Keeping Plants Alive in Extreme Heat

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(Posted on July 8, 2018, also see Tips for Native Plant Heat Wave Gardening, Sept. 18. 2018)

We are currently enjoying a balmy 99+ degree day. After the record breaking heat of 114º F on Friday, this is somewhat of an improvement. In this blog post I will 1) suggest practices to help your plants make it through the extreme heat; 2) share temperature data from my garden; and 3) give an update (and pictures) on how my plants are doing on day 3 of extreme heat.

Tips

  1. Before extreme heat arrives
    • Water deeply during the cool part of the day to hydrate plants.
    • Spray leaves in early morning to remove pests and dust. The leaves will dry off quickly as the day proceeds, reducing the chance of excessive fungal and bacterial growth.
    • Trim young, tender growth if you think the plant will have trouble maintaining hydration.
  2. It is so hot out there!
      • Shield young plants with screen, shade cloth, or white sheet.
      • If your plant is stressed, check the soil to make sure it is dry and then water deeply. If soil is moist, more water will not help. Most plants shut down during extreme heat, so if it is over 100 degrees, it may be best to wait to water even if your plant is stressed. Throw a damp sheet over it, or mist it lightly to bring the temperature down.
      • Mist leaves with a spray nozzle to cool down plant and reduce transpiration.
      • Desert plants can be watered in the late afternoon to mimic the summer monsoonal rains that they are adapted to.
  3. Post Apocalypse
    • Once it has cooled down, water plants deeply.
    • Do not remove wilted leaves just yet. They will serve as sun shield if another heat wave is on the way.
    • If plants that exhibit summer dormancy have lost most or all of their leaves, allow them to continue into dormancy by not watering. Extra water now could pull them out of dormancy. These stressed plants may then expend a lot of energy growing a new set of leaves only to be faced with the likelihood of more heat and no natural rainfall. If they do not emerge from dormancy, they will be susceptible to fungal pathogens that grow in moist, hot soil.

Please comment with your own tips and suggestions. If you disagree with mine, let me know how and why so we can learn together.

Temperature Data

A friend, Drew Ready, posted some interesting data on Facebook’s Southern California Native Plant Gardeners Group during this heat wave. He measured surface temperatures for areas that were covered with organic mulch, asphalt, gravel, cement and brick. The winner, surprisingly, was above an organic mulched bed (177.2º F)!  Next was asphalt (170.4º), followed by cement (159.2º), then brick (156.7º), and gravel (149.9º).

These surprising numbers led me to take my (actually it belongs to my husband) trusty meat thermometer outside today to see what gives. First I must make a disclaimer. A meat thermometer does not do a good job of measuring air temp. Nevertheless, I was more interested in measuring the soil temperature right below the surface. Here are my results:

Meat thermometer measurements
A) 101º F, sun, 5′ above the surface
B) 108º, sun, ~ 2″ above a mulched surface
C) 106º, sun, no mulch, 2″ below surface, dry soil*
D) 85º, part-sun, 2″ below lightly mulched surface, dry soil*
E) 77º, shade, 2″ below mulched surface, damp soil
* It should be noted that in photo D, although the area was in sun when the measurement was made, it probably was shaded earlier in the day, while photo C probably was exposed to the sun longer. It is likely that this accounts for the temperature difference as much as the thin layer of mulch in photo D.

My Garden

Finally for a preliminary report on how things look in my garden. As the heat continues more plants may succumb to the stress but here is a quick review followed by a slideshow.

Significant leaf/plant damage

Agave attenuata – likely to recover, though it will look ugly for some time
Vitis 
‘Roger’s Red’ – will recover
Ribes speciosum – leaves dried and curled, plant will go dormant and be fine
Heuchera ‘Wendy’ – very scorched, despite having been watered well before, not likely to make it
Constancea nevinii ‘Snowflake’ – some look fine, two younger plants were cooked; not sure how these will do

Some leaf damage

Avocado trees – these worry me the most, little damage, but this causes a lot of stress to these old tree
Ribes viburnifolium – not much damage, should be fine
Epilobium canum – some damage, should make it

Lookin’ good

Quercus agrifolia – never a good thing for our oaks, but they look fine now
Berberis repens – look real good
Heteromeles arbutifolia – minor amount of leaf burn, mostly fine
Fragaria chilensis – looking fine
Dudleya – potted collection in the shade is doing okay
Parkinsonia ‘Desert Museum’ – this desert plant looks downright happy
Vegetable garden – looking surprisingly good

Stay cool and let me know how your garden is doing!

12 thoughts on “Keeping Plants Alive in Extreme Heat

  1. Well, I have done everything wrong during this heat and now have my Myer lemon VERY stressed with major curling and some yellowish leaves. When temperatures soared, I panicked and watered during the heat of the day and last night removed most of the the curled and brittle leaves within reach. This morning I will hose the tree to get rid of the insects but will hold off on watering. Barbara, any advice?

