Wednesday, 25 June 2014

Probing roots


Roots absorbing water and nutrients.

Any gardening encyclopedia will tell you that the function of roots is to absorb water and nutrients, provide anchorage and sometimes store food. The first two are my subject today.

Root spread
Years ago I would present my gardening article to the editor of the village magazine in my own ineligible hand. In a piece about roots I wrote that roots might penetrate down to as much as five feet. The editor, in her wisdom, printed ‘five inches’. Just what people think when they yank a plant out of the ground and imagine what they see is all there is!


Although this bog plant may have less extensive roots when growing near water, here in a drier part of my garden I expect its roots will go as deep as two meters
In suitable conditions it is not uncommon for herbaceous plants to have roots two meters down. Some desert-tree roots have been shown to penetrate down as far as fifty meters. It is well known that the horizontal spread of tree roots is frequently the height of the tree and sometimes much more. Perhaps not unsurprisingly people imagine that the bulk of a mature tree’s roots are deeper than they actually are. In exceptional circumstance they may be as shallow as 400mm but perhaps more routinely between one and two meters. The thing to appreciate about tree and shrub roots is the huge distances they grow horizontally.

Even in the severest of droughts the deep roots of my vine enable it to grow luxuriantly (too luxuriantly for my liking!)

Taking trees as an example, their woody roots are only the actual tip of the iceberg. Fine feeding roots develop in all directions and might penetrate deeply. The life of these delicate roots might be as little as a few days but the volume of soil reached may be quite enormous. Mycorrhizal links with fungal hyphae normally enable tree roots to exploit an even greater soil profile and to extract water and nutrients from every crack and crevice.
The root systems of large plants are huge because they need massive amounts of water and they need to explore a long way to find nutrients. I am not aware that they have any special mechanism to find nutrients, they just grow towards water and well aerated moist soil.

Perhaps I should mention that all the soil’s water is not completely available to plants. As the plant extracts water the remaining amounts are more tightly held by capillarity. A clay soil for example has an extremely high water holding capacity with a great deal of available water and yet as it dehydrates to the point that the plant can ‘suck out’ no more, it might retain almost 50% of its original capacity.

The bog iris has the capacity to grow in boggy soil and sometimes actually in the water. The floating aquatic plant’s roots are exclusively in water.

Although roots grow towards wet soil rather than to dry, typical plant roots do not survive in saturated conditions. Indeed in very wet weather when water tables rise to saturate the soil and displace oxygen, roots cease to function and soon die. This puzzles some gardeners as they observe roots of aquatic plants thriving in water! The difference is that open water contains a much greater amount of dissolved oxygen absorbed from the air or produced by the photosynthesis of aquatic plants. Soil water lacks sufficient dissolved oxygen although rain itself is well oxygenated but is rapidly depleted. Root death due to lack of oxygen is swifter when temperatures are high. Flooding is more serious in Summer but fortunately less common.
My scientist friend Peter reminds me of a more subtle point about root death due to waterlogging. Ethylene is a gas produced by plants as a natural hormone. Surplus cannot freely escape from waterlogged soil and the build up is toxic to plants and this prevents them from absorbing both water and nutrients. Think of this when the houseplant you left standing in water wilts and dies!

In heavy rain sub-surface water runs down the sandy field at the bottom of our garden. Look at the effect on the farmer’s wheat! In contrast see how the aquatic grasses in the natural drainage pond thrive!


The perched water table was extremely high in Winter 2012/2013 and only surface roots survived. A severe drought in Summer 2013 consequently caused very severe dieback. Now in 2014 our catalpa is starting to recover.

Uptake of nutrients
Any gardening book will tell you that plants require nitrogen, phosphorus and potassium and significant amounts of sulphur, calcium and magnesium. Plants also require smaller amounts of trace elements. These are copper, molybdenum, boron, zinc, manganese and very significantly iron. A few more are sometimes listed as some plants can use extra trace elements such as sodium, silicon and chlorine. In nature all plant nutrients are normally absorbed direct from the soil.

1. A farmer’s list of trace elements is longer than that of the gardener. Absorbed elements such as cobalt and selenium are essential to his livestock. Such nutrients passively absorbed by the plants are of interest to fruit and vegetable growers’ customers too.

2. Nutrients are only absorbed by the plant when they are dissolved in water. They are usually absorbed in very simple inorganic forms although dissolved complex molecules such as natural chelates supply nutrients that otherwise might be unavailable. The fact that systemic pesticides can be absorbed show that certain manufactured chemicals can be taken up too. With the aid of mycorrhiza organic molecules are also exchanged.

