A conversation about Hydrophobia

by Lisa Nadler and Deb Pilkington NC State Extension Master Gardener ^SM volunteers of Durham County; and Dr. Ashley Troth, Durham County Extension Agent

Background

The Durham County Extension Master Gardener^SM (EMG) volunteers manage the container gardens at Cocoa Cinnamon Coffee Shop at 420 W Geer St. in downtown Durham. For the customers, this provides a beautiful and peaceful surrounding while they gather and drink coffee. For the EMG volunteers, this is a container garden that provides endless learning and teaching opportunities.

This past winter, our four Hardy Anise Shrubs (Illicium parviflorum ‘Florida Sunshine’) planted in the galvanized containers at the back of the seating area took a hard hit from the cold, freezing rain and shifts in temperature. Last year, they were beautiful and full with their lime green leaf color, but this year they were scraggly and tired looking, requiring pruning of dead material in the spring. They had also sunk in their containers as the soil compacted. As with many of our plantings, we used a bulk half and half mixture composed of Sand Hill soil and a mixture of aged pine bark and soil booster (a leaf compost and food waste mixture). We use soil in all of the larger containers because with this volume it’s much more affordable than bags of potting mix (which is not actually soil).

The Illicium in its prime after planting. The Illicium with winter cold damage prior to pruning and replanting (Image credit: Lisa Nadler; Deb Pilkington)

In June, we decided to take the Illicium out, add more soil to the containers and replant them in the same spots. We also planted some tiny plugs of a beautiful native groundcover in the orchid family called Rattlesnake Plantain (Goodyera pubescens). We watered everything vigorously.

Within a week, the Illicium were dead….Murder or natural death?

The Illicium aft replanting in June; The Illicium one week later. (Image credit: Lisa Nadler)

Below is part of the email discussion about what happened between two EMG volunteerss and our amazing Durham County Extension Agent, Ashley Troth.

Reviewing the Facts and Evaluating the Evidence

Over many emails, the EMG volunteers shared their thoughts and ideas about what they observed, and tried to understand what might have happened. The major takeaways at one week post planting were:

• “Wow, the Illicium looks awful – what happened? I’m scratching my head about this one. 
• The leaves look desiccated (dried out) and they are snapping, although some are still green. 
• Given that the Illicium in both containers appear to have universally wilted so quickly, I wonder if an abiotic mechanical or chemical causal agent involved. 
• Or were we over or under-watering? Looking at roots will be helpful. 
• Could something have been bad about the soil we brought in? 
• Could someone have poured some coffee or alcohol in the container?
• Were we too rough when we dug them up and replanted them?
• Let’s discuss this with Ashley to see if we can figure out the cause.”

It was especially confusing because the Goodyera planted all around them look fine. And we used the same soil in another container as well and the plants there looked fine.

Developing a Hypotheses Based on the Data

With the cause of the Illicium’s demise still a mystery, we asked Ashley to take a look at the plants and to see if there was anything we might be missing.

Ashley’s response after investigating the containers and the plants: I looked at the Illicium and I don’t think it can be resuscitated. I think the problem is the soil. I dug down about eight inches and removed a handful of the soil. It was dry and dusty. So even though you watered well, the likely culprit is the high organic content of the soil mix, which becomes ‘hydrophobic’ when dry. (According to the American Heritage Dictionary, that means ‘lacking an affinity for water; unable to absorb or be wetted by water’). The Goodyera likely survived because the top inch or two of the soil was wet. Also the Illicium can be especially sensitive to dry soil.

Hydrophobic soil repels water and is still dry after watering. (Video credit: Amber Trax)

“Often times these bulk soils are fine, but it’s been so dry, and more recently so hot that I suspect a lot of bulk mixes are just baking in the sun. A bagged potting mix would probably have been fine, because they’re often bagged with a certain moisture content, but a bulk mix, especially now, might need to be really rehydrated to be ready. All this to say, you picked a perfectly fine soil mix. It’s just gone through a drought (like all the rest of us), and needed extra care. The soil should be fine once it’s rehydrated.

