The two dragons, for some reason, are incompatible and cannot be bred together.
Cold and lightning
To date, there is no grass dragon in DragonVale. You may be thinking of the plant dragon. See the related question.
Metal and lightning
Fire Dragon and Lightning Dragon
Water dragon.
Level 11
Reported results of lightning + obsidian are:obsidianfireflyairmotleyspringaquamarinesolar eclipseequinoxmagneticcrystalstorm
Meteor+Lightning & Air+ Copper
I got it by using a Lightning and Plant Dragon.
Cactus and Ash Dragons.
a scorch dragon is made by breeding the lightning and fire dragons.
I don't know i think people just mean 'lightning'!