The furhtest any of my characters have got in the game is Marleybone. One of my wizards has her plants infected with Explosion Gnats (rank 4) and from what I hear I need access to Moo Shu to buy the level 4 pest killer.
Any other way to deal with these, short of ploughing in the plants?
I don't think there is any other way. Just hang by the world gate and ask if anyone is going to Mooshu. Someone should be willing to friend you to get you in!
The best option is to look at the plants and how hard they are to grow, and the level required. (Also going to Wizard 101 Central to see pest types for each plant) Just plant those you can handle.
If you get caught with your overalls around your feet, and you have a higher pest than you can handle, just beg to go to MS or CL and buy the spells there if you are rich.
Or just bury then, sigh.
Plan ahead, react to inndeiate situation, or give up are the options.
I found two more possible solutions. There are such things as gardening spell treasure cards - my wife got one in trade but her wizard doesn't have high enough skill to use it yet.
Two of my characters are affected and one had a plant die on her. Instead of ploughing it under she's going to leave it in the pot until she has the resources to look after it, then revive it. The pot cost a lot less than the seed did.