When It's Safe to Plant

Dear Tom,
When can I safely put my young tomato plants out without a chance of frost
killing them?
Charles Heidgen, Batavia
Dear Charles,
Noting that you live in far west suburban Batavia, we would caution to not
put out your tomato plants without protecting them on chilly nights until at
least the middle of May. If you lived in the city -- especially near Lake
Michigan -- it would be a totally different story; it would probably be safe
to plant them now. With Lake Michigan water temperatures already in the
middle and upper 40s, a combination of the lake's warming influence and the
urban heat island effect make the threat of frost negligible there after
about April 15. Further inland it is a far different story, and frost can
form on clear, calm nights well into May. Typically, areas west of the Fox
River don't cross the 90 percent safe-to-plant threshold until about May 20.