Complex sentence: He began modestly by attending the annual plant sale at the Arnold Arboretum where he would acquire woody plants to donate to the Pass Arboretum, using his yard as a nursery to develop that plant material for transplant.
Split into simpler sentences: He began modestly by attending the annual plant sale at the Arnold Arboretum where he would acquire woody plants to donate to the Pass Arboretum. He used his yard as a nursery to develop that plant material for transplant.