Hi, i planted my vinca minor on a large bank a few months ago. Now i am starting to wonder if i did the right thing planting these. a lot of them look shabby, some without leaves, some yellow. They don't seem to be growing too fast at all. Is there anything i can put on them to accelerate growth? I am thinking about pulling them out and putting something else in. It is a bank about 80 ft. Long x 10 ft. wide.