On Planting in Drought Conditions: the Relationship of Roots, Water and Soil

I had a novice gardening friend ask me the other day about planting the dry, xeric, part of her yard.  Many of you know how abnormally dry and warm a spring/June it’s been here.  Those of us with gardens requiring routine irrigation started a few weeks ago and we’re expected to be heading into an extended hot/dry period over the next 8 or 9 days with temps over 90 F. (While it is not unusual to experience 80+ deg. days here in June it is unusual when you look at our overall pattern this spring.  Remember that we can also have Junes where it is common not to get out of the 60’s with our famous Portland drizzle day after day while we wait for July and the ‘beginning’ of summer.)  She was anxious to get her new plants in the ground and was asking me about amendments as the soil was baked and hard….

Let me back up a little here in my advice to her.  Soil serves multiple functions for plants and, for terrestrial plants, those rooted into it, it is absolutely essential.  Their relationship is direct and intimate.  It serves the purpose of anchorage without which many plants would topple.  It also serves as the reservoir of water and nutrients upon which the plant is dependent.  It is the medium in which all of the biological process occur that together make plant growth possible.  It provides air at the level of the roots to provide for the gaseous exchange that is so necessary to the process of plant respiration while it goes about the process of building new tissues.  Their relationship is dynamic and organic.  There are pH requirements that, if not met, ‘lockup’ nutrients, making it difficult or impossible for the plant to draw away from the soil what it needs to grow and replace cells.  None of this can happen if there is not enough available moisture in the soil.  Water is the ‘vehicle’, the carrier, for all of the processes within the plant and each plant, has a genetic ability to draw that moisture from the soil.  Some are more efficient than others and can survive on much drier soils and have even adapted the way that they metabolize, taking different chemical pathways that require more or less water (See previous posting on CAM, C3 and C4 metabolic processing).  But even if you are planting very xeric, drought tolerant plants, they still require some level of moisture.  Without adequate moisture levels, plants that can will ‘shut down’ their metabolism, essentially waiting out the drought and each will vary in how long they can do this.  Those that can’t will continue growth until lack of water stops them.  These will begin to dessicate, shedding first their leaves and then their other softer/newer tissues, ultimately leading to the death of the plant.

Their ability to survive and grow under low water conditions is also related to how well established they are and the overall health and vigor, the condition, of the plant going into a drought stress situation.  By established, I mean, how extensive is their root system.  Have they extended out into available soil?  This volume of ‘penetrated’ soil is what is available to a particular plant.  It cannot pull from an area it’s roots don’t occupy.  Newly planted plants have a very restricted zone to draw water from.  In a sense it is much as if they were still in their pots.  But, of course, it is more complicated than that.

When we buy plants from the nursery/garden center the growers and retailers do their best to show them off.  Prior to our purchase they are grown under ideal conditions.  Every molecule of nitrogen is there to promote vigorous growth.  Water levels are scrupulously monitored.  Other nutrients are pushed to encourage flowering which can result in a much easier to sell plant.  And then we bring them home.  Any landscape will present a change to what the plant is ‘use to’.  There will be an adjustment period as the plant settles into its new conditions.  Any new growth is tender with respect to the mature growth on any given plant and because of this it is more liable to damage by various stresses.  A huge one can be drought stress.  These tissues are ‘soft’ and need to be ‘hardened’ off before unduly stressed, not unlike starting tomatoes under lights in your basement and then transitioning them to be planted outdoors.  Too harsh and quick of a change can result in the loss of all of that beautiful lush new growth and, if severe enough, the entire plant.  This makes planting out problematic in the summer time and no time is as harsh as we are experiencing now with a giant heat low settled in over Portland and dry baked ground.

