To draw out volunteers, it's critical that they know there is a an ongoing need. Also, some owners develop a suspicious nature about board motives when kept in the dark and resist being involved. Repeated pleas for help will have their effect. A newsletter and flyer distribution box the kind used by real estate agents is an inexpensive and convenient way to get the word out. Give credit where credit is due.
People love recognition.
Point out particular owners that show superior landscaping abilities. They are obvious candidates for the Landscape Committee. Consider awarding certificates of achievement at the annual homeowners meeting. Remember also that some tasks are ongoing and many faithful volunteers often get overlooked. It is a wise board president that makes a point of recognizing effort for its own merits.