Why Some Houseplants Need Filtered or Rain vs Tap Water

Rain water or filtered water is a better choice for watering your houseplants than tap water to avoid the added chemicals that improve water quality for humans but not so much for plants, like chlorine which can cause chemical burns to some of the more sensitive houseplants including Peace Lillies. Compared to rain or filtered water, tap water contains more chemicals and minerals with less beneficial elements and micronutrients that can impact your houseplants including:

• Chlorine

• Fluoride

• Less Nitrogen and Trace Nutrients (Maybe Minimal)

• Salts and Minerals (Sodium, Calcium, Carbonates, and Magnesium)

Some additional factors to consider when choosing your water source include: 

• Rainwater is more acidic

• The variable temperature of tap water

By using rainwater, buying filtered water, or filtering your own with the same systems you use in your own home for drinking water, you can reduce the additives as the activated charcoal adsorbs chemicals like chlorine without removing elements necessary for plants to perform photosynthesis like Nitrogen. 

Want to know a bit more about these added chemicals and how they impact your houseplants? We’re happy to help. Here’s some more information on each.

Your tap water is treated with chlorine to help disinfect it and prevent bacteria or parasitic growth, making it safer for human consumption. Chlorine in small amounts as chloride is also a necessary nutrient in plants, but too much chlorine exposure through tap water can cause tip burn, yellowing, stunted growth, and in extreme cases chemical burns to the roots through oxidative stress as the chemical accumulates. 

Filtered water allows you to take advantage of the fact bacteria and parasite growth was inhibited prior to the water reaching you, and now that the chlorine is no longer needed the filter can remove the chemicals. This is why some of the more delicate houseplants like Calathea Prayer Plants and Dracaneas like Lucky Bamboo require filtered water for optimal health.

Letting your water sit out overnight can help chlorine naturally evaporate from tap water, similar to what you do with fish tank water. Some areas use a different compound, chloramine which is a mix of chlorine and ammonia. This cannot be evaporated overnight or boiled out (even residual) and requires a granular carbon filter to help remove. Check your local state or city water treatment website for your information like this one for New Jersey, in their tap water. 

In addition to its toxicity over time, excess chlorine in soil can interfere with a plant's ability to absorb nitrogen. Both are anions and share some of the same transporter proteins, so high chloride levels can reduce nitrate absorption.

Fluoride is added to water to improve the dental health but it can also be absorbed by your plants. This absorption can cause tip burn to the foliage, yellowing and spotting, as well as chemical burns to the roots if there is enough concentration in the soil or potting mix. Fluoride can be absorbed through the roots and accumulate in the leaves causing the leaves to brown or yellow which prevents photosynthesis causing the plant to starve. 

This additive can also inhibit important enzymes that rely on metal cofactors (non-protein helpers) like magnesium or calcium and interfere with root metabolism. There are studies like this one that show different plants respond differently with nitrogen intake when the soil is contaminated with fluorine, so it's important to know if your houseplant is one of the more sensitive ones to fluoride. 

Unlike chlorine, fluoride does not dissipate or evaporate when you let the water sit out overnight. 

Plants like the Spider Plants, Palms, and Maranta can experience fluoride toxicity if watered with tap water. If you see this, use sterilized shears to remove the damaged foliage and switch to filtered water. 

Salts like sodium and carbonate with minerals like lead (from the pipes it travels through), calcium, and magnesium in tap water (mostly hard water) will be absorbed by the plant and can lead to leaf or root burn by increasing the pH of the potting mix. These salts are also electrolytes and when they build up overtime, they can increase the electrical conductivity making it harder for the roots to absorb nutrients and water. In addition to blocking nitrogen, high enough levels of calcium and magnesium can block other necessary minerals including potassium, iron, and phosphorus.

The salts and minerals found in tap water come from natural sources like soil and rock, as well as from corrosion of metal pipes and municipal treatment processes the water passes through on its way to your home. If you see white sediment on your plants leaves after watering with tap water, this is likely dried calcium and magnesium. 

Pro-Tip: Before using tap water, check the water quality report for your area. If the tap water has a Total Salts value (in ppm) from 175-525 with a pH of 6.5 to 6.8 it is considered good for flowering plants and foliage. 

The higher salt content in some tap water (depending on where you live) changes the pH of your potting mix to become more alkaline over time. This can lead to iron deficiency or iron chlorosis where leaves will appear yellow or white with green veins and eventually die due to lack of iron. 

The increased minerals decreases the available phosphorous as it will bind with the calcium and become insoluble (inaccessible) to your houseplant. If your leaves have turned purple, curled, appear smaller than normal or growth is stunted there might be a phosphorous deficiency and it's time to switch to filtered or rain water, and add a fertilizer as a supplement to add a fresh phosphorus source. 

Plant fact: Most plants, like Maranta and the Easter Cactus prefer a more acidic potting mixture. There are varieties that prefer alkaline soil, like Aloe Vera, Begonias, Jade plants, and herbs like thyme and rosemary. 

Rainwater may contain more carbon dioxide and be more acidic compared to tap water depending on how much is absorbed from the atmosphere when forming carbonic acid. Rain water with an acidic pH can help release micronutrients like manganese, zinc, and copper from the potting mix so they can be absorbed by the plant. This slightly more acidic pH will also help wash away excess salts in the soil outdoors improving the plants overall environment. For indoor plants you’ll want to flush or change the potting mix. Houseplants like Orchids, Ferns, and Azealas love this type of water.

Tap water, between treatments and minerals it picks up along the way, is usually more basic or alkaline. In some severe cases where there may be soap in the water, it can have a pH up to 10, so always rinse your watering cans or containers thoroughly before watering plants. Reverse osmosis (you can install a system as an extra water filter to your sink or purchase a separate filtering system) can help transition tap water to a more acidic pH, making it more suitable for house plants. 

Plants are sensitive to water temperatures and cold water from the tap can shock tropical plants causing leaf curling and slow water absorption, while hot water can damage roots and kill beneficial microbes in the potting mix or soil. 

By using filtered water that sits out for roughly 30 to 60 minutes so it is at room temperature you can help to ensure a smooth absorption of water and micronutrients to help your houseplants thrive.

But don’t immediately jump for a new water filtering system, there are plenty of tropical and non-tropical plants that do well with tap water.

Most plants can tolerate tap water including ZZ Plants, Sansevieria (Snake Plants), Pothos, Succulents, and Cacti. These plants often have infrequent watering schedules, tolerate drought, and are overall hardy so they can handle tap water better than their more sensitive counterparts. 

Tap water may be treated to be helpful to humans, but these same treatments can be harmful to certain houseplants that are sensitive to chemicals. By knowing the types of houseplants you have in your collection and their sensitivities, you can keep them growing strong and looking great by switching from tap water to filtered or collected rain water.