Blue Hydrangea in a Pot: Aluminum Sulfate, Vinegar, and the Salt Trap

You're in North Texas (Zone 7b-ish) with potted Hydrangea macrophylla, using aluminum sulfate (1/4 oz/gal) to maintain blue. You're drenching first, using 25-5-30 fertilizer, and wonder: frequency, amount, and will old blooms re-blue?

Critical Concepts

• Blue = aluminum uptake at pH 4.0–5.0. Aluminum sulfate acidifies AND supplies Al.
• Potted = balancing act. Everything concentrates: salts, pH, nutrients.
• Aluminum sulfate = sulfuric acid salt. High salt index = root burn risk.
• Old blooms don't re-color. Only new buds reflect current soil chemistry.

Better Protocol

1. Alternate: Aluminum sulfate (monthly, spring only) ↔ Vinegar water (1 tsp/gal, every watering).
2. Flush monthly: Heavy watering to leach salts.
3. Never let pot dry out. Hydrangeas wilt fast.
4. Use rainwater (no chlorine, neutral pH).

Fertilizer Fix

• 25-5-30 is wrong. High N/K, low P. Phosphorus displaces aluminum → less blue.
• Use: Aged cow manure (mildly acidic), or low-P acid fertilizer.
• French secret: Powdered slate (slow-release Al) — hard to source.

Texas Reality Check

• Your water is likely alkaline — fights your acidifying.
• Buds are frost-tender — protect in winter.
• You're pioneering — document everything.

Free Resources

• Starbucks coffee grounds (free, acidic, worm food).
• Unfinished OJ, apple cider, black coffee — pour on hydrangea.
• Peat moss + coffee grounds top-dress.

Go slow. Measure pH. Flush salts. Blue comes in seasons, not weeks.