Water is an essential part of growing vegetables, or any other type of growing or gardening,
and generally the more you grow the more water is needed, up to a point of course.
Many places around the world don’t get enough rain during the growing season, and require
irrigation in order to grow abundant crops, or we need to accept a lower yield of more
drought tolerant plants. In the maritime climate of Ireland we generally
get enough rain to maintain sufficient soil moisture for plants during most of the growing
season, for most years, though we can get problematic dry periods.
Generally we get more rain than is needed, especially during the winter, and with all
the flooding that’s going on around, it's appropriate to ask how much rain is too much?
Like so many other things the answer to this question depends on the context, and it's
not just about how much it rains over how long of a period of time, but also when it
rains and how quickly the water moves away. Here in the midlands of Ireland, we would
typically get a total of just under 1m or a bit more than 36 inches of rain each year,
and that is fairly evenly spread throughout the year.
We tend to get a bit more rain in the winter, and less in the summer, but on average August
can be surprisingly wet and February can be quite a bit drier.
Of course there can be huge variations from one season to the next, with unusually dry
periods causing problems with growth in the summer when a lot more water is needed.
And really wet periods generally happening in the winter, and that is when the water
can be the most problematic, especially with flooding.
A big factor is that here is very little evaporation from the surface of the soil in the winter,
and almost no water is being pulled up from deeper in the soil by the vegetation.
So without any significant evapotranspiration during the winter, the water can only soak
away or flow down hill, and this can take a while and cause a lot of trouble downstream.
So far this month we've had more than twice the amount of rain we would expect in a typical
February, and judging by the forecast for the next week, it will probably end up being
the wettest month in the last 4 years. Thankfully it was fairly dry for the last
2 weeks of January, and the previous few months were not excessively wet, so there was still
capacity in the soil to absorb some of this excess rainfall.
The impact of this rain really depends on the specifics of the site, how quickly the
rainfall can be absorbed by the soil, and whether water flows onto the site from somewhere
else. The RED Gardens are located on a slight hill,
with very little land further uphill, so if there is any surface water it would tend to
flow away from the gardens rather than towards them.
This is quite a bit different than a fair amount of the farmland around here, especially
those that are situated in low lying areas, that tend to receive floodwater from upstream,
and to make matters worse, typically would have soil that can’t absorb water very quickly.
The soil on this little hill is quite sandy, and it would take a lot of rain over a very
short period of time for there to be any significant water visible on the surface of the ground.
But we tend to get rain over longer periods of time, rather than the really heavy downpours
typical in other climates, so this is rarely an issue.
And based on the exploration holes that I have dug in the past, the subsoil also appears
to be quite free draining, at least for as deep as I've been able to dig.
It still takes a while for the excess water in the soil to drain away, but I have found
that I can get into the gardens after waiting a few days, without making a too much of a
mess or heavily damaging the structure of the soil.
This really helps when trying to grow vegetables year round, and is a definite benefit of this
specific site.
Really heavy rain does affect the surface of any exposed soil, breaking down the texture
or clumps of soil into smaller particles. In really bad cases this can reduce the infiltration
of water into the soil, but I haven’t found it to be much of an issue.
Only the surface of the soil is damaged, and there's enough sand involved to maintain the
ability of the water to filter into the soil, and it seems the well managed soils with higher
amounts of organic matter does even better. The leaves of overwintering plants offer some
protection and a full covering of a green manure definitely helps to protect the soil
surface. I've also noticed that the crop cover and
mesh that I use to protect overwintering plants from the wind and pests also shields the soil
from damaging rainfall. And I have found that ground cover fabric
is the best method of protecting seedbeds that are waiting for an early season sowing,
as the texture of the surface of the soil remains really good even over the full winter,
assuming that the high winds don’t blow the fabric around.
The paths in the gardens do tend to get compacted if I walk on them too much in the winter.
The process of repeatedly stepping on the paths breaks down the structure of the soil
texture and can create an impenetrable surface, which means that water can’t drain away,
which makes things even worse. The same can happen in high traffic areas
of grass that surround the gardens and the overall site, especially in front of polytunnel
entrance, and on the path towards the composting area.
This creates a real mess even without using tractors or machinery, but a heavy wheelbarrow
can cause some significant damage. Spreading straw over areas that are prone
to damage helps to reduce the impact, and wood chip is even better, but this is a big
area to cover. I generally try to stay away from the gardens
entirely during wet weather, but this has significantly delayed things in the past and
left a lot more work during the busier times later in the spring.
