There are several reasons why irises may turn brown, wilt, or die. Many are related to improper planting and/or moisture levels, which invite trouble from bacteria, fungi, and insects. Others are related to natural plant cycles and routine maintenance.
Although considered relatively easy to grow, flowers in the Iris genus are not without issues. If a stroll through the garden this season brings heartbreak and disappointment rather than full-throated blooms and healthy green foliage, you’ll want to get to the root of the problem ASAP.
Since identifying the source of an unsightly iris is the first step in taking action, we’ve gathered information on 9 issues that might zap an iris’ healthy appearance and provided some suggestions for addressing each one.
Too Much Water
Excessive saturation is often indicated by browning, soggy, or wilted leaves in irises.
If your struggling irises are in the German, Dutch, or Siberian families, look closely at planting locations and moisture levels. Irises derived from these species require well-draining soil and high planting locations that do not puddle or hold standing water. Whether your irises have bulbous or rhizomatous roots, browning, soggy or wilted leaves often indicate excessive saturation.
Established irises do not generally need supplemental irrigation unless they’re experiencing extreme drought conditions, but new transplants will need to be watered more frequently. That makes them vulnerable to overzealous watering practices. To efficiently water a bed of new irises without flooding their roots, use a soaker hose or mist spray to deliver a slow and steady soaking once or twice a week rather than a hard, fast blast every day.
Directing water at the soil surface rather than leaves and flowers will also help discourage fungal issues above ground (see potential diseases below). You should also water in the morning rather than evening so leaves can fully dry out during the day.
Not Enough Water
Insufficient moisture may cause dry, browning leaves in aquatic or semi-aquatic irises.
If a sickly, browning patch of irises is from an aquatic group (Water, Copper, or Flag) or a semi-aquatic group (Louisiana, Japanese), they may suffer from inadequate amounts of moisture rather than too much. If you’re caring for an iris from a water-loving species, and it has dry, browning leaves that are curling or wilted, you might need to step up your watering game.
Most aquatic and semi-aquatic irises need their roots constantly moist during their active season. This might mean 6 inches or so of standing water for some species.
For others, it might mean slow-draining containers or low-level planting locations that never completely dry out. The best way to know if your iris needs more water is to identify its species and adjust your watering routine to meet its specific moisture requirements.
Improper watering can cause various issues for iris plants, such as soft rot.
Not surprisingly, improper watering can open the door to a whole world of problems for iris plants. At the top of the list is soft rot, a bacterial infection that attacks a plant’s roots and turns them into mush.
An iris suffering from soft rot might have small, weak leaves that collapse suddenly or dark flower stems that lose firmness near the soil surface. Secondary symptoms include brown circular spots with yellow margins on the leaves. Sometimes black crystals will appear on lower plant tissue or surrounding soil.
The best way to determine if soft rot is the culprit is to dig up your iris and examine its roots. They will be mealy to the touch, dark in color, and foul-smelling.
Unfortunately, there is no cure for this bacterial condition, and your iris must be removed from the garden to prevent the disease from spreading to other plants. Proper iris watering practices will discourage soft rot from weakening iris plants in the future and elsewhere in the yard.
Fungi thrive in wet conditions, causing iris leaves to wilt, turn brown, and develop crown rot.
Overly moist soil and leaves are also a welcoming host to fungi, which can negatively impact an iris’ appearance. If the leaves are wilted or stunted and browning from the tips down, look for white thready spores near their bases to indicate crown rot. Round, tan structures forming may also form between yellow or rotting leaves.
The roots of an iris suffering from crown rot might be dark or decayed and will break off easily. If you suspect crown rot is present, dig the entire plant up and dispose of it.
Like the bacteria that cause root rot, the fungi that cause crown rot are discouraged through proper planting, good soil drainage, and smart watering habits.
Leaf spot is a fungal condition that can affect the appearance of irises.
Another fungal condition that affects irises is leaf spot. Fortunately, it is significantly less deadly than crown rot. Examine foliage for small brown to tan spots with blurry, watery margins to indicate the presence of leaf spot.
