You’re enjoying a lovely, peaceful day at home when you suddenly smell something foul. You search high and low for the source of the stench, only to realize it emanates from your beautiful houseplants. What can be causing the smell, and what can you do about it?
Your houseplants may smell due to overwatering, poor soil aeration, fungal and bacterial growth, rotting roots, and insufficient light. Solutions include draining the excessive water, creating drainage holes in the plant pot, exposing the plant to indirect sunlight, and changing the potting mix.
In the rest of this article, I’ll take you through the causes of odors in houseplants. I’ll also discuss how to eliminate the foul smell and safeguard your indoor plants. Let’s get started!
Most indoor plants are perennial tropical species. For this reason, they need sufficient water for survival.
However, overwatering is a common cause of odorous indoor plants. In most cases, the problem arises from the “more is better” thinking of new plant owners. Although it’s natural to think that overwatering your indoor plants will benefit their growth, this can actually suffocate the roots and cause them to rot.
The presence of yellow leaves is an easy way to know your plants are receiving excessive water. It’s also worth noting that, unlike their outdoor counterparts, indoor plants are not exposed to considerable water loss.
Overwatering outdoor plants is never an issue since these plants lose more water through more efficient soil seepage, evaporation, and transpiration. Potted houseplants also lose water this way but the indoor conditions can be less intense than the outdoors.
For instance, garden soil may contain more plant roots that can absorb water quickly. Direct heat from the sun can also facilitate faster evaporation of soil moisture and speed up the transpiration rate of outdoor plants. These factors leave you more room for error outdoors.
However, if you frequently overwater your indoor plants, they’re likely to suffer from the consequences more quickly.
How to Fix
The solution is to drain any standing water in the pot or tray and ensure proper drainage by creating holes at the bottom of the pot.
Use Pots with Drainage Holes
Keep in mind, however, that overdoing drainage holes can deprive your plant of the water it needs to survive by draining water too quickly before your plant’s roots can absorb them. Three to four 1/4-inch (0.6 cm) holes at the bottom are sufficient for pots less than 12 inches (30 cm) in diameter.
Larger pots will need larger holes (1/2 inch or 1.3 cm each) and possibly more holes. However, be mindful because apart from over-draining the water, larger holes will also drain the potting mix and deprive the plant of essential nutrients.
Use a Plant-Appropriate Potting Mix
You should also ensure that you’re using the right potting mix for your plant. Drought-tolerant plants do best in soil that drains very well. Using materials that hold too much water, such as vermiculite and peat moss, can leave the soil too moist for too long.
Adjust Your Watering Routine
Adjust your watering routine accordingly depending on your plant’s needs and the season. Each plant has a different requirement. For instance, a pothos plant will need moist soil and do well when watered as soon as the top inch (2.5 cm) of the soil is dry.
On the other hand, jade plants like a bit of dry soil because their leaves can store water. Similar plants can wait until the top 2-3 inches (5-7.6 cm) of the soil is dry before receiving water again.
2. Poor Soil Aeration
If you often overwater your plants, the potting mix can become compacted, and air cannot flow through it effectively. Poor aeration creates an anaerobic—instead of aerobic—soil environment.
An anaerobic soil environment reduces the activities of soil microbes that promote soil aggregation.
The potting mix relies on these microbes for:
- Phosphorus solubilization
- Nitrogen fixation
- Pathogen and pest suppression
- Decomposition of organic matter
Needless to say, anaerobic soil creates a conducive environment for anaerobic bacteria. These bacteria are harmful to houseplants and contribute immensely to the foul smell.
It’s also worth mentioning that poor soil aeration eliminates air pockets. Your plant needs these air pockets to breathe.
Insufficient oxygen absorption from the soil due to fewer air pockets leads to the death of root cells. Consequently, the plant loses vigor and dies.
How to Fix
You can improve soil aeration by mixing perlite or sand with your potting mix. These materials promote better drainage and prevent soil compaction.
You can also regularly turn over the top layer of the potting mixture, fluffing it up to improve air circulation.
This problem also points back to overwatering. Therefore, you should water your plant adequately to facilitate sufficient aeration.
