It’s frustrating to see the foliage of your geraniums yellow. When it does, the obvious question you’ll be asking is why are my geranium leaves turning yellow? There’s a half-dozen reasons this can happen.
The first and not so obvious is you aren’t growing a true geranium. It may be a zonal pelargonium which grows faster but the underside of the leaves yellow naturally before they die and fall off.
For true geraniums presenting with yellowing leaves, many of the causes are the growing climate. These are full sun loving plants that don’t do well in cooler temperatures or when there’s inadequacies with their watering requirements.
Generally, the better the growing climate, the better geraniums grow.
7 Known Causes of True Geranium Leaves Turning Yellow
1 – Overwatering
The leaves on geraniums turn yellow when they get too much to drink. The tell-tale sign of this is when the affected leaves are all at the bottom of the plant.
You’ll tend to notice this if you’re growing your geraniums outdoors in full sun and get several inches of rainfall during the spring or occasionally in the summer months too.
It’s best to water geraniums when they need it and the simple way to know that is with a finger test. Only give your geraniums a drink when the top 1-inch of soil is dry to the touch.
As geraniums are sun loving plants and native to South Africa, they aren’t tolerant to water and do not do well when there’s too much of it.
2 – Underwatering
Geraniums are very drought tolerant so it’s safer to underwater as they’re among the few plants to survive a heatwave. So, if you’re unsure if you should water or not, leave it be for a day or two then re-test the soil by poking your finger in it to check moisture levels.
When geraniums are affected by drought, you’ll notice that it’s just the edges of the leaves that turn yellow. It’s far easier to give your plant a drink than it is to wait for the soil to dry out. Besides, too much water also risks pesky gnats.
Proper Watering of Geraniums
When 1” of the topsoil is dry to touch, water it from below. You want to water the root ball for that to feed the plant. When the leaves are left wet for too long, it can cause Bacterial Leaf Spot (covered below), which shows as yellow spots on the leaves.
3 – Sunlight
Geraniums need to have around 4 to 6 hours of full sun each day. Ideally, morning sun as by the afternoon the temperatures are warmer and the plant will dehydrate faster.
The ideal grow climate for geraniums outdoors is somewhere with full sun in the mornings and some light shade in the afternoon.
4 – Cold Snaps
Cool snaps can cause the leaves on geraniums to yellow. In particular, when it’s wet and cool weather for an extended period of time. The effects of even a light freeze can be brutal to geraniums.
Outdoor plants are more susceptible to frost damage in the early Spring. Stay updated with the weather forecasts at this time of year and you should have enough time to prepare for a freeze by insulating your geraniums and other tender plants.
The simplest thing to do when you know there’s a frosty night forecast is to water your plants. It sounds illogical but when frost is forecast, it’s the only time to deliberately overwater your geraniums because water acts as an insulator, strengthening the foliage. When the temperatures drop, there will still be heated moisture evaporating giving some extra heat to the plants.
5 – Micronutrient Deficiency
Geraniums need to be fertilized at least once monthly or bi-weekly during the growing season. A balanced water-soluble fertilizer of 20-20-20 is ideal.
One of the core secondary nutrients geraniums need, as with all plants, is magnesium as that directly affects the chlorophyll molecule responsible for the green pigment on plant foliage. A magnesium deficiency can cause geranium leaves to turn yellow. Others than can contribute to geranium leaf discoloration are sulfur, zinc and iron deficiencies.
Check the label on your fertilizer to see if magnesium is included. If not, you can supplement it by adding some Epsom salts as the chemical name for that is magnesium sulfate heptahydrate. And you can have yourself a nice Epsom salt bath too. Good for you and your plants.
The simplest way to know if your plants are lacking vital nutrients is to test your soil with a soil test kit. Preferably one that will give you a pH reading and the Electrical Conductivity (EC) reading – a measurement of soluble salts in your growing medium.
Guidance readings from Michigan State University suggest EC readings should be between 1.0 and 2.0 with pH readings varying between different geranium species:
- pH for Zonal Geraniums: 6.0 to 6.4
- pH for Regal and Ivy Geraniums: 5.5 to 6.0
6 – Geranium Diseases
There are four types of diseases identified that cause the leaves on geraniums to yellow.
This is most noticeable on the underside of the leaves that look like V-shaped lesions and yellow/brownish water spots. Bacterial blight starts at the lower part of the plant resulting in wilting and yellow spots. As it progresses, it can cause stem rot and will eventually affect the entire plant.
It’s also fast spreading so if you suspect your geranium is affected, isolate it from all of your other plants and thoroughly sterilize any tools you’ve used on the plant to prevent cross-infection.
Bacterial Leaf Spot
This is similar to blight but without the wilting. You’ll only see yellow leaf spots. Known causes are the leaves being wet for too long, which is why you should water geraniums from below and never wet the leaves.
High humidity is also a contributing factor. Like Bacterial Blight though, it can spread and you’re best to discard the plant and sterilize your tools.
Southern Bacterial Wilt
Symptoms of Southern Bacterial Wilt are the same as Blight, only this disease always starts at the soil line and progresses upwards causing discoloration as it goes.
Blight will cause the lower part of the plant to discolor and wilt, whereas with Southern Bacterial Blight, the discoloration and wilting will always progress upward from the soil line.
There are around 15 known viruses that can affect geraniums causing the leaves to yellow. These are trickier for growers as geranium viruses can take as long as three weeks to show any symptom of an infection. By this time, it’s likely to have spread to nearby plants by water splashing or insects including aphids and thrips.
Viruses can also be spread by propagating as most geraniums are grown from the cuttings of mature plants.
A geranium affected by a viral infection can survive but it won’t produce as vibrant blooms and it can cause the leaves to yellow. While difficult to detect, early signs of a viral infection are reduced flower sizes, poor rooting and stunted growth. If you notice these, it’s best to isolate the plant to prevent spread.
7 – Herbicide Injuries
Accidents can happen when you’re using herbicides for weed control. When you’re weeding around geraniums, it’s safer to weed manually and frequently. Not all herbicides kill geraniums but they will cause a bleaching effect on the foliage.
The safer approach to weed control around your plants is to use mulch which prevents you from having to resort to herbicides that could result in accidental damage to your geraniums.
Related Reading: Geranium Care: Learn How to Plant, Grow and Care for Geraniums