Hi

Drill some holes in and use a "Brush wood killer" , just pour in and on , just google you will easily find a supplier

You need a bottle of SBK, which is a brushwood and stumpkiller. Drill holes or wells, with a larger woodbit,, preferably in the outer parts of the trunks, an inch in from the edge, if the stumps are large, and fill the wells up with the SBK, cover with a sheet of plastic or an upturned pot or something and leave.