First time outdoor auto grow, north uk

About halfway through week three now, not seeing much height growth but definitely noticeable gain to the leaves, all of them are starting there next lot of branches. The XXL is the biggest out of the three but that doesn’t surprise me, the clues in the name 😂 Decided to throw some BioBizz Bio-Grow in the mix, only to find out that it’s 100% molasses which is what i’ve been feeding it anyway, fed at a 50/50 mix of each hasn’t done any harm, and they definitely shot up a bit after feeding! Did some research on the small spots i was getting on one plant and seen that it might be a magnesium/sulfur/calcium deficiency and heard that small amount of Epsom Salts can help. Added 2tsp to my water and can see the affects already! One question, ive noticed the XXL has developed a deep purple stem (see photos) is this something i should be concerned about? It’s clearly not affecting her growth, hoping it’ll mean some purple buds are coming my way! 🤞🏻