    • weedingwildsuburbia

      I’d suggest deep watering in the morning, now that it has cooled down a small bit. Make sure you have a layer of mulch beneath the tree (keeping the base of the tree clear of mulch). If we get triple digits again (not if, when…) water thoroughly before the heat. If possible drape a white sheet over the tree during the heat wave. You can even mist the sheet – anything to try to keep the tree from getting too hot again. Meyer lemons are really tough plants – it will probably be fine. We are all learning!

      • Okay…after hosing off the lemon, it looked a little better. Then next day, as per your suggestion, I deep watered in one place at drip line and it looked a little better. However, I examined the curling and damaged leaves a little closer and I think I have an insect problem, too. Pulled some leaves and took a shot and will email it to you. Thank you for this blog!

  2. weedingwildsuburbia, thank you for your blog post.Really thank you! Awesome.

  3. LeH

    What do you plan to do with your avocado tree? Most of my tree looks pretty good but one small branch that had about a dozen ping pong ball sized avos is really bad off. Not sure how far back it needs to be pruned or when to do it.

    • weedingwildsuburbia

      I plan to leave the scorched branches where they are for now. The biggest danger is that they will crack and bring down branches beneath them. There are no structures or cars that could be damaged by fallen branches. I will then have someone come and prune the dead limbs in the fall – the tree is way too tall for me to be able to get at them.

      In the short-term, I watered both of these avocados deeply today. They are not going to look very good when the leaves turn brown!

      Don’t know how old or large your tree is, but like I said, I’d wait a bit unless the damaged branch looks dangerous or likely to crack and cause damage to the tree. If it is a mature tree you may want to consult an arborist. Avocado trees are very brittle so proceed with care.

  4. Mona Erhardt

    Two days of 113+, followed be several days of 100 F heat in Oak View (near what used to be “Lake” Casitas), all properly watered/not watered and mulched, on the SCORCHED list: Quercus agrifolia; Romneya coulteri; Salvia apiana; Heteromeles arbutifolia; Juniperus; Lavatera maritima; Agapanthus; Rhaphiolepis indica; Calandrinia grandiflora; apricot; apple; plum. Looking relatively happy: Helianthus annuus; Oenothera biennis L; Oleander; Rhamnus alaternus; Heterotheca grandiflora; Lepidospartum squamatum; Eschscholzia californica (heavily mulched); Gnaphalium microcephalum thermale; Anaphalis margaritacea; Pennisetum ‘Fairy Tails’; Ceanothus arbutifolia ‘Ray Hartman; Ceanothus thyrsiflorus; Agave americana; Nicotiana glauca; Pine trees; Albizia julibrissin; Eucalyptus;. COMPLETE GONER: Camellia japonica. There are more to report, but my brain is also scorched. All I know is that there are several natives that I’ve NEVER seen burned like this in my 55+years living in this area.

    • Mona Erhardt

      …so I’m now researching desert plants, as our zone parameters are greatly changing; e.g. lost a beautiful, mature mission fig to three consecutive winters of prolonged freezing temps. Yayy. Fun.

  5. Thank you so much for this! I’ve been trying to figure out what to do with a ceanothus – largish, but planted last fall so it hasn’t had as much winter water as it probably needed, due to low rainfall. I did water it last winter but now I’m wishing I’d done more…. it’s now looking very very stressed, with some curling and some burnt leaves. Seems like it needs water but I’m leery of watering ceanothus in summer…. Any advice?

    • weedingwildsuburbia

      Hi Heather. Do you know which Ceanothus it is? My experience is that Ray Hartman is one of the easiest ones to grow. About 10 years ago I planted 2 Conchas. The first summer after planting, one of them did what you describe. I watered it well and it promptly died. The following summer Concha number 2 did the same thing. This time I held off on watering, and it promptly died. I’m telling you this because sometimes watering or not does not help. It may be root rot or some other disease or stress.

      Having said this, I’d suggest checking the soil around the rootball carefully. If it is dry, water during the cool part of the day (early morning is best), and let the chips fall where they will. Also, while checking the soil for moisture, gently push your fingers into and around the rootball to check for gaps in the soil. If you find any, try to fill them in with you fingers. If it seems to respond positively to water, then let the area dry out and water deeply again – could be in 2 weeks depending on weather and soil type. Young plants like this usually need summer water (and winter water during dry seasons).

      Good luck and let me know how it goes.

      • Hi! Thank you so much – I’m giving this a try and fingers crossed (especially as there’s a heat wave this weekend). I’m not 100% sure but I think it is a Ray Hartman – it’s one of the larger tree-style ones, rather than a groundcover or shrub. There were indeed gaps in the soil, mostly gopher-related (although it’s planted in a large gopher basket, so hopefully the core roots are OK). I’ll try soaking it again in another week or so!

  6. Excellent post and guidance. Thanks.

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