If there is one plant or a type of vegetable that is most famous for producing an abundance,
it is the courgette or zucchini plant. And I have found that it can be incredibly
productive, providing a seemingly never-ending supply of courgettes or zucchini right through
the summer, all the way up until it starts to get too cold in the autumn.
But these plants can also produce massive courgettes, usually referred to as marrows,
if they are left on the plant for too long. It is one crop where the timing of harvests
is important, and we try to harvest more frequently, especially when the weather is
warm enough for rapid growth of the plants. But I have wondered what the impact that the
size that courgettes are harvested has on the actual productivity of the plant,
and the small trial that I did this year gave some interesting results that I wasn’t anticipating.
I use to get stuck in a cycle of only harvesting courgettes that were larger
than I wanted from the gardens, as I didn’t want to waste anything, and felt I needed to
eat these or pass them on to others before harvesting the medium sized courgettes that
most people generally preferred to eat. But by the time I got back to the gardens
to harvest, the courgettes I had left on the plant had grown bigger than I would prefer,
and the cycle of struggling to get through too many large courgettes would continue
for a lot of the summer, and I see a lot of people struggling with the same thing.
These days I tend to pick and eventually throw in the compost any courgettes that had grown too big,
and to then harvest the courgettes at a size most people seem to prefer to eat.
And we try to harvest every 2 days during the warm part of the growing season,
to try to reduce the chance that any of them will get too big, but there is always one or
two that remain hidden and continue to grow until they are way too big to miss.
By picking the courgette fruit smaller I felt that we were producing more of what people preferred,
and reducing the total amount of kilograms a plant would produce.
This is a bit counterintuitive as generally in the gardens I try to grow as much as I can,
but I felt it was a necessary to try to reduce the chance of a glut at certain times of the
year that would simply not get eaten. But I have also noticed that harvesting
courgettes when they are small, prompts the plant to produce a lot more fruit,
so the overall weight of courgettes harvested might actually end up being about the same.
And I wanted to see how the plants would respond to more frequent harvesting, compared to letting
the courgettes get really big, and to see what this would do to the overall yield.
So I decided to set up a side by side trial of six plants on the side bed of one of the larger
polytunnels, with all six plants sown at the same time, with the same variety of courgette
that I have grown for quite a few years. I planned to harvest the courgettes from
two plants when they were really small, within a day or two after flowering.
I would pick the courgettes on another two plants when they were typical or medium size.
And I would leave the courgettes to grow on the fifth plant until they were quite large,
and continue to let them grow on the sixth plant until they were full size marrows.
The seeds were sown in the middle of April, into seed balls rather than pots,
a technique I had recently seen another grower use [thanks to Jesse], and the
seedlings were transplanted into the tunnel three weeks later, spaced about 1m apart.
I began harvesting the first of the small courgettes 56 days or 8 weeks after sowing,
and tried to harvest them when the courgette fruit were between 100 and 200 grams each
The courgettes from the medium plants started to produce a few days later,
and I tried to harvest them when they were between 200 and 350 grams each.
The large courgettes were ready to harvest a few days after that,
and I tried to pick them when they were between 500 and 800 grams.
And the first marrow was picked 11 days after the first small courgette was harvested,
and the marrows harvested from this plant generally weighed between 1.5 and 3 kilograms.
I allowed the plants to grow without pruning out any of the older leaves,
or removing the side shoots that occasionally grow on these plants,
as I wanted to see how the plants woful respond to the frequency of harvesting.
It is a small sample size in this experiment, and the differences in the way the plants grew
could be a natural variation, but the plants that I was harvesting more frequently from grew
a lot larger, and with a few large side shoots. And the plant that was left to produce marrows,
stayed noticeably smaller, without any side shoots, which makes sense now that
I think of it, but this difference is not something that I anticipated.
Many of the plants eventually grew too big for the space, and I found that I was damaging the
leaves and vines when moving around the plants. And it was getting increasingly harder to find
all of the courgettes in among the abundant growth, with a few of them growing bigger
than I would have wanted for the plant, which would have affected the data a bit.
I eventually pulled all of the plants and ended the experiment in the second week of August,
about 8 weeks after the harvesting started, as some of the plants were just getting too big.
There was still a few months of possible growth in the season,
and all of the plants would have continued to produce, but I think some of the larger
ones were at risk of rotting at the base of the plant and dying back.
It would have been interesting to see what would have happened to all
of the different plants if they had been left to grow for another month or two.
I kept track of all of the harvests, weighting all the courgettes and keeping track of how many
were picked from the different types of plants. The plant left to grow the largest fruit produced
only 14 marrows, about one every 3 to 4 days on average, and this plant remained relatively small.
This was a big difference from the plants growing the smaller courgettes, which produced about 56
courgettes each, slightly more than one a day. These plants grew very large, with several side
shoots each, which made it really difficult to find all of the courgettes when harvesting.
It makes sense that if I harvested a recently pollinated courgette when it was quite small,
the plant would want to grow more to replace it, but in order to do this it would have to extend
the vine, producing more flowers and more leaves. So of course the plant would get a lot bigger
when compared to the plant that put a lot more resources into developing the marrows, which were
relying on the photosynthesis of the existing leaves rather than having to growing new ones.
I hadn’t thought about this before, but it makes sense.
Looking at when the courgettes or marrows were harvested, the marrow plant seemed
to be very consistent, producing the same number of marrows throughout the season.