3. It used to be thought that absorption of nutrients was exclusively passive. In fact plants have a measure of control over the nutrients they absorb as long as they are available in soluble form.

4. Too few nutrients lead to plant nutrient deficiencies. Too much of certain elements can be toxic to plants and  much more rarely to consumers too. Toxicity might occur if plants are grown on polluted ground.


Some interesting facts from the recent scientific press.

Toxicity from industrial pollution, especially in the form of heavy metals such as nickel and lead, can render soil completely inhospitable to normal plants. Fortunately the pool of genetic variability in populations of native plants can mean that natural selection within surprisingly few generations can lead to resistant forms. Peter Williams showed me remarkable data showing how natural vegetation down-wind from former lead mines in Wales grows healthily, whereas exactly the same species transferred to the polluted ground from elsewhere die.

Patents regarding exploiting plants ability to extract heavy metals through their roots are about to run out and release for commercial use this exciting technology. There is potential to grow certain plants that have an astounding capacity to take up heavy metals and if cropped and disposed of can be used to clean up polluted sites. A variation of this technology is to grow such plants in mineral rich soils, to harvest the plants and extract useful metals such as nickel from them. A new kind of mining! Perhaps not a good thing?

Another remarkable fact is that many plants have the ability to passively absorb soluble metal ions and thereby indicate the presence of metals below. Apparently eucalyptus trees have directed miners to nickel deposits one hundred meters down. Don’t ask me if the roots get so deep!

How knowledge of root action can be helpful to the gardener

1. The volume of soil in a container is tiny compared to what roots can find in the ground. Apart from considerations of water retentive compost and enriched nutrient supply, the frequency of watering needs to be more often than for the same plants in the ground. Except in really wet periods, the amount of water supplied by rain is not nearly enough. Last year I wrote how my bougainvillea in my conservatory needed oodles of water and was hyper-sensitive to drought. In a post about Madeira, luxuriant bougainvillea scrambled over the cliffs for many months without rain!

Where watering is skilled, frequent and includes liquid feeding it is actually quite amazing how healthy large plants will grow in very small pots.  Growing in excessively small pots is not recommended!

2. When new plants are planted into dry ground, establishment watering might be needed. Once root growth is underway plants will frequently not need watering ever again. Other than generous watering in, I almost never water my own vegetable garden.

3. It is important to avoid severe compaction such as rotavator pans that might prevent roots penetrating deeply.

4. Too frequent watering encourages roots to grow at the surface rather than deeply. This is fine as long as watering continues but it is a heck of a bind to water bedding plants every day. My bedding plants in the open ground, once established, are almost never watered.

5. Because deep roots of perennials might die in waterlogged ground, especially in winter, surface roots surviving in more oxygenated surface soil should not be cut away by digging!

6. Because nutrients might leach out of containers, extra nutrition such as liquid feeding is usually needed whereas the same plants in the ground may need no fertilizer at all.

7. Beware planting delicate plants near where aggressive tree or hedge roots roots will severely dehydrate the ground. Sometimes in such conditions planting plants in a tub might be a good tactic. This keeps any watering exclusive to your plants and shade from the trees will reduce evaporation.

In the Summer drought of 2013 our Pagoda dogwood tree suddenly showed extreme stress. We needed to leave the hose pipe running for more than an hour over about four square meters to save it!

 8. Sometimes newly planted trees and shrubs will thrive for many years as they grow. As they get bigger their water demand hugely increases and they compete with each other for limited supplies of water. Roots can explore extensively but if they eventually reach already dehydrated soil or depleted water reserves under tarmac or buildings they will suffer in dry periods. Often the weakest trees or shrubs will die.

9. Sometimes an established plant might die for no apparent reason. A very rare occurrence that I have sometimes witnessed and once personally experienced was something toxic buried deep in the ground. Most of us do not know the long term history of our site. Goodness knows what might have been buried down there and roots can go very deep.
Oh dear, I have found another reason for digging!

The roots of the royal fern do not have far to go to find water

The large fern in a large pot has a good reserve of water…..The osmunda must not dry out…. The gesneriad will lose water very quickly when it is a metre high….The Scots flames must not be watered if still very wet….
All to be considered when I go round with my hosepipe!

Previous posts that might be relevant.


