The contract follows a 10-year £13m grounds-upkeep and waste-services contract secured by the Landscape Group with the two local authorities last week.
"Hastings and Rother councils have again joined forces to secure yet another major contract and save money in the process," said a spokesman for Gristwood and Toms, which will look after trees in parks and open spaces and woods.
"As a result of putting the tender out as a partnership, the new 10-year tree maintenance contract, which began on 06 November, has resulted in reduced costs for tree work for both councils."
The contract also allowed for AmicusHorizon housing association to join the contract and take advantage of more competitive rates for tree works, he added.
Hastings councillor and head of environment Emily Westley said: "We have shown that when local authorities form partnerships to procure contracts we can get a better deal for the tax payer while maintaining excellent standards of care.
"We recently procured the grounds-maintenance contract with Rother District Council and AmicusHorizon and we are excited about continuing this partnership and welcoming Gristwood and Toms to Hastings and Rother."
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