Sunday, February 7, 2016

Drought Survivability Study Follow up

Just how much can we cut back on watering plants and still have a great looking landscape?  An experiment conducted summer of 2015 in San Antonio seeks to answer that question.  Last spring I wrote about the drought survivability study by Texas A&M Institute of Renewable Natural Resources in San Antonio to test popular landscape plants under various irrigation conditions over the summer.  When the test ended this past fall I went down to check out how some of my favorite plants had fared.

The project site is off the beaten track to say the least.  It's down that road a fair piece.

The welcoming committee was out.  Nice to know I was expected.

This house near the drought simulator features stucco over stone and seems to be quite old.

First, a quick recap and you can also read more here.  The test was designed to demonstrate plant tolerance for various levels of watering to determine how far water usage can be reduced without unacceptable loss of plant health.   Inputs from nurseries, landscapers, and the three cities involved in the study were used to select 100 different plants for the test, generally according to local popularity among consumers.  Four irrigation zones were established within the simulator.  After an initial watering in period to get all plants established, the plants in the different zones received varying amounts of hydration, including one section which received no water.  As summer approached, each plot received different amounts of water based on percentage of "evapotranspiration."  

Below is an overview of two of the four plots after a summer's worth of growth and two more are beyond the rolling roof.  This is a bit of a "dry" subject for a post since the gardens here are meant for research and not prime examples of local landscaping design.  The two plots shown below received the least amount of water.  

The shed roof moves automatically to cover the low water planted areas during rain events and control the amount of water received.  With limited rain the roof didn't move much at all during the test phase.

What looks like a donation can for the A&M folks is actually the rain sensor which closes the roof in the background over the plants when it rains.

This is the newer sensor.

Just to prove it was a very dry summer.

Drip emitters were used for the plants receiving water.

A research report based on technical evaluation is in the works, but I discovered some useful information by observation.  
The plot layout below shows the plant locations for a single quadrant, so all four quadrants were exactly the same.  Each species had four plants per quadrant shown in the guide for a total of 1600 plants in the study.  With this handy plant list for reference lets see what happened over the long hot summer.  

I'll take each of the four sections in order from driest to wettest and use my own decidedly non-scientific terminology for this post.


My primary interest in returning to the project was to see which plants survived without any water at all.  With another water rate increase for 2016, it's an incentive to save even more water in the garden.

Looking first at the No Water Zone, we see the plants have generally survived, probably better than expected.  Remember, this zone received no rain and no irrigation.  This is pretty much how my garden would have looked with no watering in the summer of 2015 and I'm suprised at how good some plants looked.  For instance, Chile Pequin in the foreground is still green, airy Pride of Barbados (center) looks great and probably bloomed shortly after this visit.  To the right and behind Pride of Barbados is Turk's Cap looking good too.

Purple fountaingrass (back center) looked outstanding.  Most ornamental grasses did quite well in this and all sections of the study.

Those four yellow plants (back right) are were boxwood.  Blue Grama Grass in the center looks pretty sad.  Blackfoot Daisy, a reliable drought-tolerant native is alive and blooming.

Silvery Cenizo or Texas Sage doing well, not a surprise.  The green plant to the right is Esperanza.

Turk's Cap proves it is a survivor yet again.  Happy in deep shade or sun, water or not, Turk's Cap is a great garden plant.  It's been hot out here, no trees and this part of town is consistently a few degrees warmer than where I live.

Dutch Iris in the foreground are singed by summer heat and sun.  Pale pink Guara to the left is blooming away with no water.  Viburnum didn't make it but Salvia 'Mystic Spires' looks great considering no water all summer.

  Henry Duelberg Salvia, Rock Rose, and especially Guara are blooming.  I've made a note to plant Guara now that the deer are not a factor in the back garden.  Overall the No Water Zone looks stunted but most plants survived.


Next I headed across the central pathway to the area which received a little more water or 20% of evapotranspiration.  For contrast, the No Water Zone is on the right and the Low Water Zone is on the left, where plants received minimal irrigation.

The low-water section looked surprisingly good considering our especially hot summer weather. I'd be quite happy with my garden plants looking this good at summer's end.  But of course not every plant did well.

Quite a contrast how well plants do with just a little irrigation.  Sabal Palms in the foreground look good.  Burford Holly looks crispy brown.

Guara receiving some water looked about the same as the one receiving no water.  Dutch Iris are much improved with limited watering.

Coneflowers bloomed and have gone to seed so this looks pretty typical.  Rosemary looks better than mine at home.

Gulf Muhly grasses look great with limited water.  The red flowers are Knockout Roses.  Belinda's Dream Rose was also in the study and did well though I don't have a photo.

Lindheimer's Muhly (center left) and Mexican Feather Grass (right) all looking good in the limited water section.  Gray Santolina is very happy back along the path.


The section receiving 40% of evapotranspiration looked good but only slightly better than the 20% section.  Of course, the key to the study is how individual plants fared with varying amounts of water.  Pink Rock Rose, Blue Plumbago, and others are blooming nicely.

Dry and heat-loving esperanza (yellow flowers) looks even better with a little water.

Fountain Grasses are standouts here.


