Science in Action: Hydraulic Lift Little Understood in Plants

Deep tree roots are important for large trees. We know that these roots help to stabilize large trees, keeping them anchored during high winds, and we know that it is important to get deep rooting established in landscape trees when transplanted into urban residential environments. Early research at the University of Arizona traced deep roots of native mesquite to depths below 200 feet. More recent research suggests that these deep roots are important for other reasons.

Sinker roots of mesquite not irrigated but close to a river

            Global research which surveyed maximum rooting depth of plants (trees, shrubs, perennial grasses) in natural settings (290 observations, 253 species) found a range in average maximum rooting depths from one foot (plants growing in northern tundra regions) to over 200 feet in deserts like the Kalahari; 194 species had roots at least six feet deep, 50 species had roots fifteen feet or more and 22 species had roots more than 30 feet deep.

When the researchers grouped these plants by similar natural habitats, they found the average maximum rooting depth to be 6 feet for cropland, 30 feet for deserts, 12 feet for conifer forests, and nine feet for deciduous forests. When plants were again regrouped into three groups based upon growth habit, then trees had an average maximum depth of 20 feet, shrubs 15 feet and herbaceous plants (nonwoody) 7.5 feet. This research showed that deep rooting is quite common in woody and herbaceous species in natural habitats, far deeper than the traditional view held up until now.

            Deep rooting is suspected, and research supports it, to be more important than just structurally anchoring plants in the landscape. Research supports that they could be very important for moving and releasing nutrients and water, both up and down, and redistributing water and nutrients among different soil profiles. Water movement up roots into drier surface soils may affect water use estimates trees and other plants growing in their vicinity.

            Many woody plants utilize deep roots for water uptake, particularly when surface soils are dry, but how they do this is not well understood. It was thought to be a combination of water “pulled” up through the tree by evapotranspiration and capillary action (like a soda straw) and little understood process called “root pressure” (like a submersible pump). Measuring water moved from deep in the soil by roots has always been difficult without disturbing the roots and accessing these roots.

Roots of irrigated fruit tree pulled from orchard

            However a plant process for moving water deep in the soil profile to upper soil profiles through plant roots has been identified more recently. Research found that during periods without rain, upward flow through deep roots was continuous during both day and night using a plant process researchers call hydraulic lift. Researchers identified that this process contributed up to 20% of daily water movement from that depth with no evidence of nighttime transpiration and no water storage inside the plant.

Research done in Texas on tree roots of two native trees found that roots growing at 20 to 60 feet below the soil surface contributed 20 – 50% of daily transpiration, depending on the water content of surface soils. As surface soils dried, more water was taken from deeper sources. All of this water from deeps soils was attributed to the plant’s hydraulic lift. Large quantities of this water are lifted at night. When hydraulic lift occurs at night then it is termed nocturnal hydraulic lift.

            The question then becomes, if available water is moved from deep sources through deep roots during nighttime, when the plant is not transpiring for transpiration the next day, then where is this water stored? Other research indicates that water lifted from the deep soil profile is redistributed to dry, shallower soils where it is stored and used in the future. Deep root water transport varies with changes in the environment. When shallow soils become wetted again due to rain and/or the plant’s need for water decreases, hydraulic lift stops or is reduced dramatically.

Hydraulic lift is the passive movement of water from roots where water is more available to roots or root compartments where the soil is drier. It does not require plant energy. While the majority of documented cases for hydraulic lift are in native plants in desert or arid climates, recent studies (such as those in the Northeast with Sugar Maple) indicate that hydraulic lift is not restricted to desert or arid species or regions.

Release of water into the upper soil layers has been shown to benefit plants neighboring roots responsible for hydraulic lift. Because soils tend to dry from upper soil profiles downward and nutrients are usually more plentiful in the upper soil layers, lifted water may provide moisture to dry surface soils and enhance mineral uptake, beneficial microorganism growth such as mycorrhizae, and uptake of nutrients by feeder roots which typically occupy shallow soils. Some researchers feel that this is a form of plant parasitism and may have been the primary selective force in the evolution of this process. Hydraulic lift may also prolong or enhance root hair activity by keeping them hydrated.

The direction of water movement in deep roots may be upward, downward or horizontal depending on where soil moisture is more limiting. The transfer of water downwards by root systems, from lets say roots growing in wet shallow soils to dry deep soils, has been termed downward siphoning or inverse hydraulic lift; the reverse of hydraulic lift.

