While most plants won't be damaged by getting hit with wafting over-spray, direct hits can do real damage. Wax or oil. Be careful considering Linseed Oil based wood protectors. In our experience they attract quickly black mildew. Professional sealants, which are typically superior, are oil-based.
Some sealants contain stain to tint the color of the deck.
Good quality sealants contain ultra-violet sunlight protection to reduce the damaging effects of the sun. Be careful in your selection of the sealant and consider the toxicity to humans when walking barefoot on treated wood. There are new decking material options. Several companies make new decking materials that don't splinter and don't require painting or sealing. Most are made of plastic or a blend of wood and plastic.