What Will Kill Irises? [7 Common Iris Killers]

Iris plants can quickly be killed by pests such as iris borers, slugs, snails, and thrips. Small insects such as aphids and leafhoppers can also spread the deadly mosaic virus to your irises. Bacterial rot—as well as fungal rot that attacks leaves and crowns of your irises—can also be deadly if your plants are not protected. Make sure to use safe methods when controlling weeds among irises, since many common weed sprays will also kill your flowers. However, proper cultivation and pest control methods will protect your irises.

What will kill irises?

7 Iris Killers and How to Stop Them

Irises are hardy flowers that can be grown in almost every zone, which makes them a great choice for gardeners around the world. However, there are several dangers that can destroy your irises. Here’s how to recognize them and protect your plants.

Iris Borers

If you notice irregularly-shaped holes in leaves and rotted iris roots, your plants are being attacked by iris borers. These pests are fat, pinkish grublike moth larvae that can grow up to 2 inches (5 cm) long. The larvae burrow into irises at the top of a fan and chew their way down to the roots, eating plant material as they go. Every iris variety can be attacked by these pests.

  • Iris borers are pinkish, grublike moth larvae that eat their way down inside iris stalks, from the fan to the roots.
  • Clear away dead and dry leaves in the fall, since this will remove iris borer eggs before they hatch in spring.
  • Look for holes in fans and leaves in spring—remove damaged fans and crush any borers you find.
  • Crush borers in iris roots when dividing irises.

In order to prevent iris borers, clear away dry and dead leaves in the fall. Iris borer moths lay their eggs in the fall but the eggs won’t hatch until spring. By clearing away dead leaves, you can often remove the eggs before they hatch. If your irises are attacked by these pests, remove badly damaged iris fans and crush any borers you find. If you see borers in the roots when you are dividing irises, crush the borers.

Slugs and Snails

If your iris leaves have large holes with ragged edges, it is most likely that your irises are being eaten by snails or slugs. These destructive mollusks love to hide among iris foliage during the daytime, so it can be hard to spot them.

  • Ragged holes in iris leaves are caused by snails and slugs that feed at night.
  • Use slug traps or this pet-safe slug and snail killer to destroy these pests.
  • Slugs and snails hide well during the day, so hunting for them yourself can be very difficult.

Since slugs and snails are nocturnal, it is better to trap them rather than hunt through your irises to kill them. You can make a homemade liquid trap for killing snails and slugs. Alternatively, you can purchase traps for slugs and snails. Keep these traps baited and clean them out when you find dead slugs and snails. This way, your irises won’t have their leaves skeletonized by slimy pests.

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Bacterial Rot

If your irises have water-soaked streaks and yellow, wilting leaves then your irises are suffering from bacterial rot. If left untreated, whole leaf clusters may fall off your irises, which can kill the plant quickly. This type of rot is most common during wet periods or if your irises are crowded in a shady portion of your garden.

  • Irises in wet, shady, and crowded conditions may develop bacterial rot.
  • Yellowed, wilting leaves with water-soaked spots signify the iris has bacterial rot.
  • If your irises have bacterial rot, dig up all the irises in the area.
  • Throw away any irises with rotted roots so the infection does not spread.
  • Replant healthy iris roots after the rotten ones have been disposed of.

Bacterial rot is more likely to attack irises that have been damaged by iris borers, so using the earlier tips on the list to spot and kill these pests will help guard your irises. If rot has already set in, dig up the irises. Then, remove any rotted rhizomes (roots) and replant the healthy iris bulbs at the correct depth. You can tell rotted rhizomes because they will be dry on the outside but wet and slimy on the inside. Rotted iris roots also have a foul smell.

Fungal Rot

Irises with black, brown, or yellow spots on the leaves are being attacked by fungal rot. This is often caused by extremely wet weather or overwatering. However, irises attacked by borers, slugs, or other pests are more susceptible to fungal infection. This infection can kill the entire iris plant if it isn’t treated.

  • Black, brown, or yellow spots on iris leaves mean the plants have developed fungal rot.
  • Remove rotted portions of the plant immediately to stop the infection from spreading.
  • Spray the leaves and stalks of irises with this neem oil spray to naturally kill fungus and insects.

