Ruling 2258
In a tree tending contract, it is not necessary to say a particular formula (ṣīghah) [for it to be valid]; rather, if the owner of the trees hands them over with the intention (qaṣd) of a tree tending contract and someone who does such work starts doing the work, the transaction is valid.
-Ayatullah Sistani, Practical Laws of Islam, Tree Tending Contract (Musaqah) and Tree Planting Contract (Mugharasah)