3. Fungal and Bacterial Growth
Fungi and bacteria thrive in moist, dark conditions with poor air circulation. These organisms can cause foul odors and harm your plant’s health by causing root rot or leaf spot disease.
Fungi use their enzymes to break down organic matter, leading to the release of foul-smelling gases.
Knowing when your potting mix has fungi and bacteria that affect your plants can be challenging. Therefore, you need to be aware of the conditions that are unfavorable for fungi survival and ensure the potting mixture meets these conditions at all times.
How to Fix
You can prevent fungal and bacterial growth by ensuring proper aeration in your potting mix and avoiding excess moisture.
Fungal diseases are also easily transmitted through contaminated pruning tools or potting mix. As such, you must sterilize your tools before each use and buy a high-quality potting mix.
Although using a fungicide can also address fungal growth in your potting mixture, it’s best to prevent the problem from occurring in the first place. Moreover, fungicides tend to be species-specific, which means not all products will be effective against all fungi species.
4. Fertilizer Application
Although fertilizers contain essential nutrients for plant growth, they can release foul odors if applied incorrectly. Fertilizers with a high nitrogen concentration can cause the release of ammonia gas, leading to a strong, unpleasant odor.
Over-fertilization can also lead to salt accumulation in the soil, especially in potted plants where the soil cannot readily flush out excess nutrients.
How to Fix
The best way to solve this problem is by dumping the over-fertilized potting mixture and repotting the plant. An already over-fertilized potting mix will take longer to fix. You may not want to wait that long if you want to restore a fresh smell in your house as soon as possible.
After repotting the plant, stick to the fertilizer packaging’s recommended application rate and frequency. You should also consider using slow-release fertilizers because, as the name suggests, they release nutrients into the soil slowly. These fertilizers also reduce the risk of leaching nutrients out of your potting mix during watering.
Here are general fertilizer guidelines for most indoor plants:
Apply Liquid Fertilizers More Frequently
It’s always best to use fertilizers sparingly because it can be more difficult to treat the consequences of fertilizer burn. That’s why I recommend using only half the strength recommended on a fertilizer’s packaging.
Since liquid fertilizers have a lower concentration of nutrients, your plant needs more of them regularly. It’s suitable to use them every 1-2 weeks during the growing season and once a month or not at all during the dormant period.
Apply Slow-Release Fertilizers Less Often
Slow-release fertilizers break down slowly, releasing nutrients over a longer period.
Spikes are popular and mess-free slow-release fertilizers you can use on your indoor plants. You can typically apply them once every 1-2 months during the growing season.
Since they have a higher concentration of nutrients, you should apply granular fertilizers once every 2-3 months during the growing season. Over-fertilizing with granular fertilizers can cause fertilizer burn.
5. Decomposing Organic Matter
Houseplants need organic materials in their potting mixture for healthy growth. These materials include compost, manure, or peat moss. However, these materials can release foul odors when they start decomposing.
The decomposition of organic matter in the soil is mainly caused by the following:
- Too much water that creates a moist environment for decomposers
- Lack of aeration in the potting mixture
- Overfeeding with organic materials
How to Fix
To address this problem, you want to ensure proper drainage and aeration in your potting mix. Mixing perlite or vermiculite in the potting mix is the best way to create a well-drained and aerated mixture.
Other fixes for this problem include:
Avoid Overfeeding With Organic Matter
Excessive organic matter in the potting mixture can lead to excessive decomposition and odor release. Thus, it’s advisable to stick to the recommended amount of organic materials on the potting mix packaging or consult a gardening expert for advice.
Remove Dead Leaves or Plant Material From the Mix
Dead leaves in contact with water and oxygen can start decomposing and release foul odors. In addition, moist and decaying plant matter can attract pests and microbes that may harm your plants. Removing them from the potting mixture is essential to prevent this issue.
Consider the Source of Your Organic Material
Before using any organic material, you should check to ensure it’s not likely to cause a poor smell. Manure from predatory animals and some plant materials, such as trimmings from a tomato plant, can cause foul odors. If you must use such materials, it’s advisable to compost them before use in your potting mixture.
