Sow, all the time
The community seeds must be sowed all the time. While sowing seeds is a simple gesture, finding them represents the bulk of the challenge at this step of community growth & care strategies. Increase your community's attractiveness, make it a magnetic pole in your sphere of influence, span its boundaries towards neighbour communities. As an example, the eZ Publish community has strong affinities with the broad PHP community (core developers, ISVs, PHP-based tools, frameworks).
Use word of mouth, reach-out to the neighbour communities' members with all due transparency, make efforts to participate to their own events, share about your community's activities and plans, find possible collaboration (on product development, for instance). It is a two-ways process between communities, increasing respective awareness in one another. Your seeds will emerge, expressing some level of interest for what your community is up to.
The harvested seeds might be trees in other communities, they nevertheless need to grow from seed to tree within your community. Foster them to the next step in the fertile ground of your community : fragile buds.