Implementing a Plan Based on the Evidence

EMG volunteers: Thanks Ashley. Can you help us understand this better? What does it mean that a soil becomes “hydrophobic.” Is this something that happens often? How would we have known this?

Ashley: I will admit this is a bit of an art (and an observation game), but here’s a bit to get you started. What you have to remember about all of this is that organic matter (and clay) holds water really well – but only to a point. Beyond that, it dries out quickly and is very hard to rehydrate. This is the point where we call it “hydrophobic,” and we have to work really hard to get it rehydrated.

EMG volunteers: Okay, we’re sad but ready to move on. Can you be specific about how you “rehydrate” a soil mix? 

• Is this done once the soil is in the container, in buckets before adding to the container, in the bed of the truck that is carrying the load? 
• How long does it take to rehydrate?
• How do you know when it’s rehydrated?
• Doesn’t it just dehydrate again once in the container? Or is there a way to know that you are adequately watering to keep the soil from dehydrating again?”

Ashley:

• Where do we “rehydrate” the soil: Rehydration can realistically take days to a week, so it’s fair enough to do it in the containers after filling. Some water will come out the bottom, but if you’re adding a fair bit of water regularly (once or twice a day), you’ll see that as the week goes on when you dig down into the soil it’ll start being wet at deeper levels. You have to have a spade or something similar to actively be checking though, because for a while it will seem like the water is going in, but it’s really moving through the soil quickly and draining out the bottom. You’re just trying to constantly re-expose the organic matter to water until it’s able to finally take it up. 
• How long does it take to rehydrate: It could take days in small containers or pots, even if they’re sitting in trays of water. It could take closer to a week or even ten days in a large container or raised bed. 
• How do you know when it’s rehydrated? Dig down regularly (and as deeply as feasible) and really just get your hands in the soil. Even if it appears dark it might not be wet yet. It should hold together in a clump when squeezed and feel at least slightly damp to the touch. 
• Doesn’t it just dehydrate again once in the container? The soil once rehydrated will hold water well unless it’s left to dry out for so long that it drops past the lower level of hydrated and becomes hydrophobic again. Luckily, things like living roots and leaf cover help protect the soil once plants are actively growing in it, so this is often more of a problem with things like bags of potting soil that have been left open too long or bulk soil media that has been left to bake in the sun.”

EMG volunteers: Okay, that makes sense! So, it wasn’t murder although this could be classified as plant-slaughter. Just an unfortunate situation that led to their early demise. We are always learning something new! That’s why it’s so fun to be a Master Gardener!

So, here’s our new plan. We have removed the Illicium and added them to our backyard brush pile (with a few tears). We moved the Goodyera into small pots and will keep them alive until we replant. We’re going to leave the containers empty for the summer, with a good layer of mulch atop. With the drought and the heat, it’s just too much to consider replanting now that we are in early July. 

When we get into the fall, we’ll water the containers/soil daily for 7-10 days. Each day, we’ll dig down into the soil to see how far down the soil is hydrated. Once we’re convinced that the soil is rehydrated, THEN we add plants. 

Summary

What did we learn? That even though organic matter is so important for overall soil health, it does have a point at which it no longer will hold water. At that point it’s “hydrophobic” so even if you water it, the water basically “bounces” off and is not taken up by the soil. This would only happen in situations where the soil is in a very dry environment for an extended period of time. This could happen in a drought like we’re in now or if you left a bag of potting mix open in your garage for an extended time. Once the soil is hydrophobic, you need a serious effort to get it rehydrated, often watering the soil daily for many days (depending on the size of the pot/container). You need to dig down into the soil daily to tell when rehydration has occurred. Once the soil is wet (forms a clump when squeezed and feels wet) you’re finally ready to plant. If you add plants before that time, you risk killing your new plants because they will not get enough water.

Hope you have learned as much as we did!!!

Additional Reading and Resources

University of California Department of Natural Resources. If your house plants or container plants seem to repel water, submerge them in a bucket of water, soak them or slowly water them.

Mathew Huchteman at the University of Missouri Extension Office has great advice on the science, prevention and treatment of hydrophobic soil.

Shortlink: https://wp.me/p2nIr1-7KW

Editor: Martha Keehner Engelke