So. Once one gets to cultivating, and gets a seed of something, for example a tree, I see no reason for that tree not to give additional seeds. A farm should be autosufficient. Not dependent on foraging to get more seeds. It's the whole point of having a farm. To not go out and forage things. It makes no sense. You have it this way in any game that allows planting things. Not in Valheim. There are many ways to make it work too.
- Have each planted tree drop only one seed (this makes you need to explore and cut down more natural trees to get more seeds to plant at the same time)
- Have Tree seeds be an upgrade to the cultivator that allows to plant that seed from now on. Redundant tree seeds can be useful in case one loses the cultivator, or something (this makes it possible to plant as many trees of the "unlocked" type as you want)
- Give trees a chance at dropping between 0 and 3 seeds, so that with only one tree you could bust your farm, but for bigger farms you'll get more seeds on the long run.
Not being able to plant more trees in your farm without going out is a big NO for such games imho.
It's not a real farm if you can't renew your crops.