Ask when the yard is clean and the customer is looking at the result, then send the link by text before leaving. Reviews mentioning cleanup and care around the property are the most persuasive to future customers, because damage is the main fear.
Reviews drive the local map results that produce most tree work, and they answer the customer's real fear, which is damage.
The method:
Encourage reviews that mention cleanup and care around the property. This is specific to tree work. A review saying they left the yard cleaner than they found it, or that they protected the fence and the flower beds, is far more persuasive to the next customer than one praising speed or price, because it addresses the damage fear directly.
Reviews that name the type of work also help you surface for those searches, so a review mentioning a large oak removal near a house does double duty.
If you are starting from few reviews, work through your last ten customers this week.
The Tree Service BrandPack is ready to use, fully customizable, and done for you, so you get all the marketing assets you need.