Why can tender plants not be kept out at night this week?
Maybe a silly question, but I'll ask it anyway.
I constantly read about "plant out when risk of frost has passed".
There's not going to be a frost over the next couple of nights but I see experienced forum members today advising people to bring in, or cover, their tender plants. Why is this if the temperature is not going to drop to freezing?
Is it because 2 or 3 degrees still presents a threat to plants, and it doesn't actually need to get to zero for tender plants to be lost?
Any help appreciated in clearing up this confusion.
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For about three weeks I have had tender plants such as runner beans, petunias, cosmos, pelargoniums and didiscus outside. True some have been in cold frames and I close the lids if temperatures threaten to fall to 3° C as they did about 10 days ago. More recently, with night time temperatures of 6 or 7°, I rarely bother closing them. Other plants are not even in a cold frame: they are placed against a raised bed and a sheet of glass is placed over them and rests on the ground making a kind of triangular tunnel. The ends of the tunnel are open. No plant has come to any damage though they might be bushier and more advanced if I had the luxury of a greenhouse. I don’t.
Often repeated is the maxim ‘wait until all danger of frost is past’. The forecast for the next 6 days suggests it will not fall below 6° so well above the frost level. This takes us to about May 19th and the longer term forecast beyond that - and in it I place little faith - says there will be almost no chance of a frost. I risked it. I planted up some wall planters with pelargoniums and verbena today. In the next few days I will be planting out the runner beans and dahlias. I would also plant out the cosmos but that means taking out beds of wallflowers which are still showing nicely.
Are people able to report back and say and say they have been caught out in past years by planting out tender stuff around now? Against the front, South facing, wall of the house I did not bother to lift the dahlias and instead covered them with leaf mould. They’ve survived, perky as ever. Also I had a 12” pot of petunias which I cut back at the end of summer and popped it in the cold frame, the lid wedged slightly open all winter to give a little air movement. They coped with temperatures of -6° C and were growing on strongly by mid spring. The petunias in the hay basket on the front of the house were left out permanently through frost and snow but most are robustly and unaffectedly growing.
In short, is the protect from frost advice overstated?
In April I took nearly 50 cuttings from the petunia and, apart from one, all are growing well. My technique was to put a 2” piece of stem into a 7 or 9 cm pot filled with compost mixed with vermiculite. I then covered the pot with a little clear plastic beaker from the canteen water dispenser. The cover stayed on for around 3 weeks and the pots were placed in a light potting shed but not in direct sunlight. Once they had obviously rooted I moved them back to the cold frame or into the triangular tunnel mentioned above.
Also in my experience some tender plants are more tender than others... while pelargoniums and petunias seem to cope perfectly well with some low temperatures, have you ever seen the leaves of a Morning Glory when they’ve turned white because of low temperatures? I have ... it’s a sad sight 😢.
In my experience planting out tomatoes, chillies, ipomeas, runner beans and sweet corn too early, before nighttime temperatures are up around 10C, causes them to sulk and stop growing ... or worse.
Better safe than sorry.
This photo is of some busy lizzies that are tucked against french windows, and I forgot to cover them last Saturday.....
Our climate is more extreme than yours admittedly. However we stick to the folklore of never planting anything out until after the Saints des Glaces have passed. Or Ice Saints in english....St Mamert 11th May, St Pancrace 12th May, St Servais 13th May. Apparently this particular bit of folklore is statistically sound!
Plants from the garden centre l tend to err on the side of caution and keep them in the coldframes or by the wall for at least a week before planting out. As with everything, it's a risk you take. I was awake at 3 am this morning, stuck my head out of the window and it was chilly with high cloud, but not frosty. My hanging basket that has been in and out of the greenhouse and was put out permanently yesterday is fine. I certainly wouldn't plant out my runner beans or tomatoes until June though.