This is a much asked question.
The garden centres and on line retailers have summer bedding ready to buy, but is now the right time?
The short answer is no, or no unless you can grow the plants on under glass. Bedding plants are tender, which means they are not frost hardy, and that means any frost or snow, which some parts of the country are suffering from, will kill the plant.
Bedding plants cannot be planted out until all risk of frost has passed, which may be mid May in sheltered parts of the country and end of May in the colder areas.
This spring, with the odd exceptional day, has been cold, windy and wet. The growing season seems to be several weeks behind the norm, and some garden centres are not awash with bedding plants because they too are struggling to grow in the low light conditions. Many of our popular bedding plants originate from sunny warm countries, like South Africa. They are grown here in warm conditions inside an insulated greenhouse with good growing conditions, the right amount of light, warmth and heat.
What do we mean by bedding? The most popular bedding plants such as Pelargoniums known as Geraniums, Verbena, Petunia, Begonia, Lobelia, Marigolds, these are all tender.
Tender veg are cucumbers, squash, courgette, tomatoes, french beans, runner beans, chilli, and herbs such as Basil, all need frost protection .
Putting them outside, even in cold but frost free conditions, is not ideal as it's just too cold, a shock and the result can be to temporarily stop the plant from growing delaying development and flowering. If you buy bedding plants now, and any of the tender veg, they need to be kept under glass in the warmth. My conservatory has been so cold some of my tender seedlings had already succumbed to the cold, and white patches were starting on the tomatoes so I had to put a heater in for a few hours to take off the worst of the chill.
Many of the summer climbing annuals are tender illustrated second image, "Cup and Saucer plant" latin name Cobaea scandens, is beautiful, but so tender that even a chill spell will severely upset it, but then it comes from Mexico and tropical South America. Another popular summer climber is "Blackeyed Susan" latin name Thunbergia, again very tender, and it hails originally from East Africa. When you consider where some of our bedding plants and summer climbers originate from it makes sense why they should not be planted out until frost has passed and the weather is warmer.
It can depend where the plant comes from. Contrast with the Cornflower which is native to Europe and hardy so although it needs warmth to germinate it will withstand much more difficult weather conditions.
Where the plant comes from helps but it is not an absolute rule of thumb and it doesn't always follow that all plants which originate from warmer climates are necessarily tender. Sweet Peas, Lathyrus odoratus, originate from temperate regions in Africa and South America, but they are hardy, and once "hardened off" can be planted out side . It is important to harden them off properly as even though frost hardy, they will have been grown in ideal conditions. Putting them outside into our Spring conditions can shock them and when this happens often their leaves turn very pale, almost white, as a result.
To harden off plants is to gently acclimatise them to the weather outside. Sweet peas would be hardened off earlier because they are frost hardy. Tender plants need to be hardened off later because they will not tolerate low temperatures. To harden off plants place outside on milder days bringing into the green house on colder nights, gradually leaving the plants out for long and longer as the conditions allow, until ready to plant out. No sudden shocks.