I’m really glad that we had a few weeks of much drier weather in January this year,
as we were able to get a lot of the gardens cleaned up, and many of the beds prepared
for planting. Now that the soil is saturated with water,
I'd prefer to wait for at least a few days without any rain before I go into the gardens
to do any work. But if I do need to get into the gardens for
harvesting or some other reason, I’m not likely do excessive amounts of damage, so
long as I don’t walk on the beds, and leave the soil alone as much as possible.
So, while the specifics of the site and soil type that I work with in these gardens can
handle a lot of rain without too much damage, so long as I stay away, all of this rain probably
creates issues with soil fertility. Any water that does drain through the soil
takes with it traces of soil nutrients that it picks up along the way, potentially moving
this soil fertility out of reach of the plant roots.
How much this happens depends on a lot of different factors, including the proportion
of clay particles that there are in the soil, but the difference between the rainfall and
evapotranspiration rates is a significant issue around here.
The potential evapotranspiration rate is a calculation of the amount of water that is
likely evaporated from the surface of the soil, added to what is pulled out of the soil
by the plant roots and incorporated into the plant itself, or more commonly transpired
into the atmosphere. Both of these increase with the heat and summer
growing season, and reduce in the winter. Ideally the amount of rainfall that we get
would roughly match the amount of evapotranspiration from the garden soil each season, but that
is rarely the case. Whenever the evapotranspiration rate is higher
than the rainfall, the garden is at risk of drying out once the soil moisture starts to
get used up, but I can irrigate the gardens to make up for this.
Whenever the amount of rainfall exceeds the combination of evaporation and transpiration,
this increases the likelihood that some of the soil fertility will wash away, and this
can be a difficult issue to address.
Looking at the typical monthly rainfall and the potential evapotranspiration rate supplied
by the national weather service, some summers have less rainfall than what is used by the
plants or evaporated. The winter shows another situation entirely,
with lots of water flushing down through the soil every winter, potentially taking fertility
with it, as it has done for thousands of years. This is a huge loss of fertility over long
periods of time, but I'm not sure how much this annual loss would compare to the amount
of fertility that I add to the gardens each season, the rate that the soil biology can
extract new nutrients from the rocks, or the amount of nutrients that are removed from
the soil through the harvest of vegetables. In addition, some types of soil nutrients
are more likely to wash away, than others, and this can lead to imbalances in the soil
fertility. I think that this is not such a big issue
in a small scale garden as it is easier to add lots of fertility, but it is something
I need to take into consideration And if I am aware of any fertility imbalances,
either by testing the soil or some other method, I can more easily correct them, but with field
scale agriculture, this can be really difficult and expensive.
Adding good quality compost helps, not just by adding more fertility to the soil, but
also by helping to increase the amount of fertility and water that the soil can hold.
Green manures can also really help as they draw some of the fertility out of the soil
before the winter and incorporate it into the plant tissue, but I don’t think they
can’t hold onto all of the fertility that was in the soil, and there is always going
to be some nutrients that can be washed away.
Overall this is quite a good site for vegetable gardens, as there really isn’t any risk
of soil erosion or flooding. And I've done a lot of things to protect the
soil, with more I can do in the future, and I tend to add lots of fertility to the gardens,
and I have been taking soil tests so that I'm not so worried about the impact of any
loss of fertility. If I make sure that I do most of the bed preparations
prior to the wet season, I can avoid needing to go into the gardens, except to harvest
the overwintering crops. So in this context these vegetable gardens
can handle quite a bit of rain, assuming that this rainy weather stops at some point in
the near future so that I can get into the gardens and start growing vegetables again.
But this wet weather has been really demoralising, especially as it's come with high winds and
cool temperatures, it's felt relentless for weeks, and it looks like next week is going
to be more of the same. I'm really glad to have polytunnels to work
and grow in during this type wet winter weather, but I still need to get into and out of these
protected microclimates, and that has been making a real mess of paths.
While I am glad I prioritised the work in the outside gardens during the drier weather
earlier in the year, this past week I have been moving a lot of compost into the two
polytunnels to prepare the beds for spring plantings, and it's been a real mess.
Especially as the high winds blew off some of the plastic covering the compost piles
and moving soaking wet compost is not much fun.
So, while the gardens may be able to handle this wet weather, and it is still quite early
in the season, I've had enough, and I’m really looking forward to this rainy weather stopping.