Need help diagnosing what's going on with my hydrangea/ what Im doing wrong with it
main concern: Stem browning, drying up, and shrinking from mid section of stem. green on the bottom where it's yet woody and has some leaves sprouting there too.
other symptoms: larger leaves on other branches yellowing around the edges. newer smaller leaves on other branches yellowing and dropping. young leaves are mostly soft and not crunchy or dry.
Location: Edmonton, Alberta
zone: 4a
Light: 12 hours of grow light (don't have access to north facing or east facing windows), and 3 additional hours of shaded afternoon sun
Watering: when the first 2-3 inches of potting soil is dry. i usually use a moisture meter all the way to the bottom of the (5") pot to read dry and water thoroughly until water starts to trickle out of the drainage holes. I mostly use RO or distilled water.
Fertilizer: fertilized last week may 23 with miracle gro water soluble acid loving plant food. I followed the instructions for concentration for potted plants.
Cultivar?: Macrophylla x Serrata (lets dance sky view) hardiness 4
My initial thoughts: Stem borers, over watering, or maybe fertilizer burns?
attempts to treat: Initially I pruned the stem back to the next unaffected green node since it had a branch that was still green and had leaves. I also I did a flush to maybe rinse out the fertilizer thinking it could be fertilizer burn. Then I sat it's nursery pot on paper towels to help drain out the excess water for several hours constantly replacing them on the hour for 6-7 hours.
Other info: Im really new to the world of plants. I've had this little guy for only a month. I've done a bit of research and I'm regretting getting this guy specially for my area and living condition (I live in an apartment and only have access to a west facing window/balcony). But I really want to make this work for the little guy and already have plans on how to over winter him (if he makes it passed summer and fall)
Also I got it with buds which I thought weren't gonna bloom but they started to bloom around a week ago so im really confused.
Help and advice are appreciated