Juniperus horizontalis 'Youngstown' Bob Fincham, Coenosium Gardens, explained about this cultivar "Juniperus horizontalis 'Youngstown' (1973) is a selection of 'Plumosa Compacta'; but lower growing, very flat, bright green: originated by Plumfield Nurseries, Fremont, Nebraska".
Iseli list this plant as Zone 2 and I would not disagree. I live in zone 3a and even during a very bad winter some 3 years back I did not observe any damage to this plant.
I will add that this cultivar exhibits an outstanding plum-purple color beginning after the first fall frost. The color deepens as the winter progresses returning to a dark green in the spring.
I have purchased some plants that were labeled 'Youngstown' but are clearly either reversions to Juniperus horizontalis 'Andora Compacta' or they were mislabeled. I believe that such mislabeling is common with this cultivar. One big distinguisher is the fall plum color. My 'Youngstown' appear a more pleasant intense color than the 'Andora Compacta' which is a plum-gray color.
This picture was taken in 2001 in my yard. The two plants had been in the ground for 2 years and were purchased as 2 gal. plants that were about a foot across. Within the 2 years they had grown to over 2 ft. across.
Photo by Ken Church
"but are clearly either reversions to Juniperus horizontalis 'Andora Compacta' or they were mislabeled"
Oh, thank you! for pointing that out. I was scratching my head over the difference between the two. Additionally confused by Monrovia's web page "Youngstown Andorra".
I'm also observing mislabeling between Seagreen juniper and Mint Julep juniper.