by Connie Oswald Stofko In a new feature this year, you can see free demonstrations on creating great mixed containers at the Great Plant Sale to be held in the Administration Building of the Buffalo and Erie County Botanical Gardens, 2655 South Park Ave., Buffalo. Admission to the sale is free. The sale will be held Thursday, Friday and Saturday, May 19, 20 and 21. (I’ll have a table there, so stop by and say hello!) Thursday is for volunteers…