Most bonsai are okay with the summer here in New Jersey, with days in the 90’s and nights in the 50’s to 60’s. This current heatwave is brutal. Roots were not adapted to be exposed to 115 degree temperatures. A few feet below the surface the earth stays a comfortable, stable temperature year-round. So here are a few ways to keep your trees cool and growing:
Give Your Bonsai A Bath
During this extreme heat (well above 100) you can place your trees in a dishpan of water during the day, pot and all. Hours of being under-water will not be enough to drown roots, just take them out to allow the soil to flush with oxygen. The water will take much longer to heat up than the soil itself. Just don’t leave them in the water for days at a time.
Place Your Trees In The Shade
This will help the most. Even here at the nursery, the plants in full-sun this summer (pre-heat-wave) have been showing signs of stress. While plants are adapted to the sun, there can be too much of a good thing. Outdoor shade, while visually not much different than indoor lighting, offers way more light than our little windows let in. The plants given shade/filtered light will often be greener and healthier than their counterparts in full-sun for the summer.
Water The Foliage
There is an old myth that watering the leaves of a plant will cook them by concentrating the light like a magnifying glass in the sun. This is couldn’t be more far removed from the truth. Watering the leaves of your bonsai will provide a buffer between them and the sun. The heat must first evaporate the water sitting on-top of the leaves before it can take the water out of the leaves, buying the tree some time to take water from the soil back up. This is especially important for trees that have been allowed to dry too much between watering as it reduces their stress.
Please keep in mind that watering the foliage in cool temperatures may cause fungal problems for certain species. In this extremely hot, dry air that is not much of a concern.
Don’t Forget To Water!!!
I cannot stress this enough, it is the most important point of all. Do not forget to water your bonsai in the summer. There is no faster way to kill your trees. Junipers and pines will not give warning by wilting in advance. If you have already forgotten to water, follow the previous three steps and hope for the best. There is not much else you can do.
Resist The Temptation To Bring Your Bonsai Inside
Due to the angle of the summer sun and the constant use of air conditioning, bringing bonsai indoors that have been growing outdoors will not be the best thing for them. Air-conditioned air, while cooler, is extremely dry. Dry air with low light is stressful for plants, and it is the worst thing you can do for hardy trees. Follow the previous four steps instead.