If covering is out, then aside from the very expensive (giant fans used by commercial growers) and the very wasteful (spraying water continuously from the time the temp dips below 32 until it recovers again) anything else you try is probably useless.

The good news is that "frost" does not necessarily mean no crop. "Hard freeze" is the scarier term (meaning below 28) but even temps in the low 20's can be tolerated for varying amounts of time by different fruit trees at different stages of development. Obviously the farther the plants are along, the more damaging a freeze will be.

Google "MSU Fruit Bud Hardiness" to get a better idea about what kind of hope you can cling toâ¦

Good luck!