植物: shokubutsu. If you want to be super-traditional, try to work "pine" or "bamboo" into your renku; these plants have special resonance in Japanese culture and, since they don't denote particular seasons, they can be used in seasonless verses.
When you mention other examples of plant life, make sure that you aren't accidentally evoking a season that doesn't fit your plan. If you're not sure, check an online guide to season words (keywords: haiku, renku, seasons, kigo).