There are both regular and alternative types of seeds. If you plant a regular seed, a monster will hide an item and give you a crop of equivalent value instead.
When you plant alternative seed, you will receive a crop approximately half of the time, and an item the other half of the time.
The lord established this system to give people more choice in what to plan in a manor.
There are four kinds of seed--coda, cobol, codran and coba, available at different levels. Coda seeds are most appropriate for level 10 - 30 farmers, relatively low levels. Cobol seeds are for farmers between level 31 and 48. Codran seeds are best for level 49 - 64 farmers. Coba seeds are reserved for high level farmers of level 65 or above.
Obviously, different seeds grow different crops, and work better with certain monsters. A weak monster will produce a disappointing crop, but, on the other hand, if it is too strong, it may damage the crop.
One secret of a successful crop is to pick a monster with approximately the same power as the sower. Trying to sow a seed on a non-compliant monster can have disastrous consequences.
You may purchase any seed at any time, but the lord reserves the right not to purchase any crop at his discretion. You should always check to make sure that your crop is currently in demand.
Oh, and keep this part a secret, but I hear that other castles also buy crops, as long as that same crop can be purchased there. That's my backup plan in case the Lord changes his mind.