There are three primary causes
- too low temperatures
- old tissues (mostly on older leaves) and
- phosphorus deficiency.
So what to do?
- Raise the growing temperature.
- Prune off old leaves.
- Feed properly
The first two are easy to do.
It’s The Feeding That’s Tricky
Some Internet Sources Say Feed Bone Meal
You won’t solve the problem by tossing some bone meal on the plant and calling it a day.
Bone meal is not readily water soluble so it won’t act immediately (usually takes a year or more in the garden)
To Complicate Things: You May Be Feeding Too Much Nitrogen
If you’re feeding a little bit too much nitrogen, you may be creating a potassium deficiency.
When you overfeed nitrogen, the potassium is tied up and the plant appears to be struggling with phosphorus deficiency.
Yeah- it can be confusing but…
If you’re not feeding too much nitrogen, then you have to get some phosphorus to the plant.
If you’re feeding too much nitrogen, then you have to cut back on the nitrogen and add potassium.
Only you know the balance of the fertilizer you’re using.
Hint: use an equally balanced fertilizer such as 10-10-10 or 20-20-20 and not one with a high first number (nitrogen)
Step One To Cure The Problem:
First step is to flush out the container with water so that water is freely flowing out the bottom of the pot. This is going to flush out any excessive salts and nitrogen.
Step Two: Feed Properly
And then being feeding properly.
Use a balanced (or close) fertilizer -and after the leaves stop turning red and the problem has been corrected.
Every second feeding use a 20-10-20 to maintain the green leaves.
Note: some geranium varietes are more susceptible to this than others and other tend to have a reddish cast to their leaves. This redness I’m talking about is clearly seen as a “red ring” and not just a blush.
But combining the reddish ring with dying edges and you clearly have a fertilizer issue.