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Polytunnels at Worsley Hall Allotments

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What Plants can be Grown in a Greenhouse & how to grow them

What needs to be grown in a greenhouse/polytunnel? - As mentioned before some plants will only need to be started off in a greenhouse and they can then be planted outside to continue growing. Others however will need to spend the whole time in a greenhouse or somewhere warm.

These plants include:-

You will find varieties within all of the veggies mentioned above that will grow outdoors, however with the recent summers in mind I would be wary of growing these outside. Just check before you buy what the instructions say about the plant/seeds, what heat it needs to germinate, if it needs a warm climate to mature and if its susceptable to frosts or cold weather. You'll then know if indoor or outdoor growing is needed.

How do I grow these plants?

The first thing you need to do is seed the plants into trays or large pots. You can buy seed trays or make your own. Ice-cream tubs cut down to about 2” in height with some small holes in the bottom for drainage are great. You will need to fill your container almost up to the top of the container with soil. Then very lightly sprinkle the seed over the surface of the soil, don’t worry about spacing at this stage.

Once you’ve sprinkled your seeds cover them with a very thin layer of soil, again just sprinkle the soil on. Give them a quick water and place them somewhere warm and light… Remember to water seeds and small seedlings with a watering can with the ‘rose’ on. You only want the water to come out in the gentle shower that the rose provides rather than a downpour that you’ll get without the rose, other wise your seeds might be washed away! Now just keep the soil damp and wait for them to germinate, the seed packet should tell you how long this will take.

Once the seeds have germinated and have what are called their first ‘true leaves’ which means they have at least 2-4 leaves they are ready to be ‘pricked’ out. Pricking out simply means pricking the seedlings out of the soil where they have been initially sown and placing then into their own pot. This pricking out needs to be done very gently as the plants will be very fragile at this stage. I find it easier to get my hand right under the plants into the soil and first loosen the soil. Then gently pull on an individual plants by the leaves and you should feel it coming away ,if you don’t loosen the soil a bit more.

Now the pot where the plant is destined needs a hole put in the soil. I find my finger is perfect for this… So pick up your seedling, gently, and place the roots into the hole you’ve made, a pencil or plant label is handy here just to make sure the roots are all well and truly in the hole. You want the plant to go a little bit deeper in the soil than it was before. Then gently push the soil around the base of the seedling and firm it down. And give a little water.

The size pot you chose depends on where the plant will end up. But remember you don’t want to make the next pot too big. So if you intend the plant to end up in a large patio pot slowly work up to that sized pot as the plant grows bigger. If your plants are destined to go in the soil outside then you may want to put them into cells, small pots or even loo roll tubes. This is because they will only need to grow a little bit bigger before they go outside. If however your plant will stay inside, so say a chilli or pepper plant then you can stop potting them on when once they're in a 25m pot or there abouts which will be its final destination.

Most of the plants mentioned above that need to be grown inside will also need some kind of support as they will grow very tall and the fruits themselves may need individual support. You can avoid this if you buy ‘shrubby’ varieties, these will grow much smaller.

But must tomatoes, peppers and melons will grow very tall. Bamboo canes pushed into the pot of these plants (carefully to avoid damaging the roots) and then tied to the plant stems is one way or sometimes just string tied to the base of the pot and then above the plant is another way. It really depends where your plants are growing and what you can work out. Try a few ways and see what works. Melon fruit may need their own support as, if you’re lucky, could grow quite large. Just be aware of this and make sure the fruit isn’t putting too much weight on its stem as it may fall off before ready. I’ve been told tights make a good supporting hammock for large fruit.

Now your plants have been potted on and are destined for outside they need to be ‘Harden off’. This needs to be done because up till now your plants have been in a lovely warm sheltered environment and if they’re suddenly shoved outside into the real world they may not do very well! So what you have to do is on niceish days take the plants outside and leave them there, then bring them back in for the night then repeat this the next day and the next for a week or so. Then your plants will be ready to be planted outside. I have to admit I’m quite lazy and a bit reluctant to comply to what the text books tell you here. I will put them outside then forget to put them in or just forget to do it for a few days. But if you’re planting out in nice weather and it stays nice for a few days, I find most plants cope OK. Just try it yourself and see what works best for you.

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How to get the best from your greenhouse/polytunnel

Watering and Ventilation

It is important to try to get these 2 things right in order to avoid many plant problems. Watering is obviously vital for your plants to live and ventilation will help keep your plants healthy.


Your plants will need the right amount of water, too much or too little will lead to many problems (and dead plants!). So always keep a close eye on your plants for signs that watering is just right. Just looking at the soil is not enough, you need to feel the soil and I mean put your finger into the soil (at the side of pot) to really get a feel for the water content. Pick up the pot if it feels very light the plant needs water ASAP if its very heavy it has been over watered and will not need anymore for that day. Its always best to water in the morning rather than the hot afternoon or evening. This gives the plant time to make use of the water before it evaporates and watering in the evening can lead to dampness added to colder evening temps, not good for the plant. When watering remember to water the soil rather than the plant, as water sat on leaves doesn’t do any good. If you have a dense area of pots with rows in front of rows etc, then check pots from all rows to make sure every plant is getting enough water.


