The red leaves on your geranium is caused by a phosphate deficiency. Gypsum only adds calcium which your water already has enough of...Do not add vinegar to potted zonal geraniums. Geranium roots emit a substance that acidifies the soil naturally this acid harms the plant over time, your hard water should prevent that... Water your plants with a water soluble fertilizer once a week and they should turn out fine... Bob.

Bob

I add calcium because the magnesium calcium balance is out of wack:

from a master garnder: Your well water is to be preferred over your softened water, even though your well water is quite high in Mg (Note how your softener removes nearly all of it?), and plants generally tolerate high Mg levels, as long as the Ca:Mg ratio is somewhere near 4or5:1. It's not, but there's an easy fix. A tablespoon of gypsum on top of the soil will add Ca and help bring the Ca:Mg back in balance.

I have added a water soluable fertilizer this past weekend so we'll see how it goes.

It might also be a result of the cool spring in the Chicago area. Geranium (pelargonium) leaves often get reddish in the fall, when the night time temps are in the 40's and 50's, because that temp range inhibits the absorption of some trace minerals like boron and magnesium. Now that the weather has finally warmed up, you may see them turn around on their own.

Schmoe, Cool weather will not cause red leaves. I grow my stock plants from november till spring in greenhouses heated to 55 degrees with night temps as low as 45 without any problems. I think geranuims grow and bloom better in the winter than in the heat of summer... Bob.

Sunnfarm, I am not disputing your results, but in fact, yes, cool temperatures do indeed cause reddening of the geranium leaves. As you state, you grow your stock plants in cool greenhouses, so it is probable that the average daily temperature is higher than plants outside with those same cool evenings. Geraniums definitely benefit from cooler nights, but mainly when there are warmer day temps.

I had the same trouble in my greenhouse. No one around here knew a cause, so searched and found one of the most common causes is Mg deficiency. I watered one time with a solution of 1 tsp epsom salt (magnesium sulfate) to a gallon of water. The old leaves didn't recover, but the new ones look very healthy. also incresed my nitrogen % in my fertilizer.