Irises are hardy perennial flowers that are some of the easiest plants to grow. However, they do have some specific requirements to keep them healthy. Some issues can cause irises to be sick which will show in the leaves, and today I will explain what the causes are and how to remedy them.
Overwatering, underwatering, the wrong pH, and too many nutrients can cause iris leaves to turn yellow. To stop it from happening only water when the top 3” (7.5 cm) of soil is dry, move them where they’ll get 6 hours of sun, and replant them in soil that has a pH of 6.8.
Slugs should also be treated with bait, or traps and the soil shouldn’t be full compost as it contains too many nutrients for irises. Below, I will explain the symptoms to look for that indicate what is causing the leaves to turn yellow, as well as, what you should do to stop it from occurring on new leaves.
Overwatering that causes leaf spot
Watering irises too much causes a condition known as leaf spot. This is where circular areas of the leaves turn yellow and eventually light brown and dry. The yellowing areas will be of different shapes and sizes. This is caused primarily by overwatering where the soil is kept too wet for too long.
- Trim off the areas of the leaves that have leaf spot.
- Only water them when the top 3 inches (7.5 cm) is bone dry.
Not enough sunlight
As a general rule, irises need a minimum of 6 hours of full sunlight throughout the day. There are short, medium, and tall irises. Tall bearded irises need maximum sun, and shouldn’t be planted where they will be in the shade for any part of the day. Whereas, short bearded irises and all other medium and short irises only need 6 hours of full sunlight a day.
Tall irises grow to 3 to 4 ft (90 to 120 cm), which is how you can tell the difference between tall bearded irises and short bearded irises. Intermediate and short irises will be quite a bit shorter than that height.
The bulb structure where the stems and roots grow out from – which is called a rhizome – needs to be planted where it will get sunlight. So, you also need to keep space around the irises so that it gets sunlight directly onto it.
- Replant tall irises where they will get 12 hours of sunlight a day.
- Replant intermediate and short irises where they will get 6 hours of sunlight a day.
- Trim branches from overhanging trees that are blocking the sun.
- Remove adjacent plants that are blocking the sun.
Soil is the wrong pH or isn’t free draining enough
The pH of soil that plants grow in needs to be quite specific. For irises, the ideal pH is 6.8. If the pH is too low (acidic) or too high (basic/alkaline) it changes what nutrients are available in the soil. This occurs due to the chemical reactions in the soil. Due to the lack of certain nutrients as a result of the pH of the soil, the leaves will show yellowing.
The pH of your soil can be tested using an inexpensive soil testing kit which is available at most garden supply stores. Then different additives can be mixed or put on top of the soil to raise or lower the pH.
Irises also prefer free-draining soil. There are anecdotal reports from people who have been able to grow irises well in thick clayey soil. However, the American Iris Society recommends free-draining soil.
- To lower the pH add sulfur.
- To raise the pH add limestone powders – known as lime.
- Replant them in soil that is the right pH, and is more free draining.
Slugs that eat the leaves and create yellow spots
Various pest insects can infest irises and cause damage to the stems and leaves which will turn them yellow. Irises typically don’t develop pest issues but it can happen. Slugs will eat holes in the leaves. Or, suck the juices out of the leaves, leaving yellow areas.
Aphids and scale will eat the stems and leaves, which will sap the irises of nutrients causing the leaves to develop yellow spots or begin to turn yellow. Aphids are small green insects that you will see crawling over around on the stems, and the most sheltered parts of the leaves. Scale appears as bumps on the stems, and excrete a black, dry, crispy residue.
- For slugs put out slug traps or slug bait.
- For scale, aphids, or other pest insects spray them with a neem oil and water mixture after sunset.
Not enough water
If the soil is too dry the roots of plants including irises cannot absorb the nutrients they need and transport them throughout the stems, leaves, and flowers. This causes yellowing of the leaves. The edges of the leaves can also become dry, brown, and crumbly.
A good test to see if the soil is too dry is to poke a finger into the soil close to the rhizome. Put your finger down 3 inches (7.5 cm) and feel to see if the soil is damp or dry to the touch. If the soil is completely dry to the touch at this depth or shallower then you should give them some water.
If the soil is too thick to do this then you can dig out a small section using a garden trowel, or a spade, and inspect the soil. Over time you’ll get feel for when it’s a good time to water them based on how much rain you’ve had, how hot the weather is, and how free draining your soil is.
- Water them when the top 3 inches (7.5 cm) of soil is dry.
- Monitor the soil once every week or to until the leaves are no longer yellow.
Too many nutrients in the soil cause soft rot
Although there are a few potential causes of iris leaves turning yellow soft rot is easy to identify. The rhizome will be soft and mushy to the touch. To identify this issue dig around the rhizome and feel it with your finger to see if it’s soft. The rhizome should be firm like a raw sweet potato.
Iris does not need a lot of nutrients in the soil, and regular potting mix or compost from a garden supply store is generally too nutrient-dense. Soft rot is also caused by the soil remaining wet too long, which is a sign of overwatering.
But, can also occur if they’re planted in a depressed area that is lower than the surrounding soil. This causes water to pool and the soil can become waterlogged. Once soft rot gets to an advanced stage the leaves will fall off and the plant will die.
- Replant them in soil that is a 50/50 mix of compost and regular garden soil.
- Replant them where water won’t pool and the soil doesn’t get waterlogged.