There’s this pervasive idea that we don’t really need to irrigate vegetable crops here
in Ireland, and many years that’s true as we tend to get enough rain during the growing
season to make due.
The recent drought that we experienced was different though, and it caused me to shift
my focus or mindset, away from the notion of watering only when it's necessary, when
things seem too dry, and towards this idea of trying to maintain ideal conditions as
much as possible.
Ideally, plants should have access to plenty of water through the entire volume of soil
that their roots occupy.
Part of this is to make sure that the soil biology across the entire garden is not stressed,
especially near the surface, where a lot of activity can be.
Nutrient availability is a related factor, and seems to be even more critical in the
calcareous soils that I’m dealing with.
So, I’m starting to ask myself the questions of should I be watering much more in general,
and how do I know when to start watering, and how do I know how much is enough?
We usually get a lot of rain during the winter, but the soil can only hold on to so much water,
and once its full, any additional rain tends to wash through the soil out of reach or across
the surface, away from the gardens.
In the spring, a lot of water will start to evaporate from the surface of the soil, and
I may need to water some of the seedlings to keep them happy.
From late spring, and right through the summer, transpiration through the plants themselves
tends to dominate, especially once the plants form a closed canopy that completely shades
the soil.
In many cases, evaporation from the surface will only be responsible for a small portion
of the total amount of water that is extracted from the soil on a summers day.
We seem to have a reasonably good intuitive sense of evaporation, as we see things drying
out all the time, but I think that we have a very poor sense of transpiration, or of
understanding the volume of water that is sucked out of the soil by the plants.
And of the huge volume that is removed by the plants through their roots, less than
5% is actually used in the plants themselves, and the rest is released to the atmosphere.
But, it’s difficult to reduce this flow of water, as it’s an essential part of growth,
and the larger and healthier the plants are, and the more abundant the garden is, the more
transpiration will take place, and the faster the soil will dry out to the full extent of the roots.
So, most of the watering that is needed while growing vegetables in the summer, is to replace
the water that has been pulled out of the soil by the plants themselves through transpiration.
But, to figure out how much water would be good to add to the soil, it would be useful
to get a sense of how much may have been removed.
A formula has been developed to calculate the potential evapotranspiration rate, which
is a combination of the estimated evaporation and transpiration, based on the real world
measurements of the amount of sunshine, windspeed, the air temperature, the relative humidity,
and other factors.
Thankfully, all of this is already being worked out by professionals, based on data from a
weather station fairly close to the gardens, which gives a useful approximation of the
amount of water that is being removed from the soils of my gardens.
It’s interesting to compare the potential evapotranspiration rate that we would typically
get in a season, to the average rainfall.
It’s usually only in May, June and July that the amount of water that is pulled out
of the soil exceeds the amount of rainfall, and even then, it’s usually less than 12mm
over an entire month.
But 2018 has been really different, with a much higher potential evapotranspiration rate,
and substantially less rain.
So, we ended up with an estimated shortfall of 35 mm in May, 80 mm in June, 75 mm in July,
or a total of almost 200 mm shortfall over those 3 months.
Looking at this in more detail, showing weekly rainfall, I can assume that my garden soil
was at full moisture holding capacity right through until the end of April, as it was
a wet spring.
From that point onwards, the estimated evapotranspiration rate exceeded the actually rainfall we received.
The amount of moisture in the soil was probably fine up until near the end of May, but after
that, I expect that the soil moisture dropped low enough to start to have negative impacts
on plants and soil biology.
I started to water the gardens around the end of May, but I can see now that it wasn’t
enough, and the soils continued to dry out more and more up until late July.
It wasn’t until the last week of July that we got any decent quantities of rain, and
since then we’ve had a number of showers, but they don’t add up to the ongoing potential
losses through evapotranspiration.
So, getting back to the question of how much I should have watered, and with many important
questions, it’s useful to have a number of different ways to figure out an answer.
Direct observation is always a good method, and I often dig holes into the soil to see
how dry things are becoming, but it’s difficult to dig deep enough or close enough to the
plants without disturbing the roots.
Soil moisture sensors are also great and it’d be really cool to have a network of these
throughout my entire gardens.
But neither of these methods really give an indication of how much I should water.
Another approach would be to keep an eye on the potential evapotranspiration rate from
the local weather station.
And then to subtract any rainfall we did receive from this figure and to water the gardens
to make up the difference.
Over the 8 week hot and dry period from the end of May through to the end of the 3rd week
of July, the potential evapotranspiration totalled about 200 mm, but we only received
27 mm of rain in that same period.
So this works out to an average of of 21 mm a week, or about 3 mm a day, of watering I
should have done to make up for the water that was pulled out of the soil.
For a 100 square meter garden, this translates to 300 L per day, or 30 of the 10 L watering
cans that I use, each day for the entire 8 week period.
But the watering that I actually did would average out to only 120 L per day.
Using this method of trying to match the potential evapotranspiration, I realise now that I should
have added two and a half times the volume of water that I did during that dry period.
And I wonder how much better my gardens would have been if I’d done this, and would it
have been worth the effort, given that I’ve been restricted to watering mostly by hand this season.
There are a few issues I need to address if I’m going to adopt this approach of trying
to maintain really high moisture levels in the soil, the biggest of which is that I’m
still watering mostly by hand which takes a lot of work.
And I need to put in the effort to improve the infrastructure so that I can get better
pressure and water from a more suitable source for irrigation.
Of course it makes sense to continue to develop the soil moisture holding capacity, and to
make better use of mulches, as these things will delay the time that I would need to intervene,
and help to smooth out the wet and dry periods of the growing season, and make the gardens
more resilient in general.
But, in a really productive garden, plants use a lot of water during the growing season,
and the more I grow, the more I need to pay attention to how much water might be removed
from the soil, and whether we’ve had enough rainfall to replace it, and to pay attention
to when I might need to intervene.
Looking back at this past season, I should have stepped in a lot earlier,
and done a lot more watering.
One of the things I like the most about this RED Gardens project is that it raises so many
interesting questions, and it’s great to have this YouTube channel as a way of exploring
and teasing out some of the answers and sharing them with a wide variety of different people
around the world.
I particularly want to thank that small but growing group of people who have gone to my
Patreon page, linked here or in the description below, and have offered support that is just so valuable.
If you’d like to join them, that’d be really cool, as it would really help me to
continue with this project, but most importantly, thanks for watching.