Water is essential to the survival of plants of all species. Getting it right can be the difference between a thriving plant and a (sadly) dead one. The importance of this task understandably leaves gardeners with a few important questions, like whether watering plants in the sun is a good idea or if you can water plants with cold or hot water.
You should not water plants with cold or hot water, as extreme temperatures can cause severe physical and cellular damage. Hot water can degrade cell walls, causing them to lose shape and die off. For cold-sensitive plants, excessively cold water can shock the roots, stunting growth and potentially killing the plant.
Luckily, these negative effects can be easily avoided by checking the water temperature and adjusting your watering through the seasons, from summer to winter.
What Are the Effects of Hot Water on Plants?
Water is typically considered ‘too hot’ for plants when the temperature exceeds 120 °F (49 °C). Any temperatures higher than this threshold will have a range of negative impacts that become more dangerous the hotter the water is.
At a cellular level, hot water damages and can destroy cell walls, potentially killing the entire cell. Plants with damaged or dead cell walls will lose their turgidity, unable to hold their shape and structure due to lack of moisture.
Hot water also negatively interferes with several critical biological processes, including respiration and photosynthesis. The heat damages the xylems of the plants (the tissues that transport water and nutrients from the roots to the parts of the plant that need it), slowing or completely stunting growth.
Using hot water even has adverse effects on the soil. High temperatures can kill beneficial microorganisms in the soil, impacting soil quality and leading to nutrient deficiencies. As soil is the foundation of successful plant growth, keeping soil healthy is essential.
Signs of Heat Damage
All these scientific explanations for problems inside the plant translate to visual signs of damage that can help you identify the problem.
Firstly, a loss of turgidity means the cells are no longer holding onto water. As cells are mostly made up of moisture, this moisture loss causes them to lose their shape, leading to wilting leaves and stems.
The main cause of wilting is usually underwatering. But if the soil is still moist and your plants are wilting, high water temperatures may be the cause.
The hot water may scald the foliage of plants too when watering overhead. This may cause parts of the leaf to die off, turning brown or black. In severe cases, leaves will begin to drop off the plant.
Hot water also damages the leaf cuticle – the waxy layer covering the leaves of plants. Loss of this protective barrier exposes the plant to the risk of illnesses, pathogens, and pests.
Plants are always at risk of pest and disease issues, not always under a gardener’s control. However, sudden and severe problems usually indicate a negative change in care or environment, including water temperature.
When planting seeds, high water temperatures can also impair seed germination by damaging the seed coat or scalding young and vulnerable roots and early leaves. The level of impact will depend on the seed type and what conditions the plant is able to withstand.
Leveraging High Water Temperatures for Pest Control and Plant Health
Hot water can be beneficial to plants in some cases. This depends on the temperature of the water, the frequency of hot water use, and the duration of hot water exposure when watering.
For example, several soil-borne plant pests can be killed with quick hot water exposure. Some plants can handle this hot water in short bursts, making it an effective and natural pest removal solution. The water shouldn’t be too hot – aim for 120 °F (49 °C) at most.
Hot water can also help remove dirt and debris from leaf surfaces that slow photosynthesis and gas exchange, especially on houseplants that aren’t regularly rinsed after rain. But rather than applying the hot water directly, it’s better to soak a cloth in the water and wipe gently to avoid scorching the foliage accidentally.
Effects of Cold Water on Plants
Cold water has slightly different effects but can still be incredibly damaging. The impact will vary widely based on what plants you are growing, as some are more cold-tolerant than others.
Tropical houseplants, for example, typically stop growing in temperatures below about 60 °F (16 °C). If the water you use is far below this temperature, it can shock the plant.
Cold shock leads to stunted growth in the roots, stopping the plant from putting out any new leaves or stems. Other plants like tulips prefer cold water and flower more often in extreme temperature fluctuations.
Cold water mostly interrupts a plant’s biological processes, causing little physical damage to the leaves and stems on contact unless the plant is exposed to below-freezing temperatures after watering.
Low temperatures cause plant cells to freeze, leading to structural damage and disrupting the flow of nutrients and water in the plant.
Signs of Cold Damage
In the winter, some plants become dormant to preserve their energy and nutrients for development in favorable conditions. Their growth rate slows down dramatically during this time, and some plants may completely stop developing to conserve resources for the future efficiently.
