I dunno, maybe this is what you are looking for?
http://docs.openclonk.org/en/sdk/script/fn/PlaceVegetation.html

I am not 100% sure, but wheat should have a Place() function. So if you want ten wheat in your scenario, you can just do Wheat->Place(10)

Don't use the [Vegetation] section of the scenario.txt, it's outdated.
All plants can be inserted via script (so Initialize in your scenario preferably) by using Plant->Place(x); where Plant is the id and x the amount (there are more parameters, just look it up in the plants library - maybe some of the plants even add something to the function in their own script).