Hendricksons start hatching in the northeast after night time water temps stop dipping below 50 degrees for three nights in a row.  After the hatch starts however, it usually will continue through most bad weather.  Find Hendrickson nymphs in moderate to fast riffles and runs with gravel to softball sized rocks.  They will tend to become active an hour or two before hatching and “drift” in the current at this time.  Since the hatch is usually an afternoon affair in the spring, using nymphs in the morning and mid-day can be effective.