as i have sprayed convolvulus with an approved spray all the top has died off but the tuba still in the ground re shoots and grows again. Is there a spray that will also kill the tuba when sprayed on the foliage.
Try painting the cut stumps with glyphosate (100ml/L) or metsulfuron-methyl 600g/kg (1g/L).
Convolvulus verecundus is actually a threatened New Zealand plant species (At Risk-Declining) http://www.nzpcn.org.nz/flora_details.aspx?ID=244 So hopefully the species that is causing you issues is not a native but one of the introduced convolvulus species.
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