A Level Biology: The Lawn Looks Dead — So How Does Grass Come Back to Life After Drought?
During a long, dry spell, one of the most obvious changes in the landscape is the colour of the grass.
Green lawns become yellow. Yellow becomes brown. Eventually, the grass can look almost like straw.
Walk across it and it may crunch under your feet.
It certainly looks dead.
And yet experience tells us something rather remarkable.
Give it enough rain and, within days, patches of green begin appearing. Soon the lawn starts growing again and, after a few weeks, it can be difficult to believe that it ever looked dead at all.
So what is happening?
Grass has not somehow been resurrected.
Instead, we are seeing a very effective plant survival strategy: drought dormancy.
For an A Level Biology student, that apparently uninteresting brown lawn is actually an excellent example of plant physiology, water potential, stomatal control, hormones, photosynthesis, respiration, meristems and natural selection all working together.
Brown Does Not Necessarily Mean Dead
The first important distinction is between a plant that is dead and one that has allowed part of itself to die back.
When drought becomes severe, many grasses stop trying to maintain all their leaves.
Keeping large quantities of leaf tissue alive is expensive.
Leaves lose water through transpiration. They also require water and nutrients to maintain their cells. If the plant continued growing normally during a prolonged drought, it could eventually lose so much water that its living tissues were permanently damaged.
Instead, the grass effectively changes priorities.
Growth slows.
Photosynthesis decreases.
Older leaves die.
The visible parts of the plant may become brown.
But crucial tissues near the base of the plant and below the soil surface can remain alive.
That is the secret of the lawn's recovery.
Grass Is Particularly Good at Surviving Damage
One reason grass can recover so effectively comes from where its growing points are located.
Many plants grow mainly from meristems near the ends of their shoots.
Grass is different.
It has important meristematic tissue close to ground level.
This makes evolutionary sense.
Grass has evolved in environments where it is regularly eaten by grazing animals, damaged by trampling, cut by fire or, much more recently, attacked by lawnmowers every weekend.
If all its important growing tissue were located at the top of the leaf, losing the leaf would be disastrous.
Instead, grasses can regenerate leaves from tissues close to the base.
That is why mowing a lawn does not normally kill the grass.
It is also one reason drought can destroy much of the visible leaf while leaving the plant capable of producing new growth later.
The Grass Detects That Water Is Becoming Scarce
Plants cannot decide to stop growing in the conscious sense, but their cells constantly respond to changes in their environment.
As soil dries, its water potential becomes more negative.
Water therefore becomes increasingly difficult for the roots to absorb.
Normally, water moves from the soil into root hair cells and then through the root towards the xylem because of differences in water potential.
During drought, that gradient becomes less favourable.
Eventually there may simply not be enough available soil water to replace the water being lost through transpiration.
The plant now has a problem.
If it keeps its stomata open, it may continue absorbing carbon dioxide for photosynthesis, but it will also continue losing water.
If it closes its stomata, it conserves water but restricts photosynthesis.
During severe drought, survival becomes more important than growth.
Abscisic Acid Helps Close the Stomata
One important plant hormone involved in the drought response is abscisic acid, usually abbreviated to ABA.
When water becomes scarce, ABA concentrations increase.
ABA affects the guard cells surrounding the stomata.
Changes in ion movement cause the guard cells to lose turgor.
The stomatal pore becomes smaller or closes.
This reduces transpiration.
From the plant's point of view this is extremely useful because it slows water loss.
But there is a price.
Carbon dioxide normally enters the leaf through the stomata.
Closing them therefore means less carbon dioxide is available for photosynthesis.
The familiar equation is:
carbon dioxide + water -> glucose + oxygen
or:
6CO2 + 6H2O -> C6H12O6
+ 6O2
Light energy and chlorophyll are required for the process.
If carbon dioxide uptake falls dramatically, the rate of photosynthesis also falls.
That means the plant cannot continue producing large amounts of glucose for growth.
Growth Becomes a Luxury
When conditions are good, a grass plant can invest energy and materials in producing new leaves, roots and reproductive structures.
During drought, that strategy changes.
Cell division decreases.
Cell expansion decreases.
Protein synthesis may be reduced.
New leaf growth slows or stops.
Photosynthesis falls.
The plant moves towards what could almost be described as a biological economy mode.
That makes sense.
Growing new leaves during a drought would create even more surface area from which water could evaporate.
Instead of investing resources in expansion, the plant concentrates on staying alive.
Why Do the Leaves Become Brown?
The green colour of a healthy lawn comes primarily from chlorophyll.
As prolonged drought damages or causes the controlled loss of leaf tissue, chlorophyll is broken down.
The leaves lose their green colour.
