The same tank as the betta? Or is the tank now empty of fish?
If there are no fish, just raise the temperature to the high 80's and let it sit for a week, consider going to 90 but keep an eye on the plants at that temp.
The higher the temp, the faster the cycle. At 86 it has been shown that ich can no longer reproduce, and at 90 they die. 86 is not too high for a betta, they are actually a warmer water fish and breeding is often easier in the low to mid 80's.