Jura's thinking was correct. This is an American saying. "To sugar" or "sugaring" refers to the process of making maple sugar from the sap of maple trees. A hole is drilled into a tree and the sap is collected, then boiled down to make syrup and/or sugar.
The sap is sweetest and starts to flow from the trees when the weather is right, which would be in late winter when the nights are cold and the days are warm (above freezing). This is also the time when the ice in the rivers melts causing the river to run free.
Don't ever sugar till the river runs free. It would be a waste of time to try and get sweet sap from a frozen maple tree in the winter. In the summer the sap does not flow and is not as sweet. You have to wait until the right time when the weather is freezing at night and warm in the day (when the river runs free).
By extension this could mean that to accomplish something you must do it when the conditions are right for success.