Low nitrogen fertlizer is unnecessary.
Don't sweat the low N stuff...
Our work has shown that fertilizer composition has no effect on cold
hardiness. Fertilization may prolong the growing season, hence may increase
early frost damage.
The safest fertilizer for end-of-season supplementation is a complete (NPK)
All trees germinate, grow, bloom, then grow again, bloom again and then again for years with the same non optimal mineral nutrition.
Why should it be different in a pot ?