Sowing in high temperatures

[Primrose]

We all know about not sowing seeds when the temperature is too low but what's the best advice about sowing when the weather is hot, i.e. like it is today at 28 degrees? I sowed some carrots yesterday morning and obviously watered them in well, but a few hours later the soil was completely dry again and very hot to the touch. I'm wondering at what point seeds actually get scorched and killed by the heat?
I've found in the past that it's useless trying to sow lettuce in very hot temperatures as it never seems to germinate in hot soil. How do you get round this problem in hot weather for successional sowing?

[Nature's Babe]

Plant in tubs or modules and find a cool shady spot in the garden perhaps?
Gee it was hot today, recent plantings were wilting a bit.

[alan refail]

I've found in the past that it's useless trying to sow lettuce in very hot temperatures as it never seems to germinate in hot soil. How do you get round this problem in hot weather for successional sowing?

As regards lettuce, you have to overcome "high temperature dormancy" - with soil/compost temperatures above 25C germination is very poor.

However, germination takes place a couple of hours after sowing, so if you sow in the late evening in wet compost or in a drill watered with cold water the germination time will fall during the cooler hours of the night.

[Johnboy]

Hi Alan,
Lettuce begin to lock up as regards germination at 20C.
When I was growing plants for sale just one period of high temperature at the begining of February rendered several thousand lettuce useless.
They got slung out of the tunnel and laid aside and forgotten then in October there was an explosion of germination which was far too late for my market but we had lettuce leaves right up until Christmas and a little beyond
The way to pursue sowing lettuce seeds in hot weather is to pop the seed in the fridge for about an hour prior to sowing and then as you say go on night work as they are best sown around dawn or just before and then kept protected in a shaded condition and certainly nowhere near a tunnel or greenhouse! Certainly quite damp compost will give a chill factor as the evaporation takes place. Always have the seed trays at ground. level.
JB.

[alan refail]

Lettuce begin to lock up as regards germination at 20C.

I would agree entirely - I always try to work to something below 20C.

[Primrose]

Johnboy - thank you for that helpful information. This certainly explains why I've often had failures sowing lettuce for succession in hot summers and I'll certainly sow my future succession lettuce seeds in trays which I can keep in a cool shady place, rather than sowing them direct into hot soil. I suspect many people may not know this and wonder why their seeds haven't germinated. One could easily be tempted to think that nice hot weather would facilitate them germinating more quickly rather than not at all.

For future reference, are there any other seeds which are affected in the same way, or is the 20 degree temperature common for most vegetable seeds?

[Johnboy]

Hi Primrose,
There are many vegetables that people use propagators to germinate who quite frankly are wasting their money.
Certainly things like Peppers, Tomatoes and Aubergines require heat but I have never used heat for growing anything that is natural to our shores.
I have just done a Compo with a batch of Streamline Runner Beans cos they have all rotted in the pot. Strange because I planted some Scarlet Emperor at the same time having given them exactly the same treatment and I have a 100% germination! Both were of newly acquired seed but I suspect that the Streamline is old seed. I have just been out and purchased some fresh seed from a different and hope a more reliable source.
It is most annoying because I shall be late in planting out, generally end of first week in June here.
The one thing that I do put in a propagator is Parsley and germination is initiated in the dark and when a few initial germination's (Wispy white ones) appear I then take them out of the propagator and put on the bench and in about a week there are hundreds lovely green seedlings.
Apart from that I do not see heat is necessary for the majority of vegetables generally grown in UK. A particular Thyme gave me a problem and I gave that the same treatment as the Parsley.
JB.

[Primrose]

Johnboy - certainly agree with you about rotting runner beans. Not a single one of my Enorma has germinated this year. I'm now also trying the wet tissue method with some Enorma and Polestar. Both varieties are last year's leftover seed and if nothing happens soon I too will have to go out and buy some fresh seed. I'm beginning to wonder about the freshness of some of the packeted seed we buy. Many of us have surplus seed left over from a previous year and if the packeted seed isn't fresh when we buy it, by the time we get round to sowing it could be three years old!

[peter]

It's not the sowing in high temperatires that got me this weekend.
It was the Squash seedlings in the lidded propagator in the greenhouse which literally cooked.
Thankfully I have a little shed behind the greenhouse and under the coppiced hazel's shade where the survivors are (hopefully) recovering.
Good job I have more seed though.

The lack of rain is causing issues outside with the lovely sunshine, even the allotment track has gone brown, which does not normally happen until July.

[alan refail]

It was the Squash seedlings in the lidded propagator in the greenhouse which literally cooked.

Hi Peter

Going back to Johnboy's informative/inspirational post re propagators - I never start squashes/courgettes in the propagator. Warm, inside (polytunnel) conditions and they always grow.

[Johnboy]

Hi Alan,
I chit all squash seeds, plant them scar down and simple leave in the tunnel. I water the compost prior to planting then sow the seed then cover with compost and spray the surface. I have a very good strike rate usually.
JB.

[alan refail]

Hi Johnboy

I have had 100% germination this year - which was actually more than I wanted - Sod's law

[Johnboy]

I will sow my Courgettes this week and there will be no check in the growth and they will produce within about a month to five weeks. Beats all the pussyfooting some people get up to. Unnecessarily I suspect.
JB.

[Mike Vogel]

Yes, absolutely, JB. Another crop which likes the hot weather for sowing is Florence Fennel, as shown last year on Gerdener's World - or it may have been one of the other programmes. I gat a lovely row by sowing in July or August - and then they bolted inthe dry spell that followed.

[John P]

Alan and Johnboy,I have known about the problem of trying to germinate lettuce seed in hot conditions for quite a few years,and have tried various ways to overcome this,not always with great success let me add. Your methods are the best and most sensible that I have come across,another example of the top tips to be had on the Forum. JP