There is a common misconception when it comes to sealing or staining your deck or fence. Pressure treated lumber is treated in such a way that will prevent major damage due to weather and insects for a short amount of time. You can stain your lumber as soon as it can properly absorb the product. Therefore when it comes to a point that dirt is being absorbed and water no longer beads off the lumber it is safe to start cleaning the wood. After the wood has had time to dry- it’s time to stain.
Although, waiting for the right weather is key. You need a very low moisture percentage on the lumber so it can properly absorb the stain. You also need at least 48 hours of warm dry weather so the stain doesn’t sit on top of the lumber for too long. If this does occur, it will attract dirt and ruin your finish.