How To Extend The Growing Season with a Polytunnel - Part 1: Spring

How To Extend The Growing Season with a Polytunnel - Part 1: Spring

Tomato seedlings in a tray - header image

As was briefly mentioned in my mail of a couple of weeks ago, changing weather patterns can challenge our habits and our 'collective wisdom' about the traditional growing season. In recent years spring weather has been staying colder and more inclement for longer; while we've also seen some unseasonably warm conditions as late in the year as November (although you wouldn't think that today). 

Of course, the truth is the seasons have never been set in stone, and the gardener's skill set has always required some adaptability and flexibility to work around whatever comes our way.

Plants protected by fabric, Magnolia Market, Texas

While I am not a fan of cold, wet weather, you only have to take a look at recent news stories to remember that, as grey as the days can sometimes be, our temperate climate has it benefits. We generally avoid the more extreme conditions that have struck other countries, where warmer air provides the energy for stronger winds and heavier rainfall. 

In terms of growing vegetables, our climate is really very good; plus, with the aid of some cover, we are able to grow many of the more exotic crops from sunnier climes. 

I have always felt that a polytunnel or greenhouse is close to being essential kit in our climate. Not only do they allow the cultivation of heat-loving crops and a longer indoor growing season, they also extend the outdoor season by offering warmth and protection for early sown vegetable seedlings. 

There are other more budget or space-friendly options which can also provide protection for young seedlings, but a walk-in structure (if possible) is much more versatile. 

Cucumbers in the polytunnel

How a Polytunnel or Greenhouse Can Maximise Your Growing Season
It's been well-established that a greenhouse or polytunnel can add 6 weeks to your growing calendar at either end of the season, which adds up to a not inconsiderable three months in a year. This extra time can be optimistic in some years, but in general a covered growing area lives up to its promises when regular sowing and good planning is practiced.

In my experience for the last 4 years spring has been very slow to arrive, with May weather being more like April and proper warmth failing to arrive until mid June. These conditions make a big difference in an already short season, as ideally we want many of our crops in the ground and growing away in early May. 

This is where a covered area comes into it's own in my opinion. Temperatures under glass or polythene start to rise by mid February, when overwintered tunnel crops begin to grow again. 

Seeds that are destined for the outdoors can be started off in late March as normal, but they may not be planted outside until late May or early June depending on the weather. It takes a bit more work, but by potting seedlings on to larger trays or pots instead of planting outside, we can have relatively large plants going out in the garden in late May when conditions are much better for growth. 

Transplanting chilli

By the time the last frost date has passed you can have built up a considerable head of steam in the garden. Strong, robust seedlings will be ready to plant out rather than starting from scratch outdoors and hoping for a run of warm weather.

It is very important that plants grown in protected indoor conditions are slowly acclimatised to the outdoors by 'hardening off' (moving outside in the daytime and back in at night or in harsh weather). Again, it is a little more work, but if managed well we can build a stock of large, sturdy plants which will romp away when planted. 

Crops to Sow in the Polytunnel in February: 

  • Aubergine
  • Broad Beans
  • Chilli
  • Sweet Pepper
  • Kohlrabi
  • Leeks
  • Beetroot
  • Early Carrot varieties
  • Radish
  • Turnip

Crops to Sow in the Polytunnel in March: 

  • Beetroot 
  • Carrots
  • Calabrese 
  • Cauliflower 
  • Cucumber (end of March)
  • Kohlrabi
  • Leeks 
  • Lettuce 
  • Peppers (ideally in February with heat) 
  • Radish
  • Spring Onions
  • Tomatoes

Crops to Sow in the Polytunnel in April:

  • Brussels sprouts
  • Cabbage
  • Calabrese
  • Sprouting broccoli
  • Cauliflower
  • Chard
  • Coriander
  • Courgette
  • Dill
  • French beans
  • Kale
  • Kohlrabi
  • Lettuce
  • Melons
  • Parsley
  • Radish
  • Perpetual spinach
  • Squash
  • Turnip


 

How a Propagation Station Can Give You a Headstart on the Growing Season
As I have said here plenty of times before, the reason why a greenhouse or polytunnel is much better (up to 3 times better) than starting seeds off on a windowsill is that they let in more light. A window only faces one direction and is shaded from all other aspects (because the rest of the house is in the way), so it only gets one third of available light.

A dedicated seed germination area can consist of a simple bench with a soil heating cable and some modular seedling trays; alternatively you can use an electrically heated propagator. The polytunnel provides stable conditions and protection from the elements during this crucial phase of seed germination. You can use horticultural fleece or some blankets to provide warmth and protection if outdoor frosts are forecast.

Vitopod stackable heated propagator

Small Vitopod Electric Propagator Single or Double Height

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Germination can be particularly successful and reliable if your electric propagator has an accurate, adjustable thermometer. One great benefit of a thermostat-controlled propagator is that it will only switch on once the inner temperature dips below that point. 18-20 degrees is generally sufficient for most seedling growth; since a polytunnel will frequently reach that level anyway once some April sunshine arrives, you can save on energy costs as it doesn’t have to be running constantly.

A very good option here is the old reliable 'Vitopod', which provides a generous 1.1m x 0.6m heated base and an adjustable height to pretty much whatever height you like. This height extension feature can really come in handy if outdoor conditions delay planting out, as relatively large seedlings can be maintained without having to move them elsewhere.

a Biogreen propagator in the Quickcrop shed/base of operations

Once heat has caused seeds to germinate, the important thing for continued growth is that they have plenty of light. A tunnel will maximise available sunlight in early spring, even compared to windowsills which are at the mercy of which way your house faces (for example).