Add Activated Charcoal
Activated charcoal is an effective way to control odors since it absorbs them. Mixing 1/3 cup of this charcoal into every four quarts (about four liters) of the potting mixture can help reduce odor release from decomposing organic matter.
6. Rotting Roots
The roots of indoor plants can rot for various reasons, including over-watering and fungal infections. These rotting roots will release a putrid smell as they break down in the soil.
Signs that your plant has rotting roots include:
- Wilting or yellowing leaves
- Mushy and discolored roots
- Stunted growth
How to Fix
The best solution for this problem is repotting. Inspect the plant roots for the extent of the damage. If less than 30% of the roots are damaged, it’s still possible to save your plant. Otherwise, you might need to dispose of the infected plant.
If you feel like your plant can still be saved, you can treat the roots by using the following steps:
- Remove as much old soil as possible. It will help make it easier to inspect the roots. In addition, the old soil is already contaminated so you’ll have to dispose of it soon.
- Trim the rotten sections using sterile shears. Cut about 1/4 inch (0.6 cm) away from the damaged sections. This helps reduce the risk that the remaining roots are carrying the pathogens.
- Remove dead or damaged stems and foliage. You can also trim a bit more to match the amount of roots you removed. That way, your roots won’t have to support too many plant parts while trying to recover.
- Rinse the roots with filtered or distilled water. The weakened roots will be less tolerant of the salts and chemicals in tap water.
- Dry the roots on a clean towel. Let them air dry for up to ten minutes before repotting.
- Repot in a fresh, moist, and plant-appropriate potting mix. If your plant needs better drainage, use a potting mix with enough perlite or sand. Ensure that the new mix is moist but not soggy. Carefully place the plant into the pot and gradually fill it in with the soil to protect the roods.
- Don’t water the plant immediately. Allow the plant to recover and just use the existing moisture in the potting mix for a while. Wait until the top 2 inches (5 cm) are dry before watering again.
More importantly, you must prevent the problem from occurring again. Overwatering is the main culprit for root rot since it creates a conducive environment for fungal growth. Moreover, it reduces aeration in the soil, making it impossible for the roots to breathe.
You should only water the plant when the soil is dry enough for your plant. A general rule is to add water when the top inch (2.5 cm) is dry for moisture-loving plants or 2-3 inches (5-7.6 cm) for drought-tolerant plants.
Alternatively, you can use a soil moisture meter to confirm the right time to water the plant. Here are the steps to getting the correct moisture readings using a soil moisture meter:
- Carefully push the meter’s metal probe into the potting mixture. You should ensure the metal probe is in at least ⅘ of the soil. Don’t force the probe into the soil to avoid breaking it or damaging the roots.
- Let the moisture meter stay in the soil for about 30 to 60 seconds. This is the amount of time the meter needs to get an accurate reading of the moisture content.
- Read and record the moisture level. Check and record the displayed moisture level. Remove the probe and clean it with a dry cloth before inserting it into a different spot in the soil. Read the results for the new spot and record them.
- Interpret the results. The meter’s scale can be on the dry, moist, or wet side. Use the scale on the package for interpretation. Usually, a reading of 4-5 indicates it’s time to water your moisture-loving plants, whereas drought-tolerant plants can wait until the soil is within the range of 2-4.
Here’s a video to take you through the above procedure:
7. Insufficient Light
Like any other plant, indoor plants need sunlight for photosynthesis. However, the good thing about most indoor plants is that they can tolerate low light conditions, a characteristic of the indoor environment. Still, your indoor plants need access to partial indirect light.
The light requirement condition doesn’t benefit the plant alone. The soil also needs light to remain fresh.
Keeping the soil in a cool space, away from sunlight, creates a suitable environment for mildew and mold growth. These microorganisms release a musty smell that can ruin your indoor plants’ scent.
How to Fix
The solution for this problem is straightforward: provide enough light for your plant and its potting mixture. For example, you should place the plant 4-8 feet (1.2-2.4 m) away from windows or in areas where it can receive indirect sunlight.
Many houseplants will do well with 4-6 hours of gentle morning sunlight from a lightly curtained eastern window. However, sun-loving plants thrive best with plenty of sunlight throughout the day near a southern window.