Such downward movement through the root system may allow growth of roots in otherwise dry soil at greater depths, permitting more rapid establishment of some plants. The amounts of water stored deep in the soil are not likely to be significant contributions if plant drought is severe. However, downward transfer of water may be important to plant establishment and the reduction of waterlogging in certain soil types. Inverse hydraulic lift may facilitate root growth into deep soil layers and transfer water away from neighboring, shallower-rooted competitors.

In addition to hydraulic lift, where water is redistributed from moist depths to dry topsoil, or inverse hydraulic lift which transfers water downward, the process of "hydraulic redistribution" includes the transfer of water horizontally, from areas that are moist to areas which are drier. In some locations and at some times of the year the subsurface transfer of water through roots may actually represent more water than the amount needed for transpiration.

            How much water can this represent? Researchers aren’t really sure but field measurements of hydraulic lift in sugar maple in the Eastern U.S. have put these estimates as high as 25 gallons per night but other studies indicate much higher values. It is suspected that there is a great deal of competition for this water by neighboring plants.

Pomegranates Can Freeze Back in Southern Nevada

Q. We live in Pahrump and our pomegranate tree looks quite dead. We purchased her as a one gallon tree in May of 2008, and planted her immediately. She has grown quite well, to a height of almost 5 feet, and even produced a few fruit last year. But she does not seem to be coming out of her winter slumber. There are no signs of growth of any kind on any of her branches, and she has developed 3 suckers (which she never did before). Any suggestions or advice? We would hate to think we've lost her.

A. We also had some significant dieback to pomegranate at our orchard as well on some varieties of pomegranate.  We have 12 varieties growing there. This was not due, in my opinion, to unusually low winter temperatures but rather the timing of some winter freezes.  We had two bad ones come through Southern Nevada; one in the fall in November and the other one late spring.  I think the one in November did the most harm. 

To my knowledge no one has published the minimum temperature for pomegranate varieties. In a general sense, it is common knowledge that 10F (-12 C) is generally regarded as pomegranates minimum temperature without freezing but this is not correct. It varies among varieties, if the plant had an application of fertilizer late in the growing season or not and what the weather was like prior to freezing temperatures. Like any other plant, its ability to withstand winter freezing has a lot to do with what the fall or spring temperatures were like prior to hitting that 10F and how the plant was managed.

Most pomegranates are grown on their own roots and so hopefully you will see some suckering from the base of the plant particularly if it was mulched during the winter.  We also had some dieback to some figs which was unusual for us.

Can I Use Pine Cones and Needles in My Desert Landscape?

Q. 1 am wondering if you have any idea about pine cones and/or pine needles: could either be used in the garden/yard/ and/or in landscaping. We have rocks in the yard with huge pine trees. There is no end to the falling pine cones and the needles. We usually dump them out in the garbage. Is there any use for them? Could the cones and/or needles be used as ground cover around the tree trunks? Thank you in advance for your kind reply.

A. We use of lots of chipped pine trees in our orchard and your landscape plantings. For our desert soils, do not be concerned with postings that the needles are too acidic or cause other problems. We have never found this to be the case with our desert soils. Pine trees are used quite often in desert landscapes particularly surrounding golf courses. If the soil is wet enough, they will decompose and be a nice addition to our alkaline desert soils.

Regarding pine cones, we have never taken the time to shred or chip pine cones that come in with our mulch at the Orchard. They do look a bit funny when you see pine cones in the orchard surrounded by peach trees. Many people who are visiting the orchard asked where the pine cones come from since they are surrounded by peach trees. We explain to them that they come in with our wood mulch from local tree services but we do nothing on purpose to decompose them.

I cannot answer what the effect of these pine needles might have on already acidic soils but my guess would be not much if they are combined with other wood chips. There is some anecdotal information warning about extreme acidity with the use of pine needles as a mulch but I would not be afraid at all to use pine needles and pine cones mixed with your mulch. If you can shred the cones, they could be added to a compost pile.

Mesquite Does Not Normally Develop Extensive Shallow Roots

Pic of native mesquite I took in Jerez, Mex., showing sinker roots.

