i have had spider mites many times on plants outside since i have a big garden, i use NEEM OIL, its extremely effective and keeps all bugs (spider mites, white fly, aphids,etc..) away for 3-4 days, kills spider mites in minutes.
You must spray hole plant, top to bottom all leafs coverd, even with up to 6 weeks in flowering you can use it since its organic you must respray every couple of days for eggs that would have hatched.
Do take caution to not spray when not needed since it slows growth a little as i experienced.
neem oil can be orderd from the internet or bought at some garden centers, i buy mine from a growshop, good luck.