Have you had the soil tested for poison? are any other plants dieing? Generally 1.8m is the max height allowed by councils.

I would look into why the plants are dieing then solve that and plant a screening hedge like Lilli pilli or bamboo for privacy.

Crickey, your neighbour sounds like a d***!

Depending on how big they are, and check with your council, they have have been protected - all trees over a certain height (with some species excluded) are protected and you need approval from council to do any works to them, or remove them. Any unlawful damage or removal of trees, if they were protected, could meet a hefty fine!

If you think it's worth the risk then yes replant the trees, or you could plant slender weaver bamboo and remove the lower branches and leaves so the bare stems are visible and will contrast with the nice timber screen you have put up and plant an understorey or your choice. Would be a nice border and give your the privacy your after.

must be the neighbours we had a lovely pine hedge that's been there since the house was built when our lovely neighbour moved we had new ones that hates trees grass anything green our hedge died mysteriously first they trimmed all their sides down then next thing we knew our whole hedge died.

I was so upset we ended moving should have known because as soon as they moved in they poisoned their lawn it went brown lol

what strikes me funny is we lived in a green wedge..parks all around..everyone in our area loved trees.. if they didn't like trees then why didn't they move somewhere else? leave the green wedge area for tree loving people? lol nutcases they spend good money to buy in the green wedge to totally destroy it lol

Are you thinking of something like a pergola or gazebo with a solid or latticed wall backing onto the neighbour's? In the circumstances, that seems like a better idea than using plants. Bamboo, while a good screen, would drop its leaves and I suspect that would cause even more grief for you.

Yes, I would not plant anymore trees. IMO anything that impacts a neighbour like overhanging branches should be pruned back anyway. That's what we do with our trees. Recently my nicer neighbour was outside when I was tidying climbing flowering plants, I was able to ask if she wanted me to cut them from her side, she said no, she liked them. Some wouldn't.

Agree with Jmm.

I don't know what size your yard is or what the rest of it looks like but you might consider bringing the planting further in, away from your neighbour's deadly reach.

The grass doesn't look healthy so creating a garden which divides the garden into 'rooms' and creates interest and mystery as well as screening can be very rewarding.

Will he poison plants if they're not touching the fence eg, with foliage 600-700mm away from the fence?

If that's OK there are some tall narrow shrubs which you could plant close to the screen, leaving some space between the shrubs and the fence.

Syzygium 'Pinnacle' (tall narrow Lilly Pilly) ...reasonably fast.

Camellia sasanqua 'Marie Young' - 2.5-3.0m tall and very narrow, initially slow but picks up once established.

Im sorry you and your other neighbour lost your trees. What an awful neighbour. I hope the police take this seriously. I think soil testing is a good idea to identify what has been used. There's risk to native wildlife, pets, children playing in the yards...