Answer: I'm sorry about your juniper. I suspect it died due to soil temperature fluctuations combined with the stress of being container grown under home conditions. Although quite winter hardy, when grown in a container these plants suffer due to the roots being exposed to severe cold in winter and also due to being exposed to oscillating soil temperatures in spring in fall. Without being in the ground, the roots are just not well insulated. Also, it may have dried out during the winter. You would need to water to keep the soil slightly moist like a wrung out sponge any time it is not frozen. Finally, container growing is extra stressful because the roots are in only a limited amount of soil. The soil may become depleted and compacted, losing its ability to hold air and moisture and nutrients. The roots eventually become too crowded and pot bound so the plant more or less strangles itself.
There really is no way to counter all of these risks in a cold winter climate such as yours. Most container evergreens such as this are lost the following summer due to accumulated stress -- if they manage to hold on through their first winter. I would suggest you use something less expensive and plan on replacing it every year, or use a deciduous shrub and plant it in your garden in the fall, then replace it the following spring. Alternatively you could use annual flowers or foliage plants, again replanting with new plants each spring. I wish I could be more encouraging.
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