Saturday, May 18, 2013

My Attempts to Grow Tomatoes in Arizona's Low Humidity

For the past several years I have been attempting to grow tomatoes in Arizona's desert climate, and I have been very successful in establishing healthy plants and setting fruit.  However, I have not been able to  grow sizable tomatoes which, I have to admit, puzzled me. Then I started to learn a little about hydroponic gardening and learned about the relationship between plants and humidity. Suddenly it dawned on me, to grow sizable tomatoes  in Arizona's dry climate, I have to find the right plant to humidity relationship.  In other words, I have either find a way to elevate the humidity in my garden or find tomato plants that will thrive in lower humidity.

Tomato plants grow well when the humidity is above 45 percent.  The humidity in Phoenix has been around 10-20 percent since about the first week in April which is right about the time the fruits began to set.  Once the humidity goes below 45 percent the plants automatically pump more water through the leaves to cool themselves; this process is called transpiration. If the humidity becomes too low, the plants begin to get stressed and if they can't transpire enough water,  their leaves begin to die.  Container tomatoes are especially susceptible to this because they cannot form a large enough root system to overcome this phenomenon.  In other words, while a large, bushy tomatoes plant with a a large amount of foliage may look great in February or March, when the warmer, drier air of April and May arrive, this same plant will struggle in a container.  The tomato fruits themselves do not receive enough moisture to gain any size before they ripen, and often the plant itself will die before this happens. This means that, in theory a plant with sparse foliage should do better in hot dry conditions and should also produce larger fruit. This idea has proven itself true in my container garden this year almost by accident!

In my February 21, 2013 post, "Container Garden Planting Spring 2013," I mentioned that I had a volunteer tomato plant which sprouted in my poinsettia plant.  I get quite a few of these every year because I compost my kitchen waste and use it to fertilize the potting soil.  Anyhow, to make a long story short, I ended up with a tall, spindly tomato plant with about half the leaves of a normal plant.  I really didn't think this plant was going to produce anything at all, but I was dead wrong.  Now, even with temperatures over 100 F, this plant is still thriving with 3 large tomatoes on it while the other plant has all but withered and died. I think this is because the plant had to transpire as much water and as a result, was able to produce larger tomatoes.  The tomatoes on this pant, although few, are about 3 times the size of the ones on the store-bought plant.  I am waiting to see what color they are once they ripen, if they are red I will call them poinsettia heirlooms and will save the seeds.  Either way though I will definitely grow them again next year.

Three Lovely Tomatoes from an Ugly Plant
Heavy Foliage equals small tomatoes in container gardening.
This spindly plant produced three good sized tomatoes
showing that less vegetation equals larger fruit.

My store bought tomato cannot take the heat and
low humidity.
On a side note, the chili peppers are doing quite well!

Speaking of next year, I will be doing away with the dozens of containers over the summer and building a single planter out of cedar.  My wife wants a functional and presentable patio so my goal is to build patio container garden that is pleasing to the eye and functional.  I also will be adding a few features to help with the humidity problem and will be adding a bird feeder to keep the insect population at bay.  In the meantime, here are a few tips to help with growing tomatoes in a hot/dry climate.  

  1. Select a variety suitable for a hot dry climate.  Look for a plant that has sparse vegetation.  These can be found at many local garden centers but you may have to ask.
  2. Tomatoes are sun lovers but in Arizona and other desert states, the sun and low humidity are just too much for them.  Use filtered sunlight by either incorporating a shades screen or planting under trees. 
  3. Set up a misting system if possible to help cool the plant and reduce the rate of transpiration.  Put it on a timer to run from sunrise to sunset.
  4. Prune away about 1/3 of the branches.  This may mean less tomatoes but it also means a lower rate of transpiration meaning larger tomatoes and a healthier plant.
  5. Start the plants indoors late in the year so they are blooming and setting fruit in early February when it is safe to set them out. This will help you to avoid the hot dry weather in late spring.
For more information on plants and humidity, you can visit Crop-King's article on relative humidity.  They go into great detail on the effects of humidity on tomatoes and other plants.   You can also visit ALL ABOUT TOMATOES for a bounty on info on growing, preserving, and cooking with tomatoes. I hope this information has been helpful and that you have much success in growing tomatoes and other plants. Until the next time, ~Happy Gardening~

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