Ripen affects either a single mature tree or all smaller plants covering a 10-foot square plot of ground. It causes the affected plants to immediately produce the normal amount of fruit (nuts, seeds, or whatever) they bear at the height of the harvest season. Thus, an apple tree will be laden with ripe apples, a patch of grain will mature, and so on. A given tree or area of ground can do this only once per season, and the spell won't work outside of the plant's normal growing season. Any produce not picked or harvested within 24 hours withers and dies.
The material component is mistletoe.
Notes: Very rare druid spell. (Updated from Dragon Magazine.)