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Best bet
The waterproof Type I formula survives constant soil moisture — interior PVA would let go.
How to do it
Step by step
- Empty the planter and let the wood dry out fully — glue won't hold on damp wood.
- Scrub off soil and algae; sand the joint faces to fresh fibers.
- Dry-fit the boards and check the joints close snugly.
- Brush waterproof wood glue onto both faces.
- Clamp or screw the joint while it cures overnight before refilling with soil.
Conditions
When the answer changes
If the joints are gappy or the wood is warped, switch to polyurethane glue — it fills and is 100% waterproof. If a board is rotted through, replace the board; glue won't save rotten wood.