Can I sub out the tree work on a city grounds contract, and do I have to tell the city first?

You need to check the specific city or state procurement rules to determine if you need prior approval to subcontract on a city grounds contract, as these rules vary significantly from federal contracts. Federal contracts follow the FAR, but state and local governments have their own procurement codes and vendor guides. Most city contracts will have clauses addressing subcontracting, even if an IFB does not explicitly state it. You will likely need to amend your insurance to add the city as an additional insured and ensure your subcontractor also has appropriate coverage naming both you and the city. Prevailing wage rules, if applicable to the contract, generally apply to all workers performing on the contract, including those employed by subcontractors. If the tree service damages property, your company, as the prime contractor, would be responsible to the city, and your insurance would need to cover the damages, potentially through a claim against the subcontractor's insurance.