Of course the 60% of evapotranspiration section looked the best although there were a few surprises. Tropical Milkweed, Gulf Muhly, and Martha Gonzales roses all doing beautifully.  The milkweed was even hosting Monarch butterflies.  Gregg's Mistflower doing great on the right, it performs well with or without irrigation.

The A&M team inspecting the irrigation system.

It all looks good, I'd be interested to know just how much water was applied here.

One of the surprises was how good the Blackfoot Daisy (low-growing white flowers) looks with irrigation since it generally does not like a lot of water.

It's a common saying among local gardeners that the "fastest way to kill Blackfoot Daisy is to water it."  Yet here it is looking great in the maximum water section.  One caveat would be that the soil here is sandy compared to my rocky clay so drainage is the key.

Bat-faced cuphea, one of my favorites, looks great with all that irrigation while the Guara (pink flowers, far right) which survived with no water looks good, but not that much better than in the other sections.  

Yellow Flowering Senna, bright red Firebush and Mexican Bush Sage all did well with irrigation though one of the Rosemary plants looks similar to several of mine which succumbed to too much spring rain.

This post is simply an overview of my observations from that day.  I'd also enjoy seeing the details on just how much irrigation was delivered to compensate for evapotranspiration.  I took tons of photos which will be useful references when deciding what to plant in the future.  The second phase of the study irrigated the plants which received the least water to see how they responded to fall rains.  I'm looking forward to the formal Texas A&M results of the study detailing each plant and learning more about just how much (or how little) water my plants need to look good in the garden.


  1. Fascinating write-up! Thank you for sharing. It is definitely a testament to drought tolerant least those that are established.

    1. The establishment period was essential and something I learned more about by following this experiment.

  2. Fascinating! Thanks for sharing your observations about the survivability of these plants under various conditions. Some (Coneflowers, Dutch Iris, Rosemary) are plants we grow here. As I add plants now, I'm attempting to add mainly native plants that survive and thrive in the extremes--drought & wet conditions, and very hot summers & very cold winters. I also enjoy visiting our local UW-Madison agricultural research stations, to discover which cultivars of raspberries perform best, for example, and to see native milkweeds growing in different soil and light conditions. Great post!

    1. Our agricultural extension services are the best resources for what to grow locally.

  3. Thanks for the follow-up, Shirley! The results are very interesting. I was particularly surprised at how well the Salvia 'Mystic Spires' did - I wouldn't have expect that based on their performance in my own garden but perhaps your average humidity levels are higher (or maybe mine get too much water!).

    1. I was surprised by that too since its cousin Indigo Spires goes dormant in the heat of summer. We are more humid than your area. The best blue salvia for hot and dry is Henry Duelberg which also did well in the test.

  4. Shirley,
    Can we assume that all these plants were newly planted at the time the experiment began last year?

    1. Yes, the links in my post will give details on how the plants looked at the beginning of the study.

  5. Amazing - thanks so much for sharing this!

    1. The official report will tell us so much more.

  6. A great post, Shirley and thanks for the update on the study. Interestingly, you mention that the Belinda's Dream Rose survived well. I pulled mine out this passed year because it struggled with my lack of watering. In my gardens, if a plant can't make it with minimal care, it goes. Perhaps soil conditions play a role as well. As for the other tough plants you mention in the post, I'm not at all surprised that they fared well.

    1. There are so many factors. Belinda's Dream struggled for a couple of years here but is quite happy now.

  7. This is such great information, Shirley. Thank you for sharing it in such detail. I'm taking notes! One thing that struck me is that a lot of these sun lovers look better in the no-irrigation plots than they do if you try to grow them in less than full sun. There's no cheating on sun levels, and if you plant in too much shade (a mistake I've made many a time), they'll fail to thrive. Also, I notice that the natives, not surprisingly, look better than the drought-tolerant non-natives, like boxwood, in the no-irrigation plot.

    Now I wish they'd do another test plot for shade or part-shade loving plants!

    1. There's zero shade here, so nowhere to hide for these plants. Most non-natives did poorly and it's not a surprise. I'll post again when the study is published.

  8. So interesting Shirley! Hope Calvin Finch comes back to Gardening Volunteers of South Texas when the study ends to give us all of the details. Just shows that most of us over water our plants needlessly!

    1. That would be great if we could have a class on the results, then we could ask questions.

  9. This was fascinating, wasn't it?! I was most surprised by the gulf muhly with zero water looking so good when I was there. And I have to say, I loved the nandina shriveled to the ground in the no water and the lowest water plots!

    1. Yes, both gulf muhly and Lindheimer's looked great. The key is getting them off to a good start each spring.

      Yep, those toasty nandina were interesting, but the sun might have had a lot to do with it as I have some that haven't been watered in 20 years.

  10. Green plant is Esperanza. Hope. I'd have to plant that one if I lived in Texas.

    I don't water plumbago or Dutch iris, but my climate is mediterranean so a little kinder than yours.

    1. Esperanza is a commercial selection of a native plant and a wonderful name for such an excellent performer in our climate. Plumbago is an excellent plant here too and I don't water it much though it seems happy either way.


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