Advanced fungal damage might be a dark reddish brown patch or a leaf that falls off the stem entirely. A close examination with a magnifying glass will also reveal the presence of some small black spores.
The condition is not usually fatal since the leaf spot attacks tissue above ground and does not typically affect iris roots. The best action for suspected leaf rot is swiftly removing and disposing of affected leaves. Pull or cut them off at the base and monitor for additional outbreaks.
Ensure your plants get at least six hours of sun, water them in the morning whenever possible, and direct your irrigation to the soil beneath the plant rather than the flowers and leaves. You can also use a fungicide from your local garden center to discourage future outbreaks.
Yellow-brown leaves with streaks, mottles, or speckles can be symptoms of mosaic viruses in irises.
If irises have streaked, mottled, or speckled yellow-brown leaves and stunted or oddly colored blooms, they may suffer from mosaic virus. Typically a virus enters a plant’s system through a wound in the tissue, often caused by insect damage or puncture. This is why careful handling and pest management are vital to prevent this from compromising your plants’ health.
If you suspect mosaic virus as the cause of your iris plant’s woes, remove affected flower parts (leaves, stems, blooms) and monitor for advancing disease.
Also, monitor the plant to ensure no visible pests are on it, as pests can be a vector for this virus to reach other plants. If no recovery is made (or if most of your iris plant shows signs of disease), dig up and remove the entire plant. Mosaic virus is not curable.
Damage to the appearance of irises can occur due to a few insects.
Irises are generally considered pest-resistant, but their appearance can be vulnerable to damage from a handful of insects. Thrips and aphids will leave tiny puncture marks and brownish-yellow spots on iris leaves after sucking out their sap, while beetles and caterpillars may chew leaf margins and leave them looking very stressed.
While the presence of insects can make irises vulnerable to other diseases, damage from these pests is typically more aesthetic than life-threatening. The exception to this rule is the iris borer, which can quickly bore through an iris’ crown and into its roots to cause rot.
The best way to address insect issues in your iris garden is through manual removal, plant shaking, or a hard, fast blast from the hose. This will not prevent borers from penetrating the crown, so monitor for borer damage and remove plants showing signs of borer tunnels.
Make sure your removal efforts are directed partly at the undersides of leaves since this is where many pests like to hang out. To prevent insects from living in an iris patch, you can also coat leaves with neem oil several times throughout the season.
They Need Deadheading
Cut the flower stalks of faded irises, keeping the leaves to photosynthesize and store energy for future blooms.
If irises have finished flowering and the faded petals (and maybe some of their leaves) are turning brown, this is probably an indication that their reproductive cycle is complete. At this point in the season, it’s important to allow the leaves to remain in place as they will harness the sun to store food the plant will use to create next year’s blooms, but you can go ahead and cut the flower stalks down.
Use a clean, sharp pruning tool to snip flower stalks off at the base.
Take care not to remove any healthy green leaves. You can also cut away any brown or dead foliage at this point, and the plants should remain firm, green, and upright for the remainder of the season.
They Are Entering Dormancy
Do not cut the iris leaf fans down completely until they are mostly brown.
When the days grow shorter, and temperatures cool off, irises will indicate that they are done photosynthesizing and are preparing for dormancy. The signal will begin with a few brown leaves here and there and pick up speed as winter approaches. Depending on your hardiness zone, this may happen from October to December.
While it’s okay to remove dead or fallen leaves from the iris garden as the season winds down, do not cut the leaf fans down completely until they are mostly browned out.
At that point, they should be cut off at a 45-degree angle roughly 4 inches above soil level and left in place for the winter. In colder zones, consider piling some mulch up around them to protect them from extreme temperature swings and freezing/thawing ice.
When a brown, wilted, or dying iris has you scratching your head, moisture issues are most likely the cause. Is the iris getting too much water or not enough? Is the soil draining properly, or could the rhizomes be rotten? Has a disease or insect moved in on vulnerable plant parts? Or has a fungal condition risen up from the soil?
Now that you know the potential causes of your irises’ lackluster appearance, you can take educated steps toward addressing and/or eliminating the problem. And even if you can’t restore them to a happy, frilly, completely healthy state, you’ll know what went wrong and what to do differently in the future.