When The Heat Is On: Understanding Temperature Tolerance For Potted Plants

Pot plants can be fickle things, and it's important to know how to care for them, especially in hot weather. While some plants like cacti and succulents are well-adapted to high temperatures, other plants can struggle when the mercury rises. In fact, even the hardiest plants will suffer if the temperature gets too high.

So, how hot is too hot? Generally, when temperatures rise above 90°F, plants will start to suffer. Leaves will wilt, flowers will drop, and plants will stop breathing to prevent water loss. This is especially true in dry heat.

If you're growing plants in pots, the temperature inside the pot can get even hotter—up to 120°F. This can cause the roots to boil, leading to plant death. To prevent this, it's important to choose the right type of pot and take steps to keep the temperature inside the pot cooler.

Soil temperature and how it affects root growth

Soil temperature is a crucial factor in plant growth. In fact, it may even be more important than air temperature, as it usually lags behind and tracks air temperature during spring and diurnal temperature fluctuations. As a result, the soil temperature may not meet the requirements for root formation, even when air temperatures are favourable.

The optimum temperature for root growth varies among species and is generally lower than that for shoot growth. For example, the optimum temperature for root growth in wheat is around 6 °C, while broadleaved seedlings tend to have a higher specific root length (SRL) than conifer species, indicating more root length per unit root mass.

Root growth is highly sensitive to temperature changes. A study on tree seedlings found that a soil temperature of 2-16 °C inhibited root expansion into deeper and colder soil layers, with maximum rooting depths occurring at temperatures between 3-6 °C. Another study on various crop types reported that increasing the root zone temperature from 12 °C to 25 °C generally improves root functions related to water and nutrient supply to shoots. However, further increases in root zone temperature can lead to growth inhibition, reduced nutrient uptake, and disturbed plant-water relations.

Additionally, low temperatures can reduce both root elongation and branching, with shoot growth being more inhibited than root growth, resulting in a higher root-to-shoot dry weight ratio. At very low temperatures, root respiration slows down due to the inhibitory effect on enzyme activity.

To optimise plant growth, it is essential to maintain the ideal soil temperature, especially during critical growth stages. For marijuana plants, for instance, a soil temperature of 80ºF is recommended to ensure warm roots and help the stems and leaves withstand bouts of colder air.

Tips for Managing Soil Temperature

• Use a soil thermometer to monitor soil temperature.
• Place heat mats on the floor to maintain soil temperature.
• Choose plant varieties that are adapted to your climate.
• Use grow tents or greenhouses to create ideal environments.
• Avoid dark-coloured containers that absorb more heat. Instead, opt for light-coloured, shiny, or white containers.
• Double-pot your plants or use fabric pots to help keep the roots cool.
• Provide shade for your plants, especially during heatwaves.
• Water your plants early in the morning or late in the evening to prevent excessive water evaporation.

Signs of heat stress in plants

Plants can be extremely sensitive to their surroundings, and this includes the temperature. Most plants thrive in temperatures between 59°F and 86°F. When temperatures exceed 90°F, plants can begin to show signs of heat stress. Above 104°F, many plants will show clear signs of heat stress, and some will not survive. Here are some of the key signs to look out for:

Leaf Rolling and Cupping

Corn and tomatoes are among the plants that commonly roll or cup their leaves in response to heat. This reduces the leaf surface area and minimises moisture loss. The stomata (microscopic openings in leaves that allow the movement of moisture and gases) close to further reduce moisture loss.

Wilting

Wilting occurs when there is a lack of water pressure within the plant due to low moisture. This is common in non-woody annuals and perennials. The longer a plant remains wilted, the more likely it is to suffer permanent damage. If a plant wilts during the hottest part of the day and recovers in the evening or early morning, it is likely suffering from heat stress.

Dry Leaf Edges

Some plants, such as squash and pumpkins, have a survival mechanism where the outer margins of the leaves dry up, but the leaf as a whole remains functional. This is because these plants have more leaf surface area to "spare" and can still maintain their function. Dry leaf edges can, however, be mistaken for disease symptoms.

