On October 7, 2001, at 2200 hours Pacific daylight time, an Aero Commander 500, N7846C, impacted trees during climb out after takeoff from Petaluma, California. The private pilot and two passengers were not injured. The left wing of the airplane was substantially damaged. The personal flight was operated by the owner under 14 CFR Part 91. The flight departed from Petaluma at 2200, and was destined for Concord, California, where it landed about 2220. Instrument meteorological conditions prevailed at Oakland, California, 36 miles southeast of Petaluma; however, the pilot reported that visual conditions prevailed at Petaluma and Concord. No flight plan was filed. In his report to the Safety Board, the noninstrument rated pilot reported that he received a weather briefing and that visual flight rules (VFR) conditions prevailed at his destination, 31 miles to the southeast. It was a dark, moonless, night but stars were visible in the sky at the departure airport with haze to the west. Lights were visible 10 miles to the south. The pilot departed on runway 29 and made a right, downwind departure. During the departure turn, he encountered instrument meteorological conditions at about 900 feet. He reported that while in the clouds he had difficulty leveling the wings and there was "some altitude fluctuation." During this time there was a "pop" sound and the aircraft became difficult to control. He broke out on top of the clouds at 2,700 feet and proceeded to landing at Concord, his original destination. Control of the aircraft required him to input full right rudder control, 90 percent of available right aileron control, and to reduce power on the right engine. Inspection of the left wing revealed damage to the outboard 5 feet of the wing leading edge and embedded tree debris.