Frequent use of cold water replicates these winter conditions, leading to slow growth (or worse, no growth at all). Stunted growth can be hard to spot, especially as there are so many potential causes. But consider the temperatures of the water you’ve used recently to determine if this may be the cause.
Similarly to hot water, cold water also damages xylem tissue, preventing the transportation of water and nutrients around the plant. This can lead to wilting and root damage. Other signs of cold damage include black leaves and other discoloration caused by nutrient deficiencies, along with lack of flowers for blooming plants.
Essentially, watering plants with cold water will cause them to act as they would in winter, and not all plants are built to survive at low temperatures. Your cold-sensitive plants may look pale and starved, indicating low nutrient uptake and use.
What Is the Ideal Water Temperature When Watering Plants?
When watering indoors and out, you’ll need to decide what temperature range is best for your plants. This differs slightly based on what you are growing, but the ideal water temperature for most plants ranges between 62 and 72 °F (17 and 22 °C). Temperatures far beyond this range could significantly stunt your plants’ growth rate.
If you live in locations with extreme temperature fluctuations or harsh weather, make an effort to measure the temperature of the water with a thermometer before you use it on your plants. You may need to let the water run for a few seconds before sticking the probe into it to ensure higher accuracy.
For those who continually struggle with temperature-related issues, it may help to invest in a soil thermometer. This ensures the plant root system and microbes are operating at the correct temperature before and after watering.
What Water Temperature Extreme Is Least Damaging?
Room temperature or tepid water is ideal for use on most plants. But when it comes to either extreme, which is less damaging?
Generally, it is better to water plants with hot water rather than cold water. Hot water comes with some benefits and less damage, while cold water usually only adversely affects plants. But the negative impacts of hot water do increase with temperature, making them equally as damaging if the temperature is well over 120 °F (49 °C).
How Does Environmental Temperature Affect Plant Growth?
It’s no surprise that environmental factors must be controlled to ensure that plants thrive. After all, even humans and animals need their immediate environment to provide certain conditions to ensure their survival. However, some plants are especially vulnerable to extreme environmental changes, impacting biological processes.
When watering, we see that cooler temperatures tend to slow down essential processes in plants, while warmer water temperatures affect turgidity and cause cellular damage.
These effects have been studied by scientists like Lori R. Tolley-Jordan, who performed experiments on wild rice. The study showed that cooler temperatures negatively affect all types of growth and reproduction in these plants.
But it’s important to note that the temperature of the environment has a more significant impact on plant development than the temperature of the water used in watering. Water temperatures change conditions in the short term, while environmental temperatures and prolonged exposure have a much larger impact.
In other words, accidentally using water that is too cold once or twice won’t do as much damage as exposing the plant to cold environmental temperatures for long periods.
In addition to water temperature, watering technique also impacts the growth and health of your plants.
It’s best to water your plant in the morning or evening, before or after the sun rises or sets. Temperatures around this time will limit evaporation and ensure that the plant can draw up as much water as it needs to thrive. Watering your plants in the morning is generally preferable to watering in the evening to help the plants manage the peak heat of the day, especially for those planted in full sun.
Soil should drain well to encourage airflow around the roots and limit chances of root rot. Take care not to overwater too as this can lead to the same problems, potentially killing the plant (depending on how much moisture the plant prefers).
The age of a plant will also influence when and how frequently you should water it. Seedlings require water more often to promote root growth deeper into the soil. On the other hand, mature plants need less water since their root systems are more extensive, giving them access to more moisture.
Here are some helpful tips to perfect your watering technique:
- Water your plants first thing in the morning.
- Don’t use too much or too little water for the specific plant you are watering.
- Water slowly and thoroughly to completely saturate the soil, preventing patches of soil from remaining dry around the roots.
- Water containers more often to combat soil drying out too quickly.
- Plant in well-draining soil to prevent root rot.
Water temperature can have a considerable effect on your plants. You should not water plants with cold or hot water, as doing so can cause severe physical and cellular damage to them. When watering plants, aim for water temperature between 62 and 72 °F (17 and 22 °C) to avoid damage and promote strong growth.