licensed in DC or elsewhere, and most would agree is objectively untrustworthy. DDOT's Urban Forestry Administration is aware of him, and considers him a bad actor. Michael Stringfellow illegally cut down a huge, healthy DC street tree on my block (not in CP) a few years ago. Felling healthy mature trees comes at a real cost to DC and to the environment - that's why DC enacted laws to discourage and prevent healthy trees from being removed.
Before cutting down the street tree, I saw him at the neighbor's and asked him if I needed any tree work. He tried high pressure to get me to pay ridiculous amounts for the work he proposed - I stuck to my guns, said I knew the prices were not reasonable and that I would get at least two other estimates. He begged and pleaded, to no avail. It's awful that I didn't think to ask what work he was looking at for the neighbor, because it was too late when he came back and cut down the tree (our, the taxpayer's, tree.) He later did work for an older family member and overcharged. His MO is to drive around to hustle up business - no reputable arborist does this, to my knowledge, as they are busy enough. He and one of his associates were convicted of defrauding a senior citizen in Potomac - Stringfellow's probation, after a short jail sentence, was set to end late last year. He pulled up and told her she must urgently remove a tree - charged 400% the going rate (charged $26K.) Montgomery County later determined Stringfellow removed yet another huge, healthy tree that did not need to come down. Here's the link - Since then, he was arrested and convicted in Montgomery County for defrauding a vulnerable senior citizen, charging her around $26K to remove a huge, healthy tree on her property. He made the evening news, as the poster boy for unlicensed "woodchucks." https://wjla.com/news/local/woodchuck-scammers-prey-on-the-elderly-for-fraudulent-property-improvements
There are easily accessible tree ratings, for example at Washington Consumer Checkbook. Some that I've hired who are licensed, capable and honest include Adirondack (provides their insurance certificate and licenses with every estimate), Ed's Trees and Northern Woods.