Mint is one of those unkillable plants that every so often likes to surprise by up and dying out of the blue. One minute it’s threatening to climb out of its pot and take over your kitchen, and the next it’s turning to mulch.
While its reputation suggests otherwise, mint is no different from any other houseplant and is vulnerable to a host of diseases, pests, and other ailments.
Over-watering is the most common cause of mint dying, creating the perfect conditions for root rot and disease. A lack of sun or poor soil condition are also likely reasons. Pest infestations, incorrect soil pH, and lighting conditions can also trigger various problems for mint which can be fatal eventually.
How to Know If Mint Is Dying?
Mint is a hardy grower, with plenty of bright green leaves. But even plants known for their vigor can suffer. Keep an eye out for these signs:
Brown Crunchy Leaves
A mint’s leaves vary a bit based on the variety. For example, spearmint is had a blue tint, and chocolate mint has blushes of burgundy. But no mint is meant to have brown leaves that are crunchy or brittle.
Soft Sickly Leaves
Soft yellowing leaves are struggling to get nutrients. This can be caused by over-watering, poor quality soil, or damaged and rotten roots.
Leaves that are soggy, blackened, or blotched are worse. That indicates disease, especially fungal rots and blights.
A healthy mint stem is bright woody brown or green and velvety depending on the variety of your mint. They are pliable, springing back when pressed.
If they snap at the slightest pressure, your mint is under-watered or sun-burned. Blackened, saggy stems are sick or rotten. If they’re soggy they’re gone and can’t be revived.
Your mint will take drastic action to conserve energy when it is sick, damaged, or dying. Dropping leaves allows the plant to conserve water and energy It’s a last-ditch effort by your plant to survive.
Total Disintegration of The Plant
Your mint should be a vigorous mass of clumping stems and runners. If they break away when touched this is a sign your plant is dead or dying. If there is no surviving growth hiding in at the base it’s game over for your poor mint.
Causes of Mint Dying And How to Fix Them
First, let’s work out what’s killing off your mint.
There’s no getting around the fact that over-watering is almost always the culprit when I’m presented with a dead houseplant. It feels like care to give your plant lots to drink, but in truth, you’re drowning it. Plants need air in their roots, and without that, the roots die and begin to rot.
Signs of Over-watering
- Yellowing leaves
- Weak, sodden stems
- Soggy potting medium
- Damp, musty aroma
- Water dripping from the base of the pot when lifted between watering.
Boggy soil encourages a host of fungal diseases that rot the roots of your plant. If over-watering has been persistent you’ve probably flushed all nutrition from the soil, too.
How to Fix
For a mostly drowned plant, our best course is to just re-pot it. This will bring new nutrients and allow you a chance to take a good look at your roots.
- Always use a pot with at least one drainage hole and the more the merrier!
- Make sure your pot is a good size for your mint. A small mint only needs a small pot. Large pots retain water away from the roots, allowing the mix to become waterlogged.
- A mix of standard potting soil with a dose of perlite and peat moss or coir is ideal (Amazon links). Too rich a mix and your plant may bolt, growing long leggy stems with few leaves that lack flavor.
- Blackened, slimy roots show your mint has root rot and needs extra attention. For mild cases trim off black, rotted roots with clean scissors or shears.
To prevent this problem from occurring again, cut back on the watering! Your mint likes moist, but not overly soggy soil. At most, your mint will need water once a week.
Larger plants will require more water, and smaller ones less. All plants need a little more during warm weather and less when it’s cool. During winter that could be as little as once a month!
To be safe, check moisture levels in the soil before watering. The top inch of soil should be allowed to dry out before you water your mint.
Under-watered mint is crispy, brown, and dry. Dry leaves, dry stems, dry soil. The medium is loose in the pot and the entire plant feels poorly secured.
If the whole thing dances about when you move the pot, odds are good your poor mint is dried out the root to tip.
How to Fix
Thankfully under-watering is much easier to correct than over-watering. Revive your under-watered mint by watering from below.
Dry soil ironically holds moisture poorly, and water poured in from the top is likely to just flow right out the bottom. Watering from below hydrates the medium more thoroughly, and will deliver water directly to the roots.
To water from below:
- Place your pot in a basin or tray that is at least half as deep as your pot.
- Fill the tray with filtered, distilled, or rainwater to half the height of your pot.
