I am playing a Harvest Moon game that is very similar to Island of Happiness and working on guides that will be available when the game is released. As in Island of Happiness, some items that are found in the Wilds are more common than others. In ANY Harvest Moon or Rune Factory game, however, you may note that there are specified 'drop' squares where items tend to appear. Keeping these squares CLEAR of any items will allow more items to appear. When the item is rather rare and is found only in a specific location, it can be vital to visit that location regularly if not daily in order to clear any other items from the spaces. Items such as stones and branches can be transformed into Materials for Building but even if you do not have time or a need to do that, pick up the stones and branches and stash them in your Rucksack. Doing so will clear all the potential 'drop' spaces, giving you a better chance of finding the rare item you seek on the following day.
This is true in Rune Factory as well as Harvest Moon. In any Rune Factory game, wild items can be found on tillable fields in the dungeons. Tillable squares can be cleared, ploughed and used to plant Crops but if you wish to continue to find wild coloured grasses as well as materials for building, you may wish to clear some fields in each Dungeon without tilling them. That way, you preserve a location for
branches and wild grasses to be 'dropped'.
In Rune Factory Frontier, Black and White Grass tend to be rare compared to wild grasses of other colours. They both can be found on your field randomly after a severe storm and throughout the Winter season. Set a tame Monster to the task of gathering them in order to collect as many as possible, and keep your field cleared of branches and stones in Winter in like fashion, with tame Monster labour. White Grass is an ingredient in Energy Drink X. It can be found in Urns in the Snow Ruins and in a Tower on the Floating Chamber of Whale Island randomly as well.