Q. I am a member of the Mesquite Club (est. 1911).  Our clubhouse is at 702 East St. Louis.  The patio concrete is fractured in many places and we are in the process of replacing it.  Inside the patio from St. Louis east side is a large mesquite tree (app. 50 yrs. Old).  The first concrete block is raised on the west side.  It has been suggested the tree is to blame and to remove it.  I spoke to concrete contractors and they felt the tree is not to blame. We have an underground stream in the St. Louis area that may have caused the problem.  The contractors, when removing the concrete, if roots are present, they will remove them.  A local nursery was contacted and said a mature mesquite tree’s root system would be the size of the tree’s canopy.  The tree is highly thought of and adds to our curb appeal.  Your advice would be appreciated. 

A. Nice to hear from you and from your historical club.  The roots of mesquite can be highly variable.  In the desert they have been traced down to over 150 feet in depth.  These are trees located close to a perennial rivers in the desert running through arroyos. 

Their root system is generally a three-tiered root system when grown naturally in the desert.  The first tier, or surface tier, utilizes water close to the surface after rains.  This is the easiest water for the mesquite to utilize and will expend very little energy to use it.  As this water disappears, deeper roots go to work in extracting water from greater depths where it still can be found. After extreme periods of drought, water may still be extracted from even greater depths. 

Dont remember the tree any more but not mesquite. surface roots.

When we grow mesquite in urban locations we seldom water them deeply.  Because of this, their roots tend to be a shallow and never develop their deep structure. 

It is possible the roots may be the problem but you will not know all until the cement is removed and the roots located.  Generally speaking I normally do not find tree roots to be a problem with lifting cement slabs or sidewalks if there is at least about 6 feet of open space between the tree and the concrete.  There are exceptions with mulberry being one of them due to its large number of surface roots. 

Desert trees normally don’t cause these problems but if the tree were watered with shallow irrigations and there was a lack of oxygen deeper in the soil, then the roots would tend to grow on the surface and be a problem.  I hope this helps. There are some possible alternatives that you might consider if you are keeping the tree.

Italian Cypress Dying in Same Spot After Three Attempts

Not the readers pic but Italian cypress dying next to each other

Q. I have a walkway about 5 feet wide, from the curb to the front door.  Approaching the entrance but not near the building, there's an Italian Cypress on either side of the walkway, simulating pillars.  One of them is lush and almost as tall as the 2 story house.  It has been in about 4 years. On the other side, the tree died after maybe 2 years in.  I watched it closely... how it died from the inside - nearest the trunk - out, and more so on the sunny side.  I had a tree expert out to access it... we tried Bayer systemic to no avail...  so we took it out and replaced it, and this time I had to get a more mature tree to match the other one. I simply can't believe it... the new one is dying to!  It barely learned to stand on its own (without its stake), and now it's dead!  There's something about the spot !  I'm amazed that whatever it is hasn't jumped over to the other tree!  It must be something there, in that spot! 

Please help me.  Do you know who and/or where I can send or take a sample of the soil from under the tree (when we take it out) and a sample of its roots so a scientist can #1, look at the soil under a microscope and

 Another problem thought to be associated with overwatering It cypress. Branch droop.

see if they see anything that's causing this; and #2, look at the roots and see if they detect some disease that 's causing this...  maybe that the trees are contracting from the soil?

My heart breaks when any  tree dies...  but these are so essential to my facade, and I love Cypresses; at Christmas I put twinkling lights on them,  ...they just bring me so much joy, and this is such an unimaginable freaky curse of a problem...   and now, I'm going to have to invest in a special ordered, really huge and lush tree to match the other one, and if I don't discover what's killing the tree in this particular spot, the new one will die too!  Imagine, if that happens, I'll have to give up and chop down the huge, beautiful one I've had from a little 5 or 6 footer 'cause the entrance looks dopey with one big Cypress on one side....  :(   !!!!!

Please don't think this has anything to do with spider mites...  I have learned all about spider mites...!  (Back about 1988 (in a different house) in the winter when you don't look at your plants that much, I finally noticed that my Cypress trees were completely embalmed in spider web!!!    ...I thoroughly washed them with dish soap and, of coarse, rinsed them well, and they survived...!  :))

When I noticed this one dying a few months ago, I washed it and washed it; I figured if it was something on the tree I might wash it away, and if it were something in the soil, maybe I would flush it away...  then I poked deep holes to give air to the roots...  it has just kept dying.

Thank you so, so much for anything you can do to help with this... 

A. Yes, it is possible there might be something in the soil remaining from the construction of the home.  If that is the case the it would be much cheaper to dig out the soil and replace it down to a depth of perhaps 2 to 3 feet in the planting area. 

