Can I use my unheated greenhouse now?
I think it depends on where you live, with experience you will find the right time, but what if you are a new gardener how will you know?
The best option is to ask some local gardeners or have a look down the local allotments and see what others are up to. You could ask online but you will get wildly varying opinions, and as it turns out some some very unhelpful advice, like you are too soon or too late.
Perhaps the third option is to use a bit of science, you can look up the average temperatures in your area using the Met office site.
All plants have a optimum temperature they are happy with and a base temperature where they will stop growing, how does this help? check the local average temperatures it will help you to plan.
For example everybody wants to grow tomatoes and is keen to get them planted as soon as possible.
So a tomato is a good example it is happiest at 22 degrees and has a base temperature of 7 degrees this is when it stops growing. So ideally when we plant, the daily average must be above 7 degrees or it will die and in practice you need double, about 14 degrees to see some growth. Our unheated greenhouse is heated by the sun and can store heat at night so typically it will be 5 degrees warmer than outside during the day and about 2-3 degrees warmer at night. Now if you look at the Met office map and check the average temperature for your area you can gauge the best time to plant.
For tomatoes we know 14 degrees is about right, so taking 3 degrees off for the greenhouse effect you will need an average of 11-12 degrees. You can still be caught out by a sharp spring frost, keep some fleece handy or grow them in pots that you can move if the weather turns nasty.
For most summer bedding you need 8-9 degrees for growth
Remember your tomato plants are happy in the house, often about 20 degrees. Keep them on a sunny window sill for 4-5 weeks, this means you can sow a good month before you plant, if the weather is unusually bad they will be happy in the greenhouse during the day and back into the house at night or have some fleece handy to cover them at night.
What about Summer bedding?
This can be split into two groups hardy and half hardy (damaged by frost)
Hardy annuals can be also be sown directly into the soil and are tough, which means they can be sown in an unheated greenhouse as early as February and can be planted out from March.
Half hardy annuals need a bit more care, now we need to consider the optimum temperature, remember you can start them off in the house.
Petunias are a good example they have a optimum of 17 degrees and a base of 4.5 degrees so ideally we need 9 degrees to allow them to grow, just check the map.
The majority of summer bedding is in this base temperature group, the main exceptions would be dahlia, impatiens, salvia, begonia, celosia and new guineas. These have a base of about 7.5 degrees so would prefer 14-15 degrees to grow properly, the sames as the tomatoes.
You can also use the unheated greenhouse to give your vegetables a boost, get some early crops, grow them in pots or trays for planting out in March they can crop a month earlier.