- You should see the level of water in the tray drop as it permeates the soil of your pot. Add water periodically to maintain the level.
- Once the water level stabilizes, leave your plant to soak for ten minutes.
- Remove your plant and allow it to drain for around half an hour before returning it to its place.
For a busy gardener, I recommend putting a reminder into your phone to check your plants once a week.
You may not need to water, but it is good to check and personally, I find the process quite soothing.
The gentle tending of my plants on a Sunday morning is one of the highlights of my week, so consider setting a reminder and take the time for yourself, too.
Lack of Nutrients
Your mint needs a reliable supply of nitrogen, phosphorus, and other minerals to support its growth.
Without them, new growth will be limited and what little your mint manages to produce will be fragile.
In time your poor mint will stop growing and eventually die. Old soil, overwatering, and damage to the roots, are all factors that limit nutrition.
How to Fix
Re-potting your plant (as discussed above) is one way of giving your mint access to new minerals.
But if your plant is in relatively fresh soil, a good starting point is to add a liquid fertilizer when watering once a month.
Mint is not a heavy feeder and isn’t fussy when it comes to fertilizer, so a standard balanced liquid fertilizer should do the trick.
Other ways to add nutrition include:
- Adding organic matter to your potting mediums. Coir, peat moss, compost, and worm castings are ideal.
- Applying slow-release granules to the surface of the soil. These release nutrients to the soil every time you water.
- Check the pH of your soil. Soil that is too acidic or alkaline will prevent your mint from accessing those vital minerals. Aim for mildly acidic to neutral (around pH 6 to 7). I go into more detail about pH further down.
Frostbite and Cold Injuries
Mint is a tough customer and can handle a bit of cold. In fact, in outdoor beds mint can endure sub-zero temperatures and extended periods of snow to pop back into weed like profusion once the spring comes.
But indoor plants have different challenges. It’s possible to kill your mint entirely with a sudden snap freeze.
If your heating drops out in the winter, or your plant is placed in a drafty part of the house, you may well freeze your plant to death or chill it into dormancy with overly aggressive air-conditioning.
If your plant does not show any damage but is shedding healthy-looking leaves, it’s too cold and is slipping into dormancy.
It looks like out and outright death, but it’s very easily reversed by simply warming your plant.
How to Fix
To revive your cold mint relocate it to a warmer part of the home. Avoid direct sunlight at first, but after a period of acclimation, it’s perfectly fine to leave your mint in direct sunlight for small periods of time.
Make sure to position it away from drafts and air conditioning vents. Your mint should soon spring back to life, sending out new growth.
You have after all just given your mint a false winter, and it may respond with the exuberance of spring.
While mint is resistant to cool temperatures, hot ones are another matter. As a plant adapted to temperate climates your mint will become limp and dehydrated if they overheat.
It’s also entirely possible to “cook” a mint plant by potting into a dark pot then leaving it in the sun, especially if the pot is made of plastic. The roots overheat and die, taking the rest of the plant down with it.
How to Fix
To prevent heat stress, avoid dark pots in direct sunlight, especially in the summer. If you’re in a warm climate keep an eye on room temperatures during the day, especially in western facing rooms.
If you have inadvertently fried your poor mint, the easiest solution is to simply move it to a cooler part of the house.
Be cautious about watering. Damp soil is often darker in color and will absorb more radiant heat. Water in the early morning or at dusk.
Thicker walled pots in lighter colors will prevent your roots from overheating. Terracotta is especially useful.
Soil-borne moisture leeches into the terracotta, which then evaporates. It’s like local air conditioning for your plant. Just make sure to keep your mint topped up!
Fungal Diseases: Diagnosis and Cure
Over-watering your mint will set the scene for fungal infections. There is a host of diseases that can destroy your mint, and all need sodden soils to become established. Ensure your mint is not boggy or in an overly humid environment.
They are temperate climate plants and don’t need the same high humidity levels as the tropicals in your collection. Fungus, on the other hand, loves the damp!
Also known as “leopard spot”, anthracnose is a fungal disease that leaves tiny brown stippling on the undersides of your leaves. On stems and flowers, these bumps form sunken lesions that quickly darken.
Rust is a group of fungi that leave characteristic dusty orange or red blotches on your mint’s leaves. Late-stage rust causes leaves to drop as they form, brown and useless.
