I want to know which crops or vegetables will withstand 40 degrees Celsius (103 degrees Fahrenheit)?
Many days this past summer were over 40 °C (103 °F) here in subtropical Queensland (Australia).
Tomatoes, capsicums/peppers, eggplants and cucumber were all really productive under shade cloth, provided they're kept well watered. It's too hot for lettuces and most other greens, however beetroot and turnip greens grow exceptionally well in the heat even though their root vegetables ultimately fail.
Also productive were sweet potatoes, chokos/chayote, malabar spinach.
I don't think it's the actual temperature in and of itself that you need to be worried about, as to whether or not plants can survive in it. For instance, in some areas, people go on about how nothing lives through the summer once it gets beyond temperature x. In other areas, it's considered the peak of the growing season when it's temperature x, and the same kinds of plants seem to thrive. I don't know of any plants that die just because it's 40° C. (104° F.) It seems a combination of factors is necessary.
Anyway, if you can figure out your Sunset climate zone (if it's been charted for your area), that should give you a lot more information than just how hot it gets. Other important factors include these:
- Night-time temperatures
- Daylight hours (the north tends to have more of those during the growing season than the south—hence such as long-day and short-day onions; long-day onions are for the north, they say, and short-day for the south).
- Your soil (soil can make a difference as to how plants act in the heat; by soil, I also mean the nutrient balance; some important factors with soil include how well it holds water, how quickly it drains, the pH, if it's high or low in minerals and which ones, if and how you fertilize/mulch/amend, etc.)
- What you're growing (this seems to be what you asked about—I'll get to that)
- Wind (and/or the lack thereof)
- Pests/diseases (temperature may favor and/or kill certain pests and diseases—which may contribute to why plants die when it gets to a certain temperature in some areas)
- The UV index
- Precipitation levels and/or drought (this isn't at all the same thing as humidity/aridity, whether or not it's correlated)
- Soil life (microbes, worms, good/bad nematodes, grubs, etc.)
- Whether the heat is inside or outside of a greenhouse
I'm sure there are many other factors. Now to your question. It is true that some plants seem to handle the heat (and/or heat and the balance of conditions that come with heat in specific areas) better than others. It's not all about the species of plant, though. Different breeds within a species can be adapted to different climates. A heat-tolerant plant in the south isn't always a heat-tolerant plant in the north and vice versa. Some plants are adapted to humid heat, and some are adapted to arid heat. Some are adapted to heat with drought and some to heat with wetness. Some may do very well in some soils, with some watering methods, in the heat, and do very poorly in others (even in the same or similar climates).
Acclimatization is also an important factor for plants. If they've situated themselves to your growing conditions, you'll probably get much better results than if they're still pretty new to them. How many generations they've been growing in them from seed is also a factor in acclimatization.
If you have unusual or harsh growing conditions, then the best crops for them just may be locally-bred landraces (if they exist). Landraces aren't terribly difficult to create, but as with most kinds of breeding, it takes some years. Landraces not only take advantage of acclimatization via seed-saving, but they also have a genetic advantage, due to mass hybridization between loads of plant breeds, and selection for the plants that do the best.
Joseph Lofthouse is probably the most popular person who breeds landraces, these days. His website talks about it a lot. I've gleaned ideas from landrace gardening, and am using them to get better performance out of watermelons in my garden (which doesn't have stereotypically ideal conditions for growing most watermelons—though it's great for the Red-seeded Citron). Most people seem to change their garden to suit their plants, but breeding plants to suit your garden is also possible (and although it may take more effort with less results to start with, in the end, I think you can get better results, especially as you can still change the garden to better suit the plants after the fact, so long as the plants haven't been bred to mind—there are tomatoes that don't seem to like to be watered much, for instance: e.g. Punta Banda and Porter).
