That is why it was suggested that you cut a groove in each thick stem and apply the herbicide. Some gardeners have had success with using a hand shovel to attack the roots and woody parts of the plant, chopping them before applying an herbicide such as Roundup. This practice is easier if one person pulls back the ivy and another uses the shovel. After this, particularly if you have sandy soil, watch for re-emergence of the plant, as it has extremely deep roots. It may take some time to get on top of the problem.
Persistence is the key.
Your geraniums are being eaten by the tobacco budworm, a striped caterpillar that may be greenish, tan or reddish in color and up to three-quarters of an inch long, says UC master gardener Bill Pierce. Budworms chew round holes in foliage and burrow into buds of geraniums, petunias, calendula, penstemon and nicotiana. Damaged buds appear dried up, fail to open or open with tattered flower petals. Handpicking can be effective if performed on a regular basis. Bacillus thuringiensis, a natural organic product, provides good control.