Hot or Cold Water For Plants? Ideal Temperature
Are you aware that hot or cold water may affect the growth of your plants? It seems odd but, yes, there is an ideal water temperature range to be sure that your plants grow healthy.
Water above 120℉ should never be used on plants due to its detrimental effect on soil microbiome and plant roots. Warm water up to 65℉ can be used to deter some type of unwanted pests, however should be used only for such purpose. Cold water is not generally an issue and can even be beneficial in hot seasons.
In other words, any water temperature that is not normally occurring in nature, will very likely harm plants.
Let us find out below how water with hot or cold temperatures can affect your plants.
Hot Water in Plants? Why Not?
Hot water, with a temperature above 120℉, should never be used directly on plants or soil as it will partially sterilize the soil removing those healthy microbiomes in the soil responsible for the production of nutrients.
Photosynthesis is the process by which plants make food substances using sunlight energy, carbon dioxide, and water. The water temperature is proven to affect it.
Hot water at a temperature above 120℉ is proven to kill unwanted soil impurities and also the soil microbiome Management of Soilborne Pathogens. This is a wide range of bacteria responsible for producing nutrients readily available to plants.
Now, if you have trouble seeing pests and insects feasting on your plant’s leaves, using warm water at 65℉-67℉ temperature can help you solve the problem. It will not kill the plants nor damage the leaves, it will only save your plants from a severe infestation.
None of us like seeing our plant’s leaves eaten by different insects, and we want to get rid of them by not using pesticides especially in culinary herbs.
Should you pour boiling water straight into your plants? Do not ever do it unless you want to kill your plants together with the insects that piss you off. So do not use water with temperatures that are above 120℉ because you will surely regret it as the plant area in touch with water will burn, especially if the plant has thin leaves.
Cold Water in Plants? Why Not?
It is beneficial for cooling soils that are used to plants that are sensitive to high soil temperatures such as radish, spinach, potatoes, herbs, cabbage, parsley, turnips, beets, and endive. This applies only if the plant is inside a planter. On outdoor soil, the cooling effect of water will be minimal.
A good water temperature is around 45℉. This is helpful during summer and mostly in tropical countries. Cold water will help you cool down and maintain the right soil temperature that is needed by your plants.
Remember that cold water can only keep the life of the plants or herbs by regulating soil temperature, but it will never help you produce high yields.
When To Use Hot Water?
Warm water can be used to a) remove stains b) pests and c) remove isolated weeds. The ideal and simple is to use water at ambient temperature.
Pests
The easiest way is to let the water boil on a pan and then let it cool down. If you use a water temperature that exceeds 120℉, it will seriously harm your plant.
If your problem is leaf-eating pests such as aphids, cutworms, caterpillars, or beetles, you may pour it directly into the leaves that are being infested. The warm temperature will not only weaken the pests but also improve their physical appearance by cleaning and removing the stains.
Another option to remove pests is to place the pot inside another larger container containing such warmth. This will keep the soil soaked so to kill the pests. My best advice is though to just replace the potting soil entirely. Faster and cleaner.
Please avoid this on plants with thin leaves such as basil and parsley as this will damage them.
Weeds
Hot water, even better boiling water, can be used to kill weeds. However, this is effective only if the weed is far from your plants or it is not in the same pot as your plants.
Indeed, if you drop hot water into a weed, this will very likely kill your dear plants if they are just a few centimeters apart. This is because weeds are often very resilient with thick (tuber-like) roots.
Based on my years of experience, I would remove the weed with my bare hand, and then pour kitchen salt into the hole and boiling water. This will probably kill any leftover roots.
Freezing Water in Winter
Freezing water that develops in the soil in cold weather can seriously damage the plant roots and prevent the uptake of macronutrients such as nitrogen and liquid water.
Look at the table below to see the different plant freezing points.
| Freezing Point | Effects |
|---|---|
| Light Freeze -29℉ to 32℉ | Kills tender plants and herbs |
| Moderate Freeze -25℉ to 28℉ | Destructive to most vegetables |
| Severe Freeze -25℉ | Heavy damage to most plants |
If you are growing cold-sensitive herbs (basil, chives, mint, oregano, parsley, sage, rosemary, and thyme) outside during winter, there is a higher chance of your herbs dying.
2 Easy Ways for Herbs to Survive in Winter
The two easy ways to keep your herbs alive and thriving during winter are 1) putting them in a cold frame, 2) placing them indoors.
1. Cold Frame
Putting the plants in a cold frame will help create a warmer temperature for the plants to survive from freezing and dying. This will also protect them from direct contact with the snowflakes.
Although the cold frame is costly, there are other resources that you can recycle such as empty gallons and black plastics. Cut the empty gallons according to the height of your plants and cover them one by one.
I recommend black plastics because they eliminate weed growth and they also absorb the warm temperature of the soil and keep it circulating together with other plants.
2. Move Indoor
Transplant your herbs into a container and place them indoors in front of a sunny window. My recommendation is to change the soil as it naturally carries out pests and small insects that will start crawling indoors.
Guidelines for Watering Indoor Plants (3 Steps)
Watering indoor plants does not require complicated practices.
First, use a well-prepared potting soil that can hold water and at the same time provide air for plant roots to absorb water. Make sure that the pot has at least one drainage hole to drain excess water.
Second, apply enough water until you see water running out of the hole of the pot. This will replenish the water supply and reduce eventual salt buildup if you are using fertilizer, especially if in liquid form. If there is excess water on the surface, pour it away.
Third, do not water plants according to your desired schedule. Water them only when the surface of the soil is already dry. This will maintain a good balance between the soil and water in the pot.
Guidelines for Watering Outdoor Plants
Water outdoor plants only if they show visible wilt signs. Unlike indoor plants, outdoor plants are exposed to sunlight and natural rain therefore the water evaporates faster. Frequent watering (twice a week) might be necessary only for vegetables over the hot season in lack of rain.
Make sure to water, the soil until excess water runs out the drainage holes of the pot. Actually, if possible, avoid watering the leave especially if you have many plants in the same planter. This will indeed cause an increase in humidity due to a lack of air ventilation.
The best time to water the outdoor plants is in the early morning, before sunrise. This will keep the plant foliage dry quickly and help plants from developing fungal diseases. Moreover, water will soak deeply into the soil with little loss due to evaporation.
Watering outdoor plants during midday is less effective because there is rapid evaporation due to sunlight exposure and higher soil temperature. It is also not advisable to water plants in the evening because the plant foliage will remain wet throughout the night and can lead to the development of different moist-related fungi.
Summary of Hot or Cold Water for Plants
1. Plants have hot and cold water temperature limits which you cannot exceed.
2. The ideal warm water temperature for plants is 65℉-67℉ and is good for killing pests eating leaves.
3. Do not use water at a temperature of more than 120℉ because it will kill your plants.
4. Water at a temperature between 115℉ and 120℉ will kill unwanted soil chemicals and impurities.
Sources
Akatsa, J., Mwaura, H., & Weru, P. (2014). Biology. Longhorn Publishers.
“Root-freezing Damage in the Containerized Nursery: Impact on Plantation Sites – A review” by Fj Bigras et.al. in Springer Link
“Effects of Low Temperature on Plants” by B. Penisi et.al., in the University of Georgia Uga Cooperative Extension
“Environmental Requirements for Indoor Plants” in Sargodha University
“When is the best time to water the garden? | Horticulture and Home Pest News” by Jauron, R. (n.d.). in Iowa State University
Yourindoorherbs.com is part of the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites like mine to earn advertising fees by promoting good quality Amazon.com products. I may receive a small commission when you buy through links on my website.