How you water can make a big difference in how plants grow in your garden (especially in heat, aridity and strong sun). In my area, with my water (maybe not yours), showering plants (foliage and all) and a broad area of the soil really makes them grow faster, bigger and healthier, whereas just watering the soil at the base of the plant and keeping the foliage dry doesn't have the same benefits. In another area, with different water, that might lead to fungal diseases, or mineral spots causing damage to your foliage, but with mine, it actually seems to ward off disease (probably because it seems to keep the aphids and whiteflies, which can spread disease, away—they don't like to be showered). Most people seem to prefer drip irrigation, though, as it doesn't get the leaves wet, or splash dirt onto them (which dirt might spread disease to the foliage).
If you don't have landraces (or even if you do), I have some recommendations for plant breeds, but I would want to know where you live in order to tailor them more to your needs. Note, for instance, that if I list cucumber breeds, I'm not recommending cucumbers generally for your area (I'm only recommending specific breeds of cucumbers, and then only if you want to grow cucumbers). You might try the following:
- Beit Alpha
- Marketmore 76
- Dark Star zucchini
Squash generally do well in the heat, although bugs or diseases may cause issues. You might try C. argyrosperma and C. moschata squash for increased insect resistance. The one cool-weather squash I know about is C. ficifolia (which grows faster when it's cool, and starts to fruit when the days get shorter), but most kinds seem to be warm-weather squash.
Watermelons and cantaloupes also tend to like the heat. However, the other factors I mentioned above may greatly influence the outcome. A lot of people with tough growing conditions recommend Yellow Doll F1 watermelon. I like Verona quite a bit, but Sugar Baby is what most people around here grow. I think focusing on acclimatization is more important with watermelon than focusing on the variety, though.
Tomatoes and eggplants: (We have a whole question dedicated to tomatoes and eggplants, although it probably needs to be updated, considering things I've learned since then, some of which I've mentioned here. Nevertheless, I'll recommend a few of my favorites. Note that my area is a northern, semi-arid region, with very hot summers and cold to very cold winters.)
- Early Girl F1
- Creole (this one likes humid heat, but I still enjoy it)
You also might try SunGold F1 and SunSugar F1. I haven't grown those, but they're generally regarded as vigorous, very sweet, prolific, and heat-tolerant.
I also recommend Aswad eggplant. I grew it in 2016 with great results.
Okra is supposed to be pretty great for the heat. Most okra is likely more adapted to southern heat than northern heat, though. However, it can flourish in the north, too (but it probably depends a lot on your soil and how you grow it).
Ground cherries seem to do pretty well in the heat, as long as your pH isn't too high and you don't transplant them too late in the season.
Some peppers do better in some kinds of hot conditions than other peppers do. However, I think how you grow them is more important than the variety, for the most part. I find that peppers often do better in containers for me (with the same soil as those in the ground; although some kind of mix free of regular garden soil is supposed to be best, to avoid diseases and allow for better drainage, it hasn't worked the best for me, so far; so, this is why I use amended garden soil; you might want to use a mix, though). Bell-types like California Wonder seem to like the ground, though. Moving totes seem to work a lot better for peppers than 5-gallon buckets, in my experience (even with two plants in the moving tote and one plant in the bucket). You'll probably have better results with smallish, productive peppers than with giant peppers (although I would seriously recommend checking out Gypsy F1 and Neapolitan for productive, large peppers; I'm not sure how well they do in the heat, though, but productivity is generally a good sign).
Amaranth seems to do well in the heat (and in drought).
You might try growing sorghum. It's supposed to be hardy, and it has more than one use. You can harvest grain. If you get sweet sorghum, you can make syrup from it. You can makes brooms from sorghum (I'm not sure what kind of sorghum you'd need, if it matters). I have Mennonite Sorghum to try out this year (for both grain and the sweet juice).
If you research it, you can find lots of plant breeds that are supposed to be heat-tolerant. That doesn't necessarily mean they're heat-tolerant in your specific growing conditions, but the odds may often be improved over those that don't mention it. However, I've found that all the heat-tolerant kinds aren't always labeled as such. So, just because a description doesn't mention that, doesn't mean it's not heat-tolerant.