Plant them behind the north face of a fence, shed or your home. Although the USDA zones are generally accurate, there are micro-zones all over the place that can be as much as one or even two whole zones warmer or colder.
You may not get as deep a cold as actually being a full zone north, but the average winter temp of the north face of your house and it's shadow will be measurably colder than the south face. Low spots in the shade will collect cooler air as well, so if you have a natural trench or valley on the north side of your house, you will get it even cooler.
In Iowa, I lived in zone 5, however I lived on the south face of the river valley (meaning the river was north of me and the sun was mostly uphill from me). My Microclimate was zone 4, and arguably zone 3 depending on where I measured temps on my land in the winter. Although the official temps in the area were as low as -28 for about 6 weeks, there were days I measured -40 degrees.