I have a "Common Purple Lilac" that I bought at Lowes and planted 2 years ago. It didn't flower at all last year. This year, it had 1 "bunch" (not sure what to call it) of flowers on it. I was told that now that it has flowered, to cut it all the way back to the ground and it will flower more next year, otherwise the bush will get bigger and bigger, but there won't be many flowers. Is this true?