You don’t want your greenhouse getting too hot or damp so its important to have good ventilation. In most cases and especially with the simple structures, this just means leave the doors open on hot days. But remember to shut the doors before the sun goes down. If you have side or top vents open these as well as more heat will escape from the top and sides. In the summer if you’re out for the day I would advise to leave the doors open as even if you leave in the rain it may brighten up and your greenhouse will be gaining record temperatures when you can’t get to it to open the door! On nice days in winter leave the doors etc open a little bit but shut them if you go out. You can buy automatic vent openers that will open the vents once the temperature has reached a certain level that you've programmed. These can only really be used on greenhouses because of their design but are relatively inexpensive and quite handy, otherwise if you dont have a greenhouse then its the manual way!

To heat or not to heat?

If you do get a greenhouse,polytunnel or similar you may decide to heat it in the colder months. It is possible to heat your greenhouse to get extra warmth in cold months however this can be costly. It is only really necessary if you are growing very delicate plants or are growing to order so need plants to grow as quickly as possible. You can buy Kerosene heaters that are cheapish to buy but will need a lot of fuel to keep going, or solar heating if you don’t have mains electric to your growing area. However I personally think its very expensive and most plants that need a bit of extra heat can be started off inside to give them the heat needed to germinate and then hopefully the spring and then summer sun will keep them nice and warm in your greenhouse.

However having said that I’ve recently read on the web (where else?!) that horse manure mixed with straw, placed in plastic bags and then put in your greenhouse/polytunnel etc can give that little bit of extra heat over the cold months. It should give off a few degrees of heat for about a month when it can be either dug into the soil or added to the compost, then bring in some fresh stuff! Brilliant idea! I shall be trying it myself this spring.

Good greenhouse/polytunnel housekeeping

In order to maintain a healthy greenhouse environment try to follow these tips-

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Some basic tips to choosing a polytunnel or greenhouse

Why might I need a polytunnel or greenhouse?

Not many of your ‘normal’ vegetables will need starting off in the greenhouse. However if you wanted to give your plants a good head start and perhaps avoid the cold snaps or if they are particular plants that will need more warmth than our barmy English weather provides then starting them off and, in some cases, growing them inside is a good idea. You don’t necessarily need a greenhouse or polytunnel to start your plants off. A light windowsill will also do the trick especially if it is a south facing window that gets plenty of sunlight. The only extra benefits from a greenhouse or polytunnel is that its keeps the mess out of the house, and it will give your young plants all the light they need, avoiding the chance of your plants becoming leggy (very long stems that aren’t strong enough to support the plant) and always leaning to one side because they don’t get enough light, which can happen on a windowsill.

An example of a plant that needs extra warmth to germinate and grow but can be done so in the house is the chilli plant. Because chilli plants come from much warmer climates than ours they will need to be kept somewhere warm to grow and hopefully bear fruit, which they will happily do on your windowsill, especially if you buy the dwarf type plants. However if you're planning to grow tomatoes, cucumbers, melons etc that will need warmth to germinate and grow and also a lot of room a polytunnel or similar, that’s discussed later, will come in handy.

So a windowsill will do fine if you're planning to start of your plants indoors to then plant out later, and also for smaller plants that won’t take over. But if you plan to grow plants the will need warmth during their whole growing season and they tend to grow big then a greenhouse or polytunnel will be needed.

What should I get?

I'm sure you've all seen and know what a greenhouse is. They come in many shapes and sizes and are usually made from glass. They are quite expensive new but if you keep an eye on the classifies, have a look at websites like 'preloved' or 'ebay' then you can often pick up a second hand one. It may need a few panes of glass replacing, usually from the drive home with it!

A Polytunnel is a structure either covered with a particular type of plastic sheeting or sometimes you will see them covered with netting. You can get various sizes from quite small for a garden to huge ones. They are alot cheaper than greenhouses, and in general easier to put up (unless in very windy conditions!) Have a look here to get some ideas. www.firsttunnels.co.uk/ there are lots of sellers out there so take a look around before you decide.

If you don’t have the room or finances for a greenhouse or polytunnel and you don’t want messy plants in the house then there are some alternatives. A lean-to greenhouse is a very good substitute (I have one of these) or shelves with a PVC covering will do the trick. There are all sorts of shapes and sizes out there with a broad price range as well, but you should be able to get a PVC one for about £14. Have a look at Wilkinsons website to get a good idea of all the different types available. www.wilkinsonplus.com/ But again theres lots of choice out there so take a look at different sellers.

Just make sure, especially with the PVC ones that its in a sheltered position or you anchor the frame down, as in gusty wind you may wake up to find the whole thing in another part of the garden!

An even simpler form of the greenhouse is cloches. These usually come in 2 forms. A bell cloche which is basically a bell shaped piece of transparent plastic or glass which goes over individual plants. Or a small tunnel of either transparent PVC or a white fleece material which can have a trade name of ‘Cosygrow’. Both of these but in particular the PVC type will keep young seedlings warm and safer from frosts. It also has the added benefit of protecting the plants in the soil where they will stay so there is no need to move them or plant out from the greenhouse and disturb roots. However they will only cover a small area and so it can get costly if you want to protect a number of rows of plants.

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