I think most of us tend to start out planting too closely in order to get more plants in the area we have. I know I did, I think I started at 18 inches, finally worked up to 36 inches center to center, decided that was over kill and am now back to thirty inches, center to center for most plants. Some plants would be better left at the 36 inch spacing and as I get more experience on which ones really tend to spread I am going to work on spacing more adjusted to the individual plants. I have several that would be just fine with the 18 inch original spacing, don't they all look so small when you first get them?