But the plants producing the smallest courgettes were definitely producing more each week as the
season progressed, no doubt due to each plant having grown several vines or side
shoots producing more flowers and more fruit. These plants also had an increasing amount of
leaves, leading to more photosynthesis. It would have been interesting to see how
much longer this expansion could have gone on, assuming the warm weather continued and we were
able to continue to supplying enough space, water and nutrients for the plants to continue to grow.
The plants that produced the medium sized courgettes produced about twice as many
courgettes as the plant designated for marrows, but about half as many
as the plants that were harvested small. This makes sense that they would be part
way between the two, but what surprised me was that the plants that I was picking the
larger courgettes from produced about the same number of courgettes as the medium size plants.
The plant with larger fruit actually produced a few more courgettes,
but I suspect this is because of damage to one of the plants dedicated to medium size harvesting.
I would have expected that there would be a gradual step change in the number
of courgettes harvested between each of the four different sizes.
But it seems, at least from this small experiment, that when the courgette is
within a certain size range it doesn’t change the amount of fruit the plant will try to produce.
If harvested below that size and the plants seemed to be triggered into producing a lot more
courgettes, but if left to grow into marrows, then the plants produced significantly fewer.
The plants for medium and large harvesting producing the same number of courgettes,
translated into a significant difference in the yield,
or total weight of the crop that was produced.
It seems that once the courgettes grow to a certain size, letting them grow bigger is
easy for the plant to do, or there doesn’t seem to be an additional cost to the plant.
But having to grow more flowers, to produce more courgettes, and growing the additional
leaves that goes along with it all, does have an additional cost to the plant,
or it diverts energy and nutrients away from simply making the existing courgettes bigger.
So it is not surprising that the total yield from the plants that
were producing the smallest fruit would have been lower,
as the plants had a lot more to do work. But it was not as low as I though it would be,
producing about 75% of the total weight that was produced by the medium fruited plants,
even though there were close to twice as many courgettes harvested.
But part of this larger than anticipated yield would have been because I missed a few courgettes
that ended up growing considerably larger than planned, but this happened on most of the plants.
At the other end of the scale, the total weight of the marrows harvested was less than 10% more that
what was harvested in large size courgettes. So it seems that after the courgettes are
left to grow to a certain size the plants don’t focus on producing more total weight, probably
because the focus shifts to putting energy and nutrients into developing the seeds inside.
So if I pick most of the courgettes quite small,
the plant will grow a lot larger, and I will end up getting a lot of small,
crisp and more flavourful courgettes, and the overall yield will be a bit lower.
But compared to a lot of other vegetables, the almost 10kg/m2 that
these plants produced is a lot, and they were only growing for part of the season.
If I had pruned a lot of the older leaves, and pulled off any of the side shoots,
the productivity of the plant would probably go down, but they would be easier to manage,
and to find all of the courgettes, and they would probably grow for longer in the season.
If I leave the courgettes to grow to a medium or more typical size
the yield increases a bit, the number of courgettes produced goes down significantly,
and the texture of the courgettes changes a bit.
Letting them grow even bigger, either intentionally or by accident,
will dramatically increase the weight of courgettes that each plant will produce.
But I feel that this is more of a watering down of the flavour that was in the medium size courgette,
and the seeds cavity starts to develop and soften. If you wanted to produce the biggest weight of
crop in the gardens, leaving the courgettes to get to this large size will definitely help,
but I find that a lot of people don’t like eating them when they get this big.
And it seems that preventing the courgettes from growing past the medium size stage is
one of the most effective ways to reduce the excessive amount of courgettes that I need
to try to use up, to pass on to other people, or to reduce the amount I end up composting,
Leaving them to grow even bigger, into what would normally be called marrows,
really reduces the number of fruit that need to be picked.
And the plant does not seem to produce a significantly heavier weight, but what is
grown will likely store for a lot longer into the winter, though they may never get eaten.
At either the large or marrow size these plants produced more than 30kg per m2 of bed space over
a 7-8 week period of the summer, with the plants only occupying the bed for a total of 3 months,
which is a massive yield even for plants in the polytunnel.
But the marrow plant was substantially smaller and I suspect it would have produced marrows for
longer of the season than any of the other plants which were just getting too crowded.
So it all depends on what I want to eat, or perhaps more accurately on
how frequently I get around to harvesting. If I wanted to produce as much to eat as
possible from a small space, then leaving them to grow to marrow size definitely
has its advantages, both with the smaller plants and the less frequent harvesting.
I think I would still avoid harvesting most of the courgettes at the larger size,
if I could help it, as the plants became quite big compared to the marrow plants,
though the yield is still impressive. The typical, medium size courgette is
probably still the best option for balancing yield and most people’s preferences,
and this is the size we usually end up harvesting from all the plants that we grow in the gardens.
But at the end of all this, I still prefer to harvest the courgettes when they are
really small, as I like the more intense flavour, even if the yield is smaller.
But I need to take into account that this will prompt the plants to become a lot bigger,
so I either need to give them a lot more space, prune the older leaves,
or to grow a replacement crop half way through the season and then to pull out the first batch when
the second batch has started to produce. I assume that a lot of this would be
similar with the courgette plants grown outside rather than in the polytunnel,
and with different varieties of courgettes, but I won’t really know until I try it out.
And I suspect that the plant spacing, the timing of sowing, and the care that we
generally use for courgette plants is geared towards producing typical medium size fruit.
And I wonder how things would change and what varieties I might grow if my focus shifts to
growing as many of the smallest courgettes as possible, over the longest part of the season.
Now that would be an interesting thing to explore.