Warm Season Crops
It may seem counter-intuitive starting off warmth-loving crops like tomatoes, peppers and cucumbers in the chilly depths of February, but this is because they need to get a good start so that they are vigorous and well established plants by the time the summer months arrive: this enables them to properly thrive, whereas if you were to start them later in the year their overall growth cycle will be curtailed.

Tomato seedlings in the polytunnel

One of the tricky things about growing tomatoes is that, because they need as a long period of sunshine as possible, we need to start them off early in the year when conditions are less than ideal. Tomatoes need a soil (or more likely, compost) temperature of around 20˚C for the seeds to germinate, so they will need to be sown in a centrally heated room (in pots or trays rather than directly into the carpet) - or using a propagator or other heat source if sowing in an unheated greenhouse or polytunnel.

The difficult part is that, while we can easily supply the heat to germinate seeds and protect young plants from cold, it is more difficult to provide enough light to prevent them getting 'leggy' (tall and gangly with more stem than leaf). The earlier you sow, the more pronounced this problem will be because daylight hours and light intensity are low in February. This is why, despite the advice on seed packs, I find it better to sow tomatoes from early to mid March. It is best to sow in pots or trays in a greenhouse or polytunnel to benefit from as much available light as possible.

Tomato seedlings on a heat bench.

Remember that freshly potted tomato plants will need to be protected from cold in the spring, especially at night. The pots or trays will need to stay on the propagator or heat bench but at the lower heat of approx 14˚C. The image above is of my plants last year doing nicely on a heat bench. On most days I left them uncovered, but I placed a layer of fleece over them at night: both to protect from frost and to help reduce the running costs of the heat cable buried under the sand. 

You may also notice that the individual tomato plants are spaced out on the bench. This is so they are not competing with each other for light and allows me to grow nice stocky plants. To be fair, this is a bit of a luxury as I have a large heat bench, if this is not possible for you, it's not essential as you can use the deep planting trick again when you either pot on to a larger pot or plant your tomatoes in their final positions.

Using Grow Lights to Extend the Season
We touched on it above, but while you can create enough heat (between 18 and 24 degrees) to germinate seeds with the aid of a propagator, low light levels and short daylight hours at this time of year make your seedlings prone to tall, weak or 'leggy' growth. One of the answers to this conundrum is to use a grow light to provide that extra light.

Grow lights are different from ordinary household bulbs; they emit a different spectrum of light which has been tweaked to mimic sunlight. There are a number of different and interesting types including halogen, flourescent and LED variants, all with different pros and cons. 

Pepper plants under grow lights

LED variants in particular might appeal to the home grower as they use much less power or wattage, and are thus more cost-effective. 

There has been a lot of fascinating work done recently on coloured LED grow lights, which has come about largely to answer the demand for food security and large urban indoor farms. These developments include a range of 'light recipes' tailored for different growing applications, from producing seedlings to growing leafy greens or ripening fruit. 

The lights produce three different spectra of light - including red, far-red, and blue light - which have different effects on root formation, leafy growth, flowering and fruiting. The concentrations of the different spectra can be altered by changing the numbers of LEDs in the light fitting. 

While this might be a bit advanced and unnecessary for the hobby grower, it's an interesting tangent - and you know we like to go off on the odd tangent!

Pepper seedlings in deep pots

Some Tips for Early Growing
Some forward planning, preparation and space management can go a long way when it comes to the early stages of the growing season.

Staging for Plants and Trays: 'Staging' in a greenhouse or polytunnel refers to an arrangement of shelves or benches with space for pots, trays and so on. With tiers of different heights or depths, this kind of system can make attending to your seedlings or plants much easier and more efficient. 

Staging keeps trays and pots off the ground and allows excess moisture to drain away easily. It also enables a more comfortable working height when it comes to watering, sowing or potting on. You can also position plants or seedlings in a position that maximises sunlight, or alternatively place more shade-tolerant crops on lower shelves.

Similarly, raised staging or benches will keep early sowings out of the reach of slugs or other critters who find their way into the polytunnel. 

Transplanting a pepper seedling

Different Sized Containers: Having a good stock of seedling trays with different cell sizes or layouts, as well as larger sized pots to transplant into, will ensure that young plants don't get pot-bound before they are planted out. This is when the roots run out of space and/or nutrients; visible signs of a pot-bound plant include roots that are congested or poke out of drainage holes.

Should You Fertilize Seedlings?: It is also worth looking at plant feeds to keep seedlings well nourished in the confines of a pot. As I think I mentioned a few weeks ago, I have had great seedling success with the seaweed-based 'Root Booster' from Better Plants which also contains fulvic and humic acid. It is an absolutely ideal feed for seedling trays and pots. 

Vermiculite and Germination: You will recognise vermiculite in the photo below, which I use to cover chilli and pepper seeds after sowing rather than using a layer of compost. The advantage is the vermiculite holds onto moisture while creating air spaces which gives a better germination rate for these slow to sprout seeds.

I am also seeing a lot more potential in the tail end of the season, especially in the tunnel where I have had pretty good growth in November. Indeed, we are close to the shortest day of the year and I still have a soil temperature in the beds of around 8-10˚C. 

Chilli seedlings with vermiculite

We will cover autumn growing in Part 2 of this blog post. There is a whole load more experimentation to do here, as I know I am not yet using the tunnel to its maximum potential. I will keep you posted on progress.

In conclusion then, I think that covered growing is becoming even more important to soften the edges of changing seasons. Our summer is short enough as it is so, in my experience, a head start is key for plants that need plenty of sunshine to do well.