Most plants brought home from the local nursery come in black plastic pots. Most of us like to leave them in their original plastic containers and simply place them in a nice spot.
But is it okay to leave those plants in their original containers? Do the black plastic containers attract excessive heat that might cause roots to burn?
Or perhaps you’re in a purchasing dilemma of choosing between black and white planters. Do black planters get too hot?
Black planters can get too hot for plants because they absorb and retain heat more quickly. The absorption of solar radiation causes the build-up and retention of heat in the root zones. This, in turn, increases soil temperatures to unfavorable conditions for plants, thereby causing adverse effects on their growth.
In this article, I explain how black planters can significantly increase soil temperature and how this affects plant growth. I’ll also provide tips on how to minimize the damage that is caused by black planters.
Why Do Soil Temperatures Matter?
Soil temperatures matter because they affect plant growth.
Soil contains bacteria, fungi, and nematodes, which have different and essential roles in boosting soil and plant health. These micro-organisms can only function within an acceptable soil temperature.
Soil temperature also affects the germination process of seeds.
A study conducted by the U.S Department of Agriculture on cotton germination over 10 days found that lateral root development begins to decrease at temperatures of 93 degrees Fahrenheit and minimal root development occurred at 104 degrees Fahrenheit. Warm soil temperatures also affect the water content in the soil. In extreme heat weather conditions, soil temperatures must be monitored and protected to ensure sufficient plant uptake of water.
Soil Temperatures Fluctuate in Black Planters
In the wild, plants grow naturally in endless volumes of ground soil that buffer fluctuations and extreme soil temperatures. When plants are restricted to pots, they lose this buffer because heat is trapped in the pots that would otherwise spread out across the ground.
Another problem with the typical nursery container is the high surface area to volume ratio. The containers are usually compact (to save space) with just enough soil to keep the plants alive.
But because the pots are so small, the large surface area that wraps the soil acts as a heat magnet that attracts and retains solar radiation. This results in significant temperature fluctuations for the plant roots.
When potted plants are positioned in an open space, the soil temperatures can consistently exceed 104 degrees Fahrenheit for 5 to 10 hours each day in some portions of the container.
During summer, temperatures near the west container wall will usually exceed 120 degrees Fahrenheit when exposed to long afternoon sun.
With such low volumes of soil contained in a pot, added with the black-colored surfaces, black planters are mini-ovens for plant roots.
How Do Black Planters Affect Soil Temperatures?
High soil temperatures will cause direct and indirect injuries to plant tissue.
Direct injuries refer to the physical plant damage caused by temperatures exceeding the critical threshold. When this occurs, the damage is inflicted on the root tissues at the cellular level. Damaged roots caused by heat damage do not recover.
Black pots can burn and cook roots by drying out the moisture in the soil. The roots that have direct contact with the outer walls of the containers are most susceptible to root burn.
Another direct injury is “transplant shock,” when plants are extracted from the ground and into their new potted home. As mentioned earlier, heat builds up a lot quicker in containers, so this sudden change of environment may cause growth to be stunned.
It is worth noting that large woody roots are more tolerant than smaller non-woody fleshy roots. This means the younger plants are more prone to direct injuries than the older ones.
Prolonged heat stress can also cause indirect injuries that damage the plant’s physiological processes. The plant functions such as photosynthesis, carbon partitioning, respiration, water relations, and nutrient uptake are disrupted by heat stress.
Observations of indirect injuries include abnormal plant growth, form, discoloration, and premature leaf abscissions.
Factors That Cause Black Planters to Heat Faster
If you currently have plants in black planters, there are a few environmental factors to be mindful of, as these will influence soil temperature fluctuations.
1. Geographical location
If you live in the warmer zones of America, your plants are more likely to experience heat stress and root burns from the longer sunny days.
High temperatures during the summer daytime, constant sunshine, and dry air contribute to the extra heat stress black planters capture.
2. Composition of soil
The material and composition of the soil will dictate how well heat conducts inside the planters. Grainy, fast-draining, and lightweight substrates will conduct heat poorly from the container walls and the rest of the soil.
Heavier soil with higher water content has higher thermal conductivity because water adheres to mineral particles. Put simply, the thicker and more dense the soil content, the more heat is generated and retained within the plant containers.
3. Material of planter
Those black containers from the nurseries are generally polypropylene plastics. These cheap plastics are essentially incubators by heating the content of the planter. The plastic material collects radiant energy from direct and reflected sunlight.
For outdoor plants, avoid metal planters as they have high thermal transference. Metallic surfaces absorb heat, drying soil content quickly and causing roots to overheat.
4. Placement of the pot
Where you place the planter will make a difference to the potential root damage caused by direct sunlight.
If you’re placing a plant in the backyard garden, balcony, or patio with plenty of sunlight, be mindful that the black planters will cause heat fluctuations for the soil. Plants that are susceptible to such changes may not survive.
The direction of where the pot is facing is essential too.
Plants facing the east will experience the morning sun, whereas the west will experience the highest temperatures during the mid to late afternoon. Plants facing the east and south are likely to experience an abundance of sunlight throughout the day.
How to Minimise Heating of Black Planters
If you’re using a black planter, there are several ways to reduce the potential heating within the pot.
1. Place the planter in a shaded area
Placing your black planter in a shaded area will eliminate concerns about an overheating pot. Fluctuations in root-zone temperatures are caused by direct sunlight. So putting the plant indoors, under the carport, window sill, or kitchen benchtop would work well.
2. Choose fiber pots
If you want to use a black colored planter, try looking for fiber-based pots. They absorb and conduct less radiant energy than plastic or metal containers.
Fibrous materials do not heat very well, so they’re generally 180 degrees Fahrenheit lower than conventional black plastic containers. Fiber pots are good insulators of heat movements.
3. Choose larger and wider containers
Wider containers are potentially less damaging for plants because the heat travels from the container walls and into the center of the pot. There is more soil content for the heat to travel to before it hits the roots at the center. This is true until the roots start reaching closer to the walls.
A short and wide container with a higher ratio of container volume to planter walls will result in less heat absorption. Consider using squat pots for growing smaller plants with shallow roots.
4. Place plants closer to each other
By placing plants next to each other, they benefit by way of mutual shading. The leaves and branches of the plants provide shade for the neighboring plants, thereby reducing solar radiation on container walls.
5. Put black planters inside a protecting pot
If you’re looking for a quick solution without changing the pot, you can simply place the black plastic container into another pot.
The protecting pot acts as a shield that drastically lowers the soil temperature extremes and fluctuations in summer. The space between the black planter and the shielding pot provides an insulating effect to reduce solar radiation.
It’s no surprise that black objects attract heat under direct sun, and you would be absolutely correct if you guessed black planters could be too hot for plants.
Black planters are a prevalent color choice due to their versatility to go with any plant. So, there’s nothing with choosing a black planter. You’d only need to be mindful of the potential root damage it can be caused under direct sun.
But if you simply place your pot under shade or follow the tips I’ve mentioned in this blog post, your plant will just as happy as being in any other colored pot!
- Reducing Heat Stress to Container-grown Plants by Dewayne L. Ingram, John Ruter and Chris A. Martin
- McMichael, B.L. and Burke, J.J. 1994. Metabolic activity of cotton roots in response to temperature. Environmental and Experimental Botany. Vol 34, No 2, pp 201-206.
- Open Oskate – Soil Thermal Properties