Once you spot fungal rot on your irises, remove any leaves and stalks with brown, black, or yellow spots. Then, spray the plant with neem oil. This organic gardening spray is made from the seeds of the neem tree. It kills fungus, as well as pest insects such as aphids.

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Silvery streaks or blotches on iris leaves are a sign that your irises are being killed by thrips. Irises being attacked by these small insects may also have dry, warped-looking leaves. You may also spot a black, sooty substance on iris leaves—this is the fecal matter left behind by thrips. On their own, thrips can decimate irises. Even worse, these bugs carry diseases that can kill irises down to the root.

  • Silvery patches or streaks on iris leaves mean your plants are suffering from harmful thrips.
  • Thrips are tiny winged insects that feed on plants and spread diseases.
  • Spray irises with this insecticidal soap to kill off thrips.
  • Continue spraying with insecticidal soap every 3–7 days until your irises have fully recovered.

Thrips are slender, winged insects that grow 1–2 millimeters long. They can easily be mistaken for gnats or flies. To treat your irises and kill thrips, spray your irises with insecticidal soap. This natural spray will destroy bugs without introducing pesticides to your garden. You can also use neem oil to kill off thrips and protect your irises.

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Mosaic Virus

Mosaic virus is an incurable disease that is spread to irises by aphids, thrips, and other plant-attacking insects. The signs of mosaic virus are mottled leaves and flowers. A pattern of dark and light green mottling on leaves is the first sign of mosaic virus. As the disease worsens, the leaves may become streaked with yellow. Even if your irises survive this disease, their growth will be stunted and the flowers will be much smaller.

  • A mottled pattern of light and dark green on iris leaves signifies your plants have the mosaic virus.
  • Mosaic virus causes yellow streaks and small, mottled flowers.
  • The mosaic virus cannot be cured, so you will have to remove and throw away infected irises.
  • Use insecticidal soap to kill aphids, thrips, and other insects before they can spread the mosaic virus to your irises.

If your irises are stricken with mosaic virus, the best thing to do is dig up and discard the infected plants. This will stop the disease from spreading. Unfortunately, there is no way to treat mosaic virus. The best course of action is to use insecticidal soap or neem oil to kill off predatory insects, so your irises don’t contract this disease.

Weed Killer Sprays

Any weed killer that is designed to kill broadleaf weeds will also kill irises. Roundup and other non-selective weed killers also kill irises quickly. This is great if you want to kill wild irises, but very dangerous if you are cultivating irises in your garden. Keep in mind that almost all weed killers will attack and kill irises if the spray lands on the iris leaves and stalks.

  • Almost all weed killers will harm and potentially kill irises.
  • Any product that kills broadleaf weeds (such as clover and dandelions) will kill irises.
  • Shield your irises from weed killer spray by placing a piece of cardboard or plywood between the spray and your irises while attacking weeds.

To protect your irises, use these tips to safely keep weeds out of iris beds. If you are spraying weed killer near your irises, put a sheet of cardboard between the irises and the spray. This way, none of the weed killer will land on your irises.

Will Roundup Kill Irises?

Roundup will kill irises, along with all other garden plants, weeds, and grasses. Do not spray Roundup on or near irises that you do not wish to kill. If you want to kill unwanted irises, mist the leaves of the plant with Roundup spray. The Roundup will be absorbed by the leaves and travel to the roots to kill the entire plant. The irises plants will be fully dead and ready for removal 2 weeks after spraying.

Will Vinegar Hurt Irises?

The acetic acid in vinegar will dry out and damage iris leaves, stalks, and flowers. Acetic acid attacks all plants, so you should never spray it on irises that you do not want to damage. The good news is, vinegar does not penetrate the soil and attack roots. So, even if your irises are hurt by vinegar spray, they may regrow from the roots in the future.

Why are Your Irises Dying?

The most common problems that will kill your irises are:

  • Iris borers burrowing from the stems to the roots.
  • Snails and slugs eating iris leaves.
  • Rot caused by deadly bacteria.
  • Fungal rot that destroys iris leaves and crowns.
  • Thrips that destroy iris leaves and spread disease.
  • Mosaic virus spread by aphids, thrips, and other pest insects.
  • Weed killer sprays that are deadly to irises.

Once you know the signs and treatment for each of these common iris killers, you can quickly identify the problem and act fast to save your plants. Then, your irises can thrive and bloom for many years.

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