12 comments:

  1. Very interesting, I've never read so much about roots. Many times weeding the garden I look at the weeds' roots trying to 'know' them or understand their nature and behaviour (if roots could have behaviour). I noticed many interesting facts, for example, that roots of couch grass have a spike-like ending for easier penetration of the soil, I once saw that the root went through a bulb of potato.

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    1. I'm another weed watcher. I was interested that my neighbour's horsetail/marestail was restricted by a retaining wall which was about 4' high, presumably with some foundations so perhaps a 6' barrier in all. On the top of the terrace it's rampant, but no sign of it below the terrace. Considering it's roots are supposed to go to Australia, why hadn't it managed to escape to the lower ground? Either it doesn't go as deep as legend would have it, or it spreads by relatively shallow side shoots.

      I haven't yet managed to do anything useful with this observation, as digging 6' trenches to contain my own horsetail seems a little bit extreme.

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    2. I think Dewfruit that roots do have behaviour as you suggest! For example if they meet compacted soil they undergo changes in their character.
      I am glad you have taken my description 'probing' further to suggest how roots might penetrate into the soil. Apparently if the root tip is smaller than the compacted space it cannot go further- although in the long term wetting and drying may give it an opportunity. It is amazing how strong the pressures roots can exert if they do penetrate and we have all witnessed heaving of hard surfaces as roots expand.
      I am going to be rather pedantic and point out that the underground rootlike structures of couch are actually underground stems (the actual roots grow from them). I do not do this to be a clever clogs lecturer but it actually raises another interesting point. Tips of stems can exert particularly strong pressures,think of bamboos- but I had better not go into the gory details!

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    3. Reply to Sarah
      I have known marestail pass under one foot foundations of walls, but am not sure it will go deeper, although I suspect it will. I have no doubt the roots do go as much as six foot down (deeper anyone?). Your observation that it might spread by relatively shallow side shoots sounds plausible.
      It would be interesting to here of anyone else's experience of this weed.

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  2. Very interesting. Here I have to have raised bed because the bed rock is very close to the surface and in winter many perennials would drown is they were not raised. In fact in spots where the bedrock is only a few inches from the surface, almost only grasses grow. There are trees in these spots but they are all aligned. We have realized that they are aligned because they can only survive when they grow over cracks in the bedrock. Looking at where the trees are tells you where the cracks are. Bulbs do very well flooded in winter and spring and baked in summer.

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    1. You always have an interesting angle on these things Alain with you own very challenging site

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  3. Didn't your editor ever try to remove a dock?

    What about foliar feeds? Are they as effective as those directed to the roots?

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  4. Thanks for the question Sue. It opens a number of interesting points! Foliar feeds are not necessary if a plant is well grown and has soluble nutrient at the roots. However sometimes nutrients are locked up in the soil, often due to extreme acidity or alkalinity, but other factors such as soil temperature can be involved. Nutrients in the form of chelates/sequestrenes for example can be quickly absorbed by the foliage bypassing the roots (chelates are also root absorbed if watered on to the soil)
    Some foliar feeds consist of more complex chemicals such as amino acids, effectively making up for bad plant management or very poor growing conditions. Best to manage your plants well to get optimum results rather than hope for a fix!
    I liked your comment about the editor, I was confused at first because I thought of Cathi who used to make sure my prose was pristine. I also had a flutter that one of my pictures had shown a large dock!

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    1. Large dock i your garden - it wouldn't dare.

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  5. Very good article Roger. Very thorough. I do have a question. I learned in our Master Gardening class that when temperatures reach into the 90's F that the roots of plants stop taking up water from the soil. The problem being they transpire to much too fast. And rather than lose the water shut down the stomata. True?

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    1. I would think different plants might vary in such a response to and the critical temperatures might differ..
      I would rather put it differently that if anything inhibits water uptake to less than what they potentially might transpire, then such stress causes them to close their stomata. So very rapid transpiration at high temperatures might very well be in excess of what water they can find and transport. Similar closing of stomata might occur in strong wind. It’s interesting that in still conditions or a mild breeze plants are able to transpire more than when they have been induced to close their stomata by strong wind.
      If plants are subject to drought such stress similarly inhibits stomatal opening
      As you know Donna, if the stomata are closed this reduces gaseous exchange and inhibits photosynthesis - not a good thing for the gardener albeit life saving for the plant in terms of surviving drought.

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  6. Fascinating stuff as ever Roger, I think you ought to put your posts together to form a book. I was once told that the pressure exerted by a large tree to draw water and nutrients to its full height was enough to implode a glass milk bottle. Have you ever heard this and if so has it ever been verified?

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