To have a soil examined for an unknown problem would be terribly expensive and you would probably learn nothing.  So if you are this concerned there is something in the soil, then replace it. 

Let’s assume that your soil is fine.  If an entire plant dies then the problem is either in the soil, the roots, or lower trunk.  Then it might have something to do with how the plant is managed once it has been planted or even how it is planted.  I do think you could plant a smaller tree and through some pruning over the next couple of years they could become matched fairly well.

Planting.  Never allow Italian cypress to be planted deeper than it was in the container.  Don’t dig the hole deeper than you need to in case of settling.  If existing soil is piled around the trunk of the tree or even a wood mulch and kept moist it can rot the base of the tree causing what we call collar rot and very quickly die, usually in the heat of the summer.  When the tree is removed, the trunk of the tree at the location where it enters the soil will be brown and rotten.  This is not easily seen unless a trained eye is looking for it.

Once disease organisms causing collar rot have attacked plant roots and the lower trunk in a spot, the disease organism can be a big problem for plants put in the same spot. Some fungal disease organisms that cause collar rot (there are at least five) can be extremely virulant to the same or similar plants planted in the same hole.

Italian cypress can be very sensitive to soils which do not drain easily or are watered too frequently and the roots kept wet.  It might not be a bad idea to replace the soil and make sure that the soil being used drains freely after an irrigation.  This would eliminate that problem.  Any soil that you are bringing in should be amended with organic matter such as compost.

Irrigation.  Italian cypress likes to enjoy the wet winters and hot dry summers of a Mediterranean climate.  Water them deeply but infrequently.  Make sure the soil drains freely.  I would rather have them under-watered than watered too often. 

Make sure enough water is applied during an irrigation to wet the soil to a depth of 24 inches.  Do not water until the soil begins to become dry again.  During the winter time, you might want to water once every 10 days to two weeks.  Around February you would increase the frequency to once a week.  Around the first of May you might increase this to twice a week.  I do not believe you should water more frequently than twice a week, ever, unless these trees are planted in pure sand.  So, when irrigating, water with a large volume and then withhold watering until the next large volume of irrigation.

It does not sound like an insect or spider mite problem to me either but spider mites commonly become a problem during the heat of summer, not during cooler or winter months. Spider mites may or may not cause webbing AND there are spiders that cause webbing in cypress without causing problems... in fact, these spiders can help keep the cypress free of some bugs.

A better indicator of spider mite problems is a "dusty appearance" to the leaves or foliage. This may or may not be actual dirt or dust. When spider mites become a problem during the summer months they leave behind hundreds of thousands of dead spider mites which appear like dust or dirt on the foliage. Frequently washing helps but sometimes you have to resort to chemicals unfortunately.

Oleanders Not the Health Concern That Many People Think

Petite oleander growing in rock mulch. They do nicely in rock mulch.

Q. Saw your article in the Southwest View. My association has several petite oleanders. I like them and they are very pretty and hardy and never have to replace them. Some people with allergies and pets think they are very poisonous and bad for their allergies. Just how bad are they and will they kill pets????

A. I personally love oleanders. Yes, they are poisonous along with hundreds of other plants in our landscapes. If we tried to outlaw all the plants that are poisonous we would not have much left to use for landscaping. As far as animals eating the oleander leaves, I think this is a bit stretched. There are some reports in California of suspected animal poisonings from oleanders and it is probably not a good idea to have them close to livestock that eat landscape plants like any other poisionous plants.

Closeup of salmon colored flowers of this petite oleander.

Oleanders have been unfairly treated in my opinion. They do not cause heart problems due to their pollen as has been reported in anecdotes. I remember back in the 1980's there was a cardiologist in the Las Vegas area who tried to have them banned claiming that they caused heart problems. But this was an overzealous physician who based his opinions on his opinions, no facts were involved. I remember that we (the University) challenged his position with hard science and his proposal was turned down by the government officials that were approached about their possible ban. If this opinion is flairing up again then it needs to be challenged with research documentation that this has been proven in peer-reviewed research published in scientific journals. I doubt anything has changed.

Oleander pollen is heavy and sticky and not carried by wind any considerable distance at all compared to plants like olive, pine, mulberry and many, many others which we know do cause allergy problems. Poisonous, yes. Contribute to health problems? No. Prove it differently.