Ask Mr. Smarty Plants

There are 11 species of the genus Equisetum native to Michigan; we have chosen Equisetum hyemale (scouringrush horsetail) as a representative species to use as an example. Blue Rug juniper is the cultivar 'Wiltonii' of Juniperus horizontalis (creeping juniper), also native to Michigan.  So, we can understand your having both plants in your garden, as both are native; what we can't understand is why they are growing close enough together to be interacting. Equisetum hyemale (scouringrush horsetail) (and the rest of the genus) are very much wetland plants-they tolerate shade, poor drainage, occur in wet places and even standing in water. They are considered very aggressive, which you have already found out. Juniperus horizontalis (creeping juniper) is intolerant of shade and poor drainage, needs dry soil and does best on dry, sunny hillsides.  If the Equisetum is competing successfully with the juniper, then it's obvious the juniper is in the wrong place.

What to do? The first and most obvious solution would seem to be move the juniper, if that is at all feasible. If it is too large to move or there is no other more suitable spot in your garden, you will need to amend the soil, and perhaps prune back trees to provide more sunlight. Either way, you are going to have the get the horsetail out of the juniper, and the only way to do that is by pulling it out, and keep on pulling it out. It spreads by rhizomes, shallow underground roots. Most plants that spread that way are happy to snap off at the ground when you pull them, because those underground rhizomes are down there with food stored up ready to resprout the plant. 

To amend the soil for the juniper, you must improve the drainage. The best way to do this (without digging them up and getting them into a raised bed) is to work compost or some other organic material into the soil and continue to do so periodically. Also, mulch the soil, without covering the crowns of the juniper, with a good quality shredded hardwood bark mulch. This could help suppress the horsetail, although it's unlikely. The main benefit of the mulch is that it will protect the roots of the juniper from heat and cold, and as it decomposes it will continue to add to the organic content of the soil and improve drainage. 

Pictures of Blue Rug juniper

Equisetum hyemale

Equisetum arvense

Equisetum hyemale var. affine

Equisetum laevigatum