Your mint can’t feed itself without leaves, so it’s important to keep this one under control.
Mint blight causes irregularly shaped black spots on leaves that spread until the whole leaf is black. It sneaks in via other injuries, like insect bites and damage from pruning or poor handling.
It often infects plants that have other fungal diseases like mint rust, pairing up into a horrifically destructive team.
Mint spreads via underground runners and through stolons – long stems that rest on the ground and eventually sprout roots along their length.
Stolon rot is another fungal disease, this time afflicting the stolons and stems. It causes them to brown then blacken, and eventually rot away.
Treating Fungal Disease
First quarantine infected plants. Fungal diseases spread, and you want to contain them. Trim away diseased leaves and stems with clean shears. Burn trimmings or double bag them before disposing of them in household garbage.
Practice good hygiene. Never compost diseased plants or their cuttings to limit the chances of re-infection. Sterilize your shears, and never re-use potting mediums or pots from infected plants.
Sadly, for many diseases, once you see the signs there’s not much to be done but to toss the plant.
Mint blight and stolon rot are very resistant to treatment, and the risk to your other plants is great. Just toss the lot and start again. Mint is not hard to replace.
Now, here are some other diseases that can cause the death of mint plants.
Mosaic Virus Infections
Mint is vulnerable to infection by viral diseases from the mosaic virus family. This group of viruses includes Tobacco Ringspot Virus, Cucumber Mosaic Virus, and Tomato Aspermy Virus.
In general, these diseases appear as a patchwork of discoloration on the leaves, giving the group its name.
The viruses destroy the chloroplasts or the green organs within the leaf that produce food from sunlight via photosynthesis. With this important ability destroyed, your mint will wither and die.
There is no real way to cure a mosaic disease. If you catch it in time you can perhaps cut and destroy the infected leaves, but if the disease has progressed to the point of die-back you’re out of luck. Toss the entire plant, pot and all.
Phytophthora And Pythium Stem and Root Rot
These two villains are oomycetes or water molds. They are primitive but deadly, attacking the cell walls inside the plant and afflicting every part from the root to the leaf.
They cause a sudden softening and rotting of the plant, with leaves developing patches of well-defined black or brown patches that quickly spread.
As a soil-borne pathogen, they attack stems first, and younger mint may rot from the base of the stems up. Phytophthora in particular loves attacking roots.
Slimy black or red roots are an indication that you have an infection that needs immediate treatment.
If you’re growing in a soil-less medium, like Lecia beads, a sulfurous rotting egg smell is a dead giveaway you have an infection.
Prevention is generally better than cure. Always use a fresh, clean potting medium whenever you repot your mint. Make sure pots are clean and do not water with stagnant water.
If your mint starts to show signs of phytophthora or pythium infection, you must act quickly. Immediately quarantine it from your other houseplants, and quarantine any nearby plants as they may be infected too.
Next, prune and destroy infected parts of the plant. Do not compost, but burn or dispose of it by double bagging and tossing it with household garbage.
Root rot caused by these two is trickier to diagnose. Roots are tough to inspect, and many other pathogens can cause rot, too.
Some gardeners swear by hydrogen peroxide, applying a dilute solution directly to the roots themselves either as a bath or via a spray.
Personally, I think if your mint has gone that far it’s best to just toss it pot and all, and try again.
You and I are not the only ones to recognize mint’s delicious flavor, and they are just as prone to pest infestation as any other plant. Here’s your guide to dealing with mint pests.
Aphids (Peach aphid)
Aphids are small, soft-bodied sapsuckers. They are tiny, able to hide in nooks and crannies.
For a heavily textured leaf-like one found on many types of mint, it can be next to impossible to spot them.
Keep an eye out for ants! While the ants won’t attack your mint, many species ‘farm’ aphids, placing them on the best parts of your plant.
A well-fed aphid oozes a sugary liquid called “honeydew”, which ants love to eat. If your mint is crawling with ants, aphids will almost always be hiding there too.
Mealybugs are tiny oval-shaped insects that cover themselves in a fluffy white powder to deter predators. They pierce the leaves and stems of your plant and eat the sweet sap within.
Like aphids, some species of mealybugs secrete honeydew and are often accompanied by ants. Personally, I’ve had better luck spotting the ants than I have the mealybugs!