When water enters the soil, whether via precipitation or irrigation, it generally starts at the surface and is pulled down through the profile by gravity.  The rate at which it does this is dependent on the moisture already in the soil and the soil’s ability to ‘hang on to’ moisture, the later of which is determined by the soil’s texture or particle size, the proportion of organic material in the soil and geo-morpholgy which includes things like physical barriers, e.g., compacted layers, rock.  As water moves down through the profile it passes soil particles, sand, loam and clay, passing aggregates of these, utilizing pore space between them and the various passageways provided by roots, decaying organic matter as well as fractures and ‘tunnels’ created by creatures great and tiny.  Water molecules possess two abilities/forces that make all of plant growth possible, they are both adhesive and cohesive.  They tend to stick to each other forming droplets, (like water beading on your waxed car, does anyone still do that?) and are drawn to other particles, like soil, and held there.  Smaller particles, like clay, in a given volume of soil, have more surface area and so can ‘attach’ to more water molecules.  This is why sandy soils are more quickly draining.  So, while gravity pulls it down through the soil profile, it spreads out wetting even more of it (this is why drip irrigation works!)  Water’s ability to continue moving through the soil is somewhat dependent on how wet or dry the soil already is.  The higher the soil’s water content, the lower the effect of adhesion to the soil particles and the deeper the water moves.  The lower it’s water content, the more likely the water will more strongly attach to soil and organic particles.  It is somewhere in between here where water is available to plants and can be pulled ‘osmotically’ from the soil particle into the root hair and on into the plant where it can be utilized.  Physically water acts much like a sponge, put too much into it and it will drain away, but when dry, a given volume of water must be applied before the sponge will release any.

Once their garden soil is dry, I’ve heard many people complain about how they water and water and everything is still dry.  Their plants are still wilted and perhaps dying in the heat.  This is because moisture has been reduced to levels below which their garden plants can draw moisture away.  None is available to them.  Keep in mind that other plants may still be doing relatively well with that level of soil moisture, but the message is that once soil is too dry for ‘your’ plants, you must raise it throughout your garden’s soil profile.  We are talking about its volume not just the surface.  (Many desert plants have evolved to take quick advantage of any rains that fall and may be very efficient at gathering moisture near the surface during monsoonal downpours.  Others may have deeply penetrating roots that seek out water at some depth.)  Raising your garden soil’s moisture content at this point will require more water than you might think…because you must not only provide the amount of water that your plant currently needs daily, but you need to bring your soil’s content up into the range in which it will be available to your plants.  By and large plants from temperate regions have evolved in climates that provide them with adequate soil moisture, in the upper profile, at the times of the year they require it.

Keep in mind that while your plants are out there enduring the heat, they are not doing so passively.  Even when not actively growing they are drawing water from the soil in a process known as ‘evapotranspiration’…they are essentially pumping water from the ground through their tissues and then into the air as they respirate.  Plants are alive.  They must continue their interior processes to maintain their structure and vitality, much like adult animals that are no longer growing in stature.  As temperatures rise, they use more water, losing it into the atmosphere.  Remember, this is one of the ways plants help cool the air around them making them more habitable for us.  The demand for water increases with the temperature.  Hot days, dry soil, means even more water.

There is another frustrating factor at work here, actually I’ve already mentioned it but in an other context.  Capillary action, that utilizes the adhesive and cohesive forces that characterize water, enabling plant growth, and the ‘sponginess’ of soil, also means that when you apply water to dry soil it is ‘pulled’ away from the point of application to drier adjacent soil.  So, that couple of gallons of water you give to your new tree in your baked parking strip goes to the tree and to the surrounding volume of soil pulling it ever ‘thinner’ until it ‘evens’ out.  The same happens when you plant into dry summer soils.  You may wet the hole, even the pot itself when you plant, but the surrounding soil will draw away moisture inexorably.  Your new plants can end up dry and suffering very quickly.  The answer, again, is to water the whole space to raise the soil moisture within the profile, or, to wait until the fall rains do it for us.  The cooling fall temperatures will further aid you by reducing the evapotranspiration rates of your new plants as well as those thirsty plants already established in your landscape.


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