Is it unseasonably hot right now where you live? If so, make sure your bonsai are getting the care they need to stay healthy.
The actual temperature during a heat wave is often less important than how much warmer it is than normal. This past weekend, San Francisco set a new all-time high of 104 F – quite a ways above the average of 70 F.
Because our trees aren’t used to such heat, we can expect to see signs of stress on any bonsai that weren’t well protected.
Heat stress can cause leaves to lose their luster or become yellow. More severe damage will result in brown spots or branch dieback.
Typical signs of heat stress – dull green and yellow foliage
Some discoloration on deciduous bonsai isn’t much to worry about this time of year, especially if it’s the older leaves that are affected.
Older leaves yellowing on Chinese quince
What I don’t like to see are areas where the foliage actually burned and turned brown.
Sunburn on Chinese quince
When properly protected, trees can weather unseasonable temperatures and retain their deep, green foliage.
Healthy quince foliage
Conifers can show similar signs of stress.
Healthy cryptomeria foliage
Signs of too much sun – pale, yellow foliage
If a tree heats up too quickly or dries out when temps rise, shoot dieback is common.
Limited dieback on branch tips
The longer a tree goes without water, the more severe the damage.
Moderate dieback on cryptomeria
What to do if a tree is stressed by hot weather? If the tree is dry, water it! If the rootball is wet, mist the tree and water again when the soil dries out a bit.
Trees can shut down and become dormant in hot weather. If a tree shows signs of heat stress, watch the water carefully and mist often.
Tips for protecting bonsai during heat waves
The following tips can help bonsai survive unseasonably hot weather. Not all tips may make sense – or be an option – where you live. As always, checking with experienced enthusiasts where you live is a great place to start.
- Make sure trees get adequate water. Watering before temperatures rise and preventing them from drying out will go a long way to helping bonsai survive heat waves.
- Cool down trees – and the garden – between waterings. I don’t usually fill the pot with water more than 2-3 times a day, but I might sprinkle trees lightly an additional 3-5 times a day to cool them down. Take care, however, if fungus is a problem where you live, especially when watering in the afternoon as warm, moist weather is conducive for fungus growth.
- Protect the surface of the soil by adding a layer of white sphagnum moss (or mountain moss/yamagoke). Moss can increase moisture retention and prevent surface roots from drying out.
- Protect roots from overheating by shielding the the pot from the sun. This can be done by moving pots closer together (the combined canopies can shade the pots), turning trees so lower branches better shade the pots where the sun exposure is direct (typically south facing in the U.S.), or propping a board against the pot to provide shade: (see examples and more tips for beating the heat from Michael Hagedorn at crataegus.com). Pots can also be wrapped with a wet towel or aluminum foil.
- Move sensitive trees into the shade. When temps are high, I’ll move trees under benches or to corners of the garden where there’s less sun to help them stay cool. Moving trees to the ground can insulate the pot and help protect roots from overheating.
- Move trees away from fences, walls or other structures that reflect light and heat.
I can also add that spraying pesticides or fungicides on hot days can be particularly stressful for bonsai. Read and follow instructions on labels before spraying and if at all possible, avoid spraying during heat waves.
If heat spikes make growing bonsai difficult where you live, the following strategies can help.
- Install shade cloth that covers the top and sides of trees that need additional protection. Shade cloth positioned above trees provides shade below, and fully enclosed shade cloth structures increase the relative humidity around trees.
- Setting trees on a tray with pumice provides extra water and keeps pots cool. In time, roots may grow into the pumice which can help trees withstand warmer temperatures. I do this frequently for accents and smaller trees, and sometimes I’ll bury the entire pot in a larger pot filled with pumice in an effort to keep the roots cool.
- Use larger containers for growing. Many heat-related problems can be avoided by using larger containers as they offer better insulation and provide more room for roots to grow.
Have additional suggestions? Share them in the comments below.
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