These pests are the caterpillars of several different species of moth. Also known as “loopers” or “inchworms”, they have long smooth bodies that vary in color from greens to browns.
They are voracious eaters, devouring your mint from the leaves to the stems. Nibble marks or clean, neat holes in your mint leaves are clear signs of a cutworm infestation.
Thrips are small, slender flying insects, so tiny that they appear almost like dark slivers scattered on the leaves of your mint.
Damage caused by thrips is quite obvious, forming streaks, speckling, or patches of whiteness left after the thrips eat everything of value.
Not only do they damage your mint, but they can also transmit diseases like mosaic virus and rust.
A tiny arachnid, the spider mite is a common pest to the indoor gardener. These tiny nuisances hide in the crevasse of your plants, and in mint, they enjoy hiding out in the textured undersides of the leaves. Spider mites pierce leaves and stem to dine on the fluids inside.
While they themselves are hard to see, their webs are not. If your mint seems covered in gossamer threads, it’s a good sign there are spider mites about.
The first step to dealing with pests is to quarantine your infected mint. Move it away from other plants so the little monsters can’t jump ship to another victim.
If you are facing just a few bugs, simply wipe them away with a cotton tip soaked in spirits.
Spider mites, thrips, and aphids can also be gently washed off under a shower or garden hose. Cutworms retreat if sprayed with a 50/50 solution of vinegar and water.
But given most pests are adept at hiding on your plant, you may need to act aggressively and poison them. Neem oil sprayed over your mint will kill most pests, and is readily available and easy to use.
A dilute solution sprayed on your mint twice a week in the morning should clear most infections in a month or so.
Incorrect Soil pH
Mint prefers soils that are mildly acidic to neutral, around a pH of 6 to 7. This is the ideal band that allows them to access nutrients in the soil, and to protect their roots from pathogens. You can buy a home pH testing kit here, or from your local garden supply store.
How to Fix
Acidic potting medium will benefit from a dose of garden lime. This is a white powder and can be sprinkled onto the soil.
For alkaline soils, a sulfate salt such as aluminum sulfate can bring your pH back into line, though it takes a while to become effective.
Steeping poultry manure in water for a few days will leave you with a “tea” that is excellent for raising acidity while also nourishing your mint.
Of course, if you’re rather skipping the fuss it may be best to just repot as outlined above. Adding peat moss or used coffee grounds to the medium will help keep your potting medium mildly acidic.
Too Much Sun Exposure
Mint does well with a lot of good light, but it’s possible to have too much of a good thing. Plants become used to a certain amount of light.
If you suddenly move a plant from shade to sun, it won’t have time to adjust and will become sunburned.
Sunburned mint will show crispy edges and browned or blackened patches on the over-exposed part of the plant.
It differs from disease or malnutrition in how fast those patches appear. The damage will develop during sun exposure, or in a day or two after.
While burnt leaves are unsightly if there is green they can still function to provide your mint with energy. But given mint enjoys regular pruning, a wee haircut to remove the unsightly leaves is not going to harm it.
Not Enough Sunlight
Mint thrives in bright light. It is not a suitable plant for low light. Your mint will become leggy – long stems with few leaves, often on the side closest to its source of light. This is your mint doing its best to reach what little sun it can find.
This is a very easy problem to fix – simply move your plant to a brighter area of your home. A northern-facing window is ideal.
Your potted mint is an ideal plant for those dangerously bright areas that would scorch other plants.
Take your time acclimatizing your poor shadow mint. Moving it directly from darkness to light and you risk sunburn. Spend a week or so moving it closer every other day, until it’s in its new home.
How to Keep Mint Alive and Thriving
I love growing mint. It’s incredibly rewarding and rewards with sprawls of fragrant leaves that are pleasing to the eye and delicious to eat. To keep your mint thriving, remember:
- Water only when the top inch of soil is dry.
- Grow your mint in a well-draining, moderately rich mixture.
- Make sure your mint gets plenty of light.
- Fertilize once a month with a dilute, balanced fertilizer.
- Trim regularly, and treat yourself to a mojito or a cup of mint tea.
Follow my advice and you won’t have to wait long until your dying plant is back to its best self, threatening to spill out of its pot in a deliciously fragrant wave.