The first and last frost dates are one of the key defining characteristic of different
regions that need to be taken into consideration by those of us who are growing vegetables.
Lots of different types of vegetables are fine with freezing temperatures, and it’s
amazing how frost tolerant some plants can be.
But the popular summer fruiting crops such as tomatoes and cucumbers are very different,
and many of them can be killed off or severely affected by even a light frost.
And here in Ireland, it’s the frost tender potato plant that many of us grow a lot of
that is the crop that is most likely to be impacted by frosts, especially in the spring.
As a grower I'm still figuring out the best ways to work around the possibility of frost
damage, and I've been quite successful for the most part, but I was disappointed by the
impact of frost that we had earlier this month.
The RED Gardens are located in the midlands of Ireland, about as far away as you can get
from the oceans and seas that surround the island.
While the warming effect of the Gulf Stream provides us with quite a good climate to grow
in, despite being so far north, but it does have less of an impact this far inland, leading
to a slightly greater range of temperatures, including more frosts.
The typical last expected frost date in the spring for this area would be about the middle
of April, with the first frosts of the autumn generally expected around the last week of
October.
This theoretically gives us around 185 to 200 days of frost fee growth in the summer.
But my personal experience is that these frost dates can vary a lot and we have experienced
frosts as late as the end of May, and as early as in late September and I need to work around
this possibility.
Traditional gardening lore around here has lots of growers planting early potatoes on
St Patrick’s day, which would mean that the plants could be really well established
to be badly affected by these late frosts.
But I suspect that this gardening advise is better suited for growers who are located
closer to the coasts, where they don’t get frosts this late in the season.
Over the years I've learned to not plant courgettes or zucchini, and pumpkins or squash until
late May, even though this reduces the yield and the plants miss out on the warmth and
good growing conditions that we often get in the spring.
There are also issues with tomatoes plants, as well as the related aubergine or eggplants,
as well as peppers because of the long time it takes them to produce a crop, and I try
to get them planted into the polytunnel as early as possible.
These plants are usually quite safe from frost under the protection of the plastic, or that’s
what I thought.
The frost that we usually get in late spring are typically quite light, and generally develop
on clear nights with no cloud cover, very little wind, and often following a sunny day.
On these nights the temperature of the air gradually falls over the night time and reaches
the freezing point just before dawn.
This short amount of time can be just enough to cause the water in some leaves to freeze,
which bursts the cell wall, killing off sections of leaves that have been affected.
I have become use to these lighter frosts, but earlier this month I saw a forecast for
the 4th or 5th of May, that indicated that temperatures could drop several degrees below
the freezing point overnight, due to an unusual cold air mass over most of Europe.
That night the temperatures dropped below freezing just after midnight, and stayed below
freezing for 5-6 hours before warming up again just after dawn.
This longer period of time and colder temperatures would allow more heat to dissipate and the
possibility of greater frost damage to set in.
It seems like a very subtle difference of a few degrees but it could be the difference
between success and failure for some crops, and it reduces the effectiveness of some of
the protections that I usually put in place.
Whenever a frost is forecast I always look around my gardens and try to protect whatever
plants that might be growing that could be at risk.
I knew that all the seedlings and young plants in the heated propogation space would be fine.
But the early potato plants in the Polytunnel Garden were really well established, and a
deep enough frost could kill off a lot of the leaves and destroy the chance of an early
yield from these plants.
I decided to covered the long rows of these large plants with a length of horticultural
fleece, which is a fabric that has quite good insulating value but still lets a lot fo the
light in.
I am really glad that I had invested in a full roll of they horticultural fleece few
years ago so that I had it in hand when I needed it.
I knew that all of the other crops in this Polytunnel Garden would be fine with this
level of frost as I hadn’t transplanted in any tomato plants or the other tender crops yet.
Most of the plants in the outside gardens were quite hardy, except for the early crop
of potatoes which had only just started to appear in four of the outside gardens.
I knew that these plants were small enough to be able to recover from any frost damage,
but it could delay the crop and reduce the yield a bit, so I thought that it was better
to protect them if I could.
I ended up covering two of the batches with fleece, and the other two with a different
kind of crop cover.
I was also concerned about the tomato plants that I had recently transplanted into the
larger Polytunnel that I have, so I decided to set up a full tank of water and filled
a number of buckets with water and set them around the space of the polytunnel as a precaution.
