I've experimented with growing rhododendrons from seed, and it's a rewarding process, even if it's a bit of a test of patience. Timing is crucial, so you're on point about collecting seeds in the fall after the pods brown and open slightly. This is when they're mature enough for propagation.
For germination, recreating their natural habitat can make a big difference. Use a seed tray with a mix of peat and sand or a commercial seed-starting mix that's acidic and well-draining. These seeds prefer a cool environment, so maintaining a temperature around 60-70°F (15-21°C) is ideal.
You won't need to bury the seeds deeply—just a light sprinkle of soil on top. Keep the medium consistently moist but not waterlogged to prevent damping-off disease. Covering the tray with a clear lid or plastic wrap can help retain humidity, which is beneficial for germination.
Lighting is another factor. They do well with bright, indirect light, so placing them near a window with filtered sunlight works well. Some people use fluorescent grow lights to ensure consistent light exposure, especially during shorter winter days.
This process takes time—sometimes several weeks to a few months for germination—so don't be discouraged if you don't see immediate results. Have you considered any specific rhododendron varieties to start with, or are you open to trying a mix?