Ozone Damage

Ozone damage can occur when high temperatures combine with poor air quality. In tomatoes, this can appear as dry brown spots between the leaf veins. In cucumbers, squash and pumpkins, the leaves become bleached and very dry.

Blossom and Fruit Drop

Many ornamental plants will drop their buds and blossoms after prolonged exposure to high temperatures. This is the plant's way of conserving resources for its survival. Blossom and fruit drop are common in peppers, squash and cucumbers.

Bolting

Bolting is the premature flowering of plants before they are usually harvested. It is induced by high temperatures in cool-season vegetables. A heatwave can mean the end for many cool-weather plants such as broccoli, cauliflower, lettuce and spinach.

Sunscald

Fruits such as apples, tomatoes and melons can scorch in extreme heat, usually on the side most exposed to the sun. This can appear as watery areas, discolouration, blisters, dried-out patches or sunken, hardened areas on the fruit surface.

Blossom-End Rot

Blossom-end rot is common in tomatoes and is made worse by heat and high solar radiation. It appears as a dark, watery stain on the fruit.

How to cool down plants

Extreme heat can be very damaging to plants, but there are several ways to cool them down and protect them from the sun. Here are some detailed and direct instructions on how to cool down plants:

Watering Plants

Watering plants is essential for their survival during a heatwave, but it should be done at the right time of day to be most effective. The best times to water plants are early morning or evening, as this gives water a chance to soak into the soil before the heat of the midday sun causes it to evaporate. For container plants, water twice a day as soil dries out much quicker in pots. Water generously, so that the water penetrates the root zone, but be careful not to overwater, as this can cause plants to leach nutrients and cut off the oxygen supply to the roots. Avoid getting the foliage too wet, as this can cause leaf burn. If using a sprinkler, water early in the morning so that any moisture on leaves will dry quickly and minimise the risk of fungal infections.

Provide Shade

Move container plants into a shady spot, especially those in terracotta pots, which are more susceptible to evaporation. For plants that can't be moved, create shade using shade cloth or garden shade ideas such as pergolas. You can also use companion planting to create shade by planting tall, heat-tolerant plants around more tender, shade-loving plants.

Mulching

Mulching borders and containers will help plants' roots retain moisture and protect them from heat. A layer around 3 inches deep will stop heat from penetrating the soil. Light-coloured mulches offer more protection from heat than dark-coloured ones.

Windbreaks

Wind can damage plants and cause soil moisture to evaporate. Install or grow windbreaks in your garden to protect your plants. Windbreaks can be made from snow fencing, pallet fencing, or living windbreaks such as tall annual crops or dwarf fruit trees.

Trellises and Intensive Planting

Build a trellis for climbing plants, which can then be planted with a cool-weather-loving crop. Intensive planting, where plants are grown in beds rather than rows, can also help to shade the soil, reduce moisture loss, and block weeds.

Other Tips

• Feed heat-fatigued plants at the right time – fertiliser should never be added to dry soil as it can cause 'burning'.
• Choose drought-resistant plants such as rosemary, marjoram, and lavender, which love the heat.
• When transplanting crops, do so in the early morning, late evening, or on a cloudy day to avoid transplant shock.
• Catch rainwater by digging a basin around each plant to funnel water towards the roots.

How to prevent heat problems

Heat stress can be detrimental to plants, causing them to wilt, develop brown leaves, and even die. Here are some ways to prevent heat problems and protect your plants during a heatwave:

• Understand your climate: Know how hot your region typically gets and how long the high temperatures will last. Monitor local weather forecasts and research average high temperatures and heatwave durations. This knowledge will help you prepare and implement protective measures for your plants.
• Plant in the right position: Observe sunlight patterns and identify areas in your garden that receive morning sun and afternoon shade. Plant vulnerable plants in these spots, or under the shade of larger, heat-resistant plants. Avoid planting heat-sensitive plants in areas that receive harsh afternoon sun.
• Water properly: Water slowly and deeply to encourage deep root growth and enhance plant resilience. Deep watering saturates the lower levels of the soil, allowing roots to grow downward and access deeper moisture reserves. Water in the early morning or late evening to minimize evaporation and give your plants a chance to absorb water before temperatures rise. Avoid overwatering, as it can lead to root rot and fungal diseases.
• Install drip irrigation: Consider installing a drip irrigation system, which delivers water directly to the roots. This ensures efficient water usage and helps prevent diseases caused by water-splashed foliage.
• Mulch your plants: Mulching is essential during extreme heat. It helps conserve moisture, regulate soil temperature, and suppress weeds. Use organic materials like straw, wood chips, or compost, spreading a layer about 2 inches thick.
• Choose the right containers: For container plants, opt for light-colored, larger clay or ceramic pots. Dark-colored containers absorb and retain heat, which can overheat the root system. Light-colored pots reflect sunlight, keeping the soil and plants cooler. Additionally, choose breathable materials like clay or ceramic to improve air circulation.
• Relocate container plants: Move container plants to cooler, shadier spots during the hottest parts of the day. Place them under large trees, on the east side of buildings, or under a pergola or patio.
• Create temporary or permanent shade: Use shade cloth, old bedsheets, or umbrellas to provide temporary shade for your plants during a heatwave. For a more permanent solution, install shade structures like pergolas, shade sails, or awnings to provide lasting protection from extreme heat.
• Use plants to create shade: Larger plants, such as trees and shrubs, can provide shade for smaller, delicate plants. Consider the eventual size and shape of these shade-creating plants to ensure they cast their shadow where needed without completely blocking sunlight.
• Hold off on pruning: Avoid pruning during extreme heat, as it stimulates new growth and puts additional stress on the plant. Mature leaves are better equipped to handle hot conditions than new, tender growth.
• Keep up with weeding: Weeds compete with your desired plants for resources, including water and nutrients. Regular weeding reduces this competition and helps protect your plants from heat stress.

How to water plants in hot weather

Watering plants in hot weather can be challenging, but with the right tools and knowledge, you can keep your plants healthy and thriving. Here are some detailed instructions to help you successfully water your plants during hot weather:

Timing is Everything

Water your plants in the morning when it's cooler. This allows water to reach the root system before it evaporates in the heat. If morning watering is not possible, water in the late evening before bedtime. Avoid watering at midday when the sun is at its peak, as this leads to rapid evaporation and reduces water reaching the roots.

Water Deeply and Efficiently

Instead of light, frequent watering, opt for less frequent but deeper watering. This encourages plants to develop stronger and deeper roots, making them more resilient to dry conditions. Aim to water the base of the plants, avoiding the foliage, to prevent leaf scorch and reduce susceptibility to fungal diseases.

Use the Right Tools

Utilize soaker hoses or drip irrigation to slowly and steadily apply water to the root zone of your plants. This method ensures efficient watering and reduces the risk of runoff and evaporation.

Mulch for Moisture Retention

Apply mulch, such as compost, wood chips, or straw, to retain moisture in the soil for longer. Mulching helps keep the soil moist, cools the roots, and adds nutrients to the soil as it decomposes.

Check and Adjust Watering Schedules

Monitor the moisture level around the base of your plants to determine the appropriate watering schedule. Watering consistently is more important than the frequency. Adjust your schedule if you notice signs of drought or overwatering, such as wilting or drooping leaves.

Move Potted Plants to Shadier Spots

During hot weather, move container plants to shadier locations to reduce their heat exposure. Black planters, in particular, can get incredibly hot, baking the roots of the plants inside. Protect your plants from excessive sun by positioning them in partial shade while watering.

Avoid Overwatering

Overwatering can be just as harmful as underwatering. Always check the soil's moisture level before watering. Use your fingers to test the moisture a few inches below the surface, or utilize a moisture meter. Wait to water if the soil is still damp to avoid overwatering, which can lead to root rot and other issues.

By following these guidelines, you can effectively water your plants during hot weather, ensuring their health and vitality.

Frequently asked questions