When the sun gets too hot…

There are various ways of helping our bonsai cope with sun and not literally cooking them on our benches in the summertime. They are in pots, but it doesn’t mean we want to fry a special root dish, in a soil sauce…

Because bonsai are in pots, they are very unlike trees in the ground. We want to reduce any similarities to a dog in a car on a hot day.


There are two situations… A cooler climate with rare spikes in temperature to 100 F / 38 C or higher that might last a few days, and then there are the hot summer areas that are always that high:

Sudden, rare spikes in cooler climates: Simply relocate your trees temporarily. Don’t bring them inside, but on the ground is a good start, under benches maybe, in light shade. Try to avoid full shade. Place them close together, but still retaining ease of watering. Plants near other plants cool one another with transpiration. Placing bonsai on grass is going to be a lot cooler than on a sidewalk.

Hot weather areas: These areas need site modification. Shadecloth over your growing area is a great option, reducing the ambient and radiant heat. Try 30% shadecloth for general use. Plant more vegetation around your benches, and reduce gravel or concrete areas. Put up fogger/misters that use very little water for cooling down during the heat of the day. Another, less attractive but very effective option is protecting individual pots with either towels or aluminum foil. Yet another is the same as cold weather protection, sinking your pots in bark or gravel. Gravel holds heat, though, so bark or something similar is better protection and is less likely to damage pots. Simply turning your trees so that the lowest branches shield the pot in the late afternoon can be significant. Be aware that entire site modification is much more effective than shielding individual pots.

In general terms (very general!), conifers will be more resistant to the effects of heat. Most conifer bonsai are happy up to 90 F. Most deciduous will do fine up to 80 F. If you’re consistently much higher than that, then you need site modification.

The advantages to any sort of heat reduction are:

  • Moisture needs of the bonsai greatly decrease
  • Foliage remains in better condition
  • Less damage to roots
  • Bonsai show less stress

Most importantly, do not ignore hot spells! Wing into action… They can seriously put your tree back, as badly as if they’d been hosting a very successful pest or disease.

Especially those who live in areas that are currently seeing water restrictions, reducing your water needs might be really significant if you’ve a lot of plants to take care of. Protecting bonsai from intense sun can make a huge difference to health and the ability of the tree to avoid other stresses. Trees can generally handle one stress, but multiple stresses get dicey. Bonsai already have one…they’re in a pot.

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  1. Paul says:

    Hi Mike, we’ve had an uncharacteristic hot spell in Portland with temperatures 95+. The sun has been very bright and some of the leaves and/or needles on my trees have changed color. I make sure to water every day and twice on plus 85 days. My atlas cedar has its needle tips turning brown. At first I thought it was a disease and treated it but then I thought the needles were burning so I put it in the shade and the browning stopped. Your thoughts on this.

    • crataegus says:

      For our average Portland summer days of 80 F, placement in full sun is fine for almost everything. Above that and some things will need protection. Light shade—very light, just to take the intensity down a notch—is best for deciduous and anything under stress. Your cedar should do best with just being in light shade on the hot days, and then moving it out in the sun after that has passed.

  2. Frank Hovenden says:

    Great advice Michael;
    Many of the older bonsai guides often advise full sun for your bonsai. This has to be tempered in light of climate change. Here on Vancouver Island the summers are far hotter and drier from our historic short drought period which occured in late summer for a month or two.
    I have started moving many of my bonsai into area closer to the shade of my house and away from the hot afternoon sun.
    My pines remain on sunny benches but with a shade cloth covering.
    My trees seem happier and later in the season I will move them back.
    Frank Hovenden

  3. garyswiech says:

    Great advice. Although I live in WI, there are clear days in the high 80’s and 90s where I do have to move my maples especially. Hornbeams too.

  4. nicholai hel says:

    It’s been 110F+ for the last 4 days in here, plus the scorching winds. Being out is even risky for myself. 🙁
    Few hours of sun in the morning and full shade for the rest of the day is the way i go now but shadecloth sounds really great for these hard times.

    Handy post. as always…

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