I was hoping that the thermal mass of large volume of water would be enough to keep the
temperatures above freezing in this space but I wasn't sure if it was enough.
It's always a judgement call in this type of situation, to figure out how much effort
to put in to avoid a potential risk, and in this case didn’t do enough.
After the strong frost that one night I was glad to see that the potatoes in the polytunnel
were not effected at all, as the double layer of the plastic of the polytunnel and fleece
over the plants worked really well in combination.
The potatoes in the outside gardens that I had covered with double layer of fleece were
all fine, apart from a bit of damage to some of leaves on a few of the plants at the end
of the row, where obviously a bit of cold air had gotten in.
I suspect that it helped that the young plants were at the base of shallow trench, which
left an air space between leaves and fleece above.
All of these plants are fine, and a few weeks later are growing really strongly, and they
seem to have benefited from the extra warmth of the fleece that was covering them for those
few days.
The other type of crop covering was less effective, and a fair number of the potato leaves in
two of my other gardens being burnt.
These plants will recover, and a few weeks later they're all growing strongly again,
but it is hard to know what the impact on yield will be, thought I suspect it will be
fairly light.
It was interesting to observe that a few of the plants that had been covered with a double
layer of this crop cover material were not affected at all and now seem to be a little
bit more advanced than the other plants right beside them.
While it was good news for the potato crop, unfortunately the tomato plants in my larger
polytunnel were hit fairly hard by the frost.
I think that things might have been a lot better if I had filled up the tanks and buckets
with water a lot earlier in the day so the water would have had a chance to warm up a
lot more, and this was my first mistake.
The tomato plants in the middle of the space, away from the doors at either end were untouched.
Closer to the end doors the plants started to get a bit of frost burnt patches on the
edges of leaves, which will slow the plants down a bit but they should recover reasonably well.
The tomato plants closest to the end doors were badly hit, with most of the leaves on
the plant getting frost damage.
It seems that a draft through the gap at the base of the door was an issue, and I just
didn’t think of that.
Normally on these frosty nights we don’t have any wind, but apparently there was enough
to cause the damage, and that was my second mistake.
At one end of the polytunnel the damage was a bit less and I suspect that this was due
to the larger volume of water in the large tank.
A few weeks later these plants are showing signs of decent regrowth so I should still
get a reasonable crop of tomatoes out of them.
The plants at the other end were in really bad shape and I decided to replace some of
them with surplus tomato plants that I was growing for a slightly later crop for the
other Polytunnel garden.
I will still get a lot of tomatoes out of this polytunnel, but I'm more disappointed
that it disrupts the variety trial that I was undertaking, where I have been growing
32 different varieties of tomatoes.
I still have most of the varieties of tomatoes growing, but because some of them were badly
affected by the frost, and others weren’t, I won’t be able to do yield comparison that
I was planning to do.
But at least I’ll be able to taste most of the varieties.
As with so many experiences in the gardens, there's a bit of learning in all of this,
or reinforcing what I already knew.
There is definitely a risk of transplanting tomato plants into a polytunnel too early,
and the early crop of potato crops are also at risk, but these can be protected by horticultural
fleece, and a double layer is definitely better than a single layer
Thermal mass works, up to a point, but only if it has a chance to warm up a lot, and keeping
out the drafts is a big learning point for me, and the one thing I’ll definitely be
adding to my list of things to check whenever there’s a risk of frost.
In the future I think I’ll make an effort to cover the tomato plants, probbably by attaching
fleece to the support strings on either side of the row of plants, as I think this could
really help.
Having an active source of heat in these polytunnels on those cold nights could be of real benefit,
as all it has to do is raise the temperature by a few degrees, and this is something I
am going to look into in the future.
But of course all these things take more work, preparation and material, and it would be
much easier to simply delay the plantings until the risk of frost is over.
But I’m willing to do a certain amount of work in order to get an earlier crop from
my gardens, or at least to reduce the risk of frost damage.
I’m really interested in exploring a few ways of boosting the temperature in this polytunnel
overnight, and there seem to be a number of different options for doing this, fro the
simple and passive, to the complex and mechanised.
But I think it would be beneficial to explore a few of these as raising the nigh time temperature
by even a few degrees not only reduces the risk of frost, but it seems to be able to
boost the growth rate of plants in the spring, and even extend the harvesting into the autumn.
I’m definitely interested in experimenting with a lot of this, and I will be sure to
make more videos about it all in the future, and this is where support from people like
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