Eventually many of the exposed leaves dry out completely.
At this point they may genuinely be dead.
But that does not mean that the entire grass plant is dead.
The brown leaves we see are therefore somewhat misleading.
They are the disposable parts of the system.
The living crown, basal meristems, roots and other protected tissues can survive much longer.
That distinction is one of the most important parts of understanding drought survival in grass.
The Crown Is Crucial
Near the base of a grass plant is an area often described as the crown.
This region contains important growing tissues and connects the leaves with the roots.
Because it lies at or close to soil level, it is much better protected from extreme drying than the exposed leaf blades.
Some grass species may also spread through underground or surface structures such as rhizomes and stolons.
These can contain living buds capable of producing new shoots.
Consequently, even when the lawn above ground appears devastated, considerable living tissue may remain hidden.
This is why looking only at leaf colour is a poor way of deciding whether grass is actually dead.
The Roots Matter Too
Roots do more than simply hold a plant in the ground.
They provide access to the remaining soil water.
A well-established lawn may have a surprisingly extensive root system.
As the upper layers of soil dry, deeper roots can sometimes continue obtaining water from further below the surface.
Different grass species vary considerably in their drought tolerance.
Those capable of producing deeper or more extensive root systems are often better able to survive prolonged dry periods.
This gives us a very clear connection with natural selection.
In environments that regularly experience drought, plants possessing characteristics that improve drought survival are more likely to remain alive, reproduce and pass their alleles to the next generation.
Over many generations, drought-resistant characteristics can become increasingly common.
Osmotic Adjustment Can Help Cells Retain Water
Plants also have cellular mechanisms that can help them tolerate water shortage.
Some cells accumulate dissolved substances such as sugars, ions and certain organic molecules.
Increasing solute concentration lowers the water potential of the cell.
That can help the cell retain water and may allow water to continue entering from surrounding tissues.
At A Level, this connects nicely with the concept that:
Water moves from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
The detailed drought responses of different grass species vary, but the general principle is important.
Plants are not simply passive objects that dry out.
Their metabolism changes in response to environmental conditions.
What Happens to Respiration?
Even when photosynthesis becomes extremely limited, living cells still require energy.
That energy comes from respiration.
The general aerobic respiration equation is:
C6H12O6 + 6O2
-> 6CO2 + 6H2O + energy
The energy released is used to produce ATP.
During dormancy, the plant's metabolic rate is greatly reduced, so its energy requirements are much lower than during active growth.
Stored carbohydrates can therefore help keep essential cells alive.
It is rather like reducing the number of things running from a battery.
If energy supply is limited, you do not continue powering everything at full capacity.
You shut down non-essential processes and preserve enough energy to maintain critical systems.
Grass does something biologically similar.
Then the Rain Arrives
Eventually there is a proper period of rainfall.
Not simply a few drops that wet the surface, but enough water to begin soaking into the soil.
Suddenly the situation changes.
The water potential of the soil becomes less negative.
Roots can absorb water more readily.
Water enters root hair cells.
It moves across the root and into the xylem.
The xylem transports water upwards through the plant.
Cells that had lost some of their water regain turgor.
Guard cells can function normally again.
Stomata reopen.
Carbon dioxide can once again diffuse into the leaves.
Photosynthesis increases.
Respiration has a renewed supply of carbohydrate.
Meristematic cells divide.
Cells expand.
New leaves emerge from the base of the plant.
And the lawn starts turning green.
Why Can the Change Appear So Fast?
What always impresses me is how quickly this recovery can appear to happen.
After weeks of staring at a brown lawn, several days of decent rain can produce visible green growth.
That is because the plant is not starting again from a seed.
There is already a root system.
There are already living meristems.
There are already transport tissues.
There may already be buds waiting to grow.
The biological infrastructure has survived.
Once water is available again, the plant can reactivate it.
That makes recovery dramatically faster than replacing the entire lawn with newly germinating plants.
It Is Not Really the Old Brown Grass Turning Green Again
There is another useful misconception to clear up.
When a lawn becomes green again, the individual dead brown leaves are not necessarily repairing themselves and turning green.
Much of what we see is new growth emerging among the dead material.
New leaves contain functioning chloroplasts and chlorophyll.
As they elongate, the lawn gradually appears greener.
Meanwhile, the old dead material becomes less obvious and eventually decomposes or is removed by mowing.
So the apparent transformation from brown to green is largely the result of surviving tissues producing replacement leaves.
Why Watering Little and Often Is Not Always Ideal
This biology also has practical consequences.
Suppose someone lightly sprays a lawn every evening.
The upper few millimetres of soil may become wet, but very little water penetrates deeper.
That encourages roots to remain relatively close to the surface.
Those roots are particularly vulnerable when the surface dries.
