I fought the buggers for over 2 years and have finally won the battle. Almost everything I used worked little if at all. Each time I thought I got rid of them, they'd show back up a month latter, worse than before. The problem is, and what most people don't know or don't talk about is- there are different types of spider mites. Many things work for one or two types but not all of them. If you read most of the label for mite killers, they list the mites they kill but often I see some mites listed but others not. So, it's important to identify the type of mite(s) you have.
OR, go this root and I can almost guarantee you better results than most if not all of the products previously mentioned.
Go into flowering because you have 2 weeks or so before any buds have begun. You're problem will be solved by using the following recommendations:
1. With lights on or off, but no ventilation, go to Walmart and buy a pack or two of the No Pest Strips and hang them in the room for two days. It releases a vapor that will kill all spider mites. Make sure you shut off ventilation and seal the room very well. Let the vapor build in the room do it's stuff. I do it for 2 days to be sure. I always use 2 strips in a 10 X 12 room.to
2. Then in 2 days, vent the room well before you re-enter. The vapor is no healthy to breath obviously. Then as the second part, go buy some Dawn Ultra dish detergent and mix 2 teaspoons per quart of warter and spray the plants well for 3 days in a row or every other day for 3X. This soapy water will smother all mites. Make sure you do it very thoroughly as if you miss 1 mite, they'll be back!!!!! I rinse the plants off well about 20 minutes after each spraying. The mix does not have much soap in it and won't hurt the plants, but you don't want it to build up over time so a nice drenching is good and it also rinses the bugs off the leaves.
If you don't believe me, go to youtube and watch some of the videos about Dawn killing spider mites, along with many other insects including white flies, thrips, gnats, etc. It's very effective. Many companys will try to sell you the latest and greatest mite killer for mucho bucks, and many of them aren't very effective, no matter what they say. If many of these products worked, you wouldn't keep seeing so many spider mites issues from growers everywhere. None of the other products killed off my problem totally and I tried them all. Probably spend $500-$600 over a couple of crops. My way will cost you $15 tops, and works better than all of them.
Lastly, DO Not Knock it until you've tried it. It's very effective if you spray every plant well, including the pots, walls, floor, etc. But especially the plants from top to bottom.