Great question. Fortunately, the law is relatively straightforward on what you can and can’t do in your circumstances. Generally, Colorado law supports your right to prune your neighbor’s tree branches back to the property line, as long as you don’t harm the tree in the process. While you don’t need your neighbor’s permission to prune branches encroaching on your property, I suggest providing advance notice of your intentions. 

Since it appears the tree belongs to your neighbor, pruning beyond the property line could result in unwanted legal challenges, such as:

• Trespassing accusations: Damages for trespassing on your neighbor’s property can include paying restoration costs, the difference in the property’s market value if it’s proven to have decreased, and compensation for any discomfort or annoyance the neighbor claims you caused

• Criminal charges and penalties: Punishment for defacing a “boundary tree” in Colorado can include imprisonment, hefty fines, and more, depending on how many times you engage in pruning. C.R.S. 18-4-508; C.R.S. 18-1.3-501

• Harvesting a forest product: Depending on where the tree is, your neighbor could invoke the Colorado statute making it unlawful to harvest or remove a forest product from another person’s property without consent. C.R.S. 23-31-402

Giving your neighbor a heads-up can go a long way toward avoiding legal disputes and maintaining good relations. 

Alternatively, you could offer to help take care of the tree for your neighbor. While you may not want to take on the work, doing so could protect you from retaliation if your pruning efforts cause irreparable damage to the tree’s health. 

I recommend talking with a real estate litigation attorney with experience handling property rights and boundary cases.