Good Morning Cheryl, 

Thank you for contacting the U of M Extension Service. 

Please click on the following link about gardening in the shade: https://extension.umn.edu/planting-and-growing-guides/gardening-shade

This information states: 

Growing plants beneath large trees or under the overhang of a building is challenging because they prevent even plentiful rainfall from reaching the plants. The dry soil and lack of sunlight create difficult growing conditions for plants.

The drip line of a tree is where the rain drips off the furtest out branches. Any plants you plant within the drip line of the tree will be competing with your trees for light and soil nutrients.

I would consider adding wood mulch under your trees, a few inches from the trunk, going out in a circle for a few feet. The wood mulch will help to keep your trees healthy, the soil cool and moist and protect the roots. Plant your shade plants outside the dripline. 

There are many current diseases of pines and spruce in Minnesota. If you search for pine tree diseases and spruce tree disease on the U of M Extension Yard and Garden website, there are many. If your double row of tree are healthy and vigorous, I'd be careful in disturbing the roots with plantings. 

 If you start digging in the soil under the trees, you will also be injuring the roots of the trees. You don't want to add too many inches of soil under the trees as this might smother the existing roots. Many of the tree roots are within 18" of the top of the soil. 

Here is another link from the website on ornamental grasses for the shade: https://extension.umn.edu/landscape-design/ornamental-grasses-shady-sites

Good Luck!