If the plant is already affected by mold or mildew growth, remove the top layer of soil and replace it with a fresh potting mixture. You may also introduce beneficial microorganisms in the form of compost tea to improve the overall health of your houseplant’s soil.
8. Old Potting Mix
An ammonia, or excrement-like smell signifies an old potting mixture. You may also smell traces of sulfur (which smells like rotten eggs), indicating bacterial activity.
This smell is not a good sign as it means that the potting mixture has broken down, losing its ability to retain water and supply nutrients for the plant’s growth.
Although the potting soil doesn’t go bad, its additional components do. For instance, peat moss has a lifespan of one to two years. When this component expires, it reduces the soil’s water-draining ability, and the potting mixture may produce a foul smell.
How to Fix
Repot the plant regularly with fresh potting mix depending on their growth rate and nutritional requirement.
Fast-growing indoor plants need repotting every 1-2 years to accommodate new growth and replenish the soil with fresh nutrients.
Slow-growing ones, such as cacti and succulents, may require less frequent changes of about once every 2-4 years. They can tolerate their potting mix longer because cacti and succulent mixes tend to be rich in perlite, which is mainly unaffected by soil microbes. Desert cacti also have a low-nutrient requirement.
You must buy indoor plant potting mixes from reputable gardeners or nurseries. Avoid using an old potting mix from other plants, as it may contain diseases, pests, and harmful microorganisms that can affect your indoor plant.
9. Under-Processed Manure
Manure processing is vital in removing harmful microorganisms and weed seeds. If the manure is not processed correctly, it can release ammonia which smells like cat urine or rotten eggs.
Apart from the smell, the type of manure you use affects the plant’s quality. Fresh manure is the last thing you want to go for when planting indoor plants. Such manure may contain bacteria that can affect the health of your plant.
How to Fix
Avoid using unprocessed manure as a fertilizer for your indoor plants. Instead, opt for organically-composted manure with minimal odor, such as kelp meal or fish emulsion.
If you have already used unprocessed manure, remove it and replace the top layer of soil with fresh compost or organic fertilizer to improve your plant’s health and eliminate foul odor.
10. Insect and Pest Infestation
Indoor plants are not immune to pests and insects.
Stink bugs are the worst when it comes to odors in plants. These insects are sap feeders that are attracted to the plant’s sap. When under attack, they release a putrid smell similar to pungent cilantro as a defense.
Aphids and mealybugs also produce an unpleasant odor when squashed or disturbed. These pests damage the plant by sucking its fluids, leaving behind sticky honeydew conducive to mold growth.
Snails and slugs produce a musty and fishy odor when they crawl through the soil. These pests have voracious feeding habits that create holes in the leaves and stems of plants like ZZ plants.
How to Fix
Inspect your plants regularly for any indications of pest infestation.
Common signs that your plants have been infested include:
- Presence of sticky honeydew
- Holes in the leaves
- Damaged or wilted leaves
- Discoloration and distortion of new growth
If you notice any of the above signs, spray your plant with organic insecticidal soap or horticultural oil. Alternatively, you can introduce beneficial insects, such as ladybugs to manage pests naturally.
11. Chemical Pollutants
Most indoor plant owners prefer to use insecticides and pesticides to keep pests and insects away. However, some of these chemicals can harm your plants and release a strong odor.
Apart from insecticides, other household chemicals such as chlorine and bleach can also cause damage to indoor plants if accidentally spilled on the soil or leaves.
How to Fix
Avoid using chemical pesticides and insecticides as much as possible. It’s advisable to use organic solutions, such as insecticidal soap or horticultural oil to manage pests.
If your plants have been exposed to chemical pollutants, flush the soil with clean water and trim off any damaged leaves or stems.
Odorous houseplants signify a mess that must be addressed urgently. If left unchecked, the condition will worsen and make your house inhabitable.
A foul smell can be a sign that your plant is slowly dying from problems like root rot and suffocation.
If you notice your houseplants don’t smell right, inspect them for waterlogging. This is the leading cause of odors in indoor plants. The presence of yellow leaves confirms that your plants are overwatered.