A deeper watering, where appropriate and where local water restrictions allow it, can encourage water to penetrate further into the soil.
The biology therefore gives us an interesting general principle:
A plant's response depends not simply on whether water has been added, but where that water becomes available to its roots.
Of course, during drought conditions we should also question whether maintaining a perfectly green lawn is a sensible use of drinking-quality water at all.
A brown lawn is not necessarily a lawn that needs rescuing.
It may simply be doing what grass has evolved to do.
Not Every Brown Lawn Will Recover
There is, however, a limit to dormancy.
If drought is sufficiently severe or lasts for too long, the crown and roots themselves may die.
Once the meristems are dead, the plant cannot simply restart growth.
High temperatures can make the problem worse because they increase evaporation and can directly damage cells.
Soil type matters too.
Sandy soils tend to drain quickly and hold less available water than many clay-rich soils.
Grass species matter.
Root depth matters.
Shade matters.
Previous management matters.
And the length and severity of the drought matter.
Some lawns therefore recover almost completely, while others develop permanently dead patches.
A Brilliant Example of Structure and Function
For A Level Biology, grass survival provides an excellent example of the relationship between structure and function.
Consider just how many features contribute:
The stomata regulate gas exchange and water loss.
Guard cells control stomatal opening.
ABA contributes to the drought response.
Roots absorb water from the soil.
Root hairs increase the surface area for absorption.
Xylem transports water.
Meristems produce new cells.
The crown protects important growing tissue.
Stored carbohydrates provide an energy reserve.
Dormancy reduces metabolic demands.
All these features contribute to the plant's ability to survive environmental stress.
It Also Demonstrates a Biological Trade-Off
One of the most important ideas in biology is that organisms constantly face trade-offs.
For grass during drought, the trade-off is particularly clear.
Open stomata:
More carbon dioxide enters.
Photosynthesis can continue.
But water loss increases.
Closed stomata:
Water loss decreases.
But carbon dioxide uptake decreases.
Photosynthesis falls.
The plant cannot maximise both growth and water conservation simultaneously.
During drought, survival wins.
When water returns, growth can become the priority again.
Evolution Has Produced a Remarkably Resilient Plant
It is easy to dismiss grass because it is so familiar.
Yet grasses are among the most successful groups of flowering plants on Earth.
They survive grazing.
They survive cutting.
They survive trampling.
Many tolerate fire.
Some tolerate flooding.
Others tolerate drought.
Humans depend heavily on members of the grass family too.
Wheat, rice, maize, barley, oats and rye are all grasses.
So understanding how the grass on a lawn responds to drought is not merely gardening trivia.
It introduces us to the biology of one of the most economically and ecologically important plant families on the planet.
A Simple Observation for Biology Students
The next prolonged dry period provides an opportunity for a simple observational investigation.
Choose one small area of lawn and photograph it from exactly the same position every few days.
Record:
soil moisture if you have a suitable probe;
air temperature;
recent rainfall;
percentage green cover;
average grass height;
and the date when significant rainfall eventually occurs.
Then continue recording as the lawn recovers.
You could produce a graph showing percentage green cover against time.
If weather data are available, rainfall could be plotted alongside it.
This turns a familiar environmental change into a useful biological investigation.
You could even compare:
shaded and exposed areas;
long and closely mown grass;
different soil types;
or lawns receiving different amounts of water.
Care should be taken not to waste water simply to conduct the experiment.
The Lawn Is Telling a Biological Story
I think this is what makes everyday biology so interesting.
We can walk past a brown lawn and simply think:
"It needs rain."
Or we can look more closely and realise that an extraordinary series of physiological processes is taking place.
Water potential is changing.
Hormones are signalling.
Stomata are closing.
Photosynthesis is slowing.
Metabolism is being reduced.
Leaves are being sacrificed.
Meristems are surviving.
Roots are searching the soil for water.
Stored resources are maintaining essential cells.
And the whole plant is effectively waiting for conditions to improve.
Conclusion: Brown Grass Is Often Grass Waiting for Better Times
A drought-stressed lawn looks lifeless because much of its visible leaf material may indeed be dead or dying.
But underneath that brown surface, the important parts of the plant can remain very much alive.
By reducing water loss, slowing its metabolism, protecting its meristems and relying on its roots and stored resources, grass can survive conditions in which active growth would be impossible.
Then the rain arrives.
Water enters the roots.
Cells regain their turgor.
Stomata reopen.
Photosynthesis increases.
Meristems become active.
New leaves grow.
The green lawn returns.
So the next time you see grass that looks more like straw than a living plant, remember that you may not be looking at death.
You may be looking at dormancy — one of nature's most effective survival strategies.

