The start of construction of $11.6 million in improvements to the Greater Des Moines Botanical Garden will be marked with a groundbreaking ceremony at 8:45 a.m. Thursday in the geodesic dome. Improvements include upgrading the original 1979 electrical and mechanical systems, and renovating the program and meeting rooms and lobby. The only addition to the building will be a café with an exterior terrace overlooking a new water garden and the Des Moines River. The Botanical Garden will be open seven days a week through June 30. After a two-month closure for interior renovations, the facility will reopen in September.