Hi

We bought a house last year and 4 syacmores growing on the borders just inside the neighbours gardens. They have not been planted, just growing basically as weeds. The previous owner cut them down to the ground and tried to poison the stump, but failed and now 3 have grown up as lots of thin trunks, the tallest about 10-12 foot tall. The other was not cut and is therefore only one trunk

So my questions are - for the single trunk, how low down do I have to ring it to kill it?
For the others - how can I kill them?! Is poisoning the only way, and if so whats the best (legal) one, and how do I apply it?

Thanks
Katherine