While "first planted doesn't give seed" is true, it's good to understand the underlying reason. When you are planting seed B, it has a chance to produce seeds based on what it is placed besides. For this reason, the first planted can't yield seed because it's planted besides nothing, into an empty garden. However, you can very well plant seeds besides other plants planted at different time. Even if they are harvest able, just first pick up corners, so you have four empty slots that are all besides some existing plants. Sow something there and pick the other 4, leaving again 4 empties that are besides something.