Tree services are like other services you are availing. For example, how many times do you need your car to be washed? How often do you do your laundry? The number of times you will need this kind of services depends on the following factors:
Most people do not have the time to tend to their trees and this results in them hiring a tree service provider. If you have some free time to tend to your trees then you might need the service every once in a while. It can also be done once a month or twice a month depending on your preference.
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Busy people tend to hire a service provider very often because of their busy schedules that they can't even pause for a second just to cut some leaves on the trees.
Your budget is also one of the factors that determine how often you will need a service provider. It can be costly to hire this kind of service especially if you have other bills to pay on time. If you can allocate a specific budget for this service, then you might be available to avail it at least once or twice a month.
For those who cannot set up a budget for a tree service then it might be a good idea to set avail it every 3 months or depending on your financial status.