I have several yucca plants located around the back yard which gets mid-day and afternoon sun. I have had some for over three years but none have ever produced a flower spike much less flowers. Around town I see many yucca's that have spikes and even a few have flowered. What is going on? How old does the plant have to be to flower here in WI? Should I be doing something I am not? I water weekly if there is no rain and use fertilizers twice a year. Soil is sandy with compose added.