Hardening-off question

[michael4spoons]

Eventually I will be taking my seedlings from under the lights and setting them outside to harden-off. Question: after each day's round of hardening, should I bring them back inside and put them back under the lights, or instead should I bring them inside but NOT put them back under the lights?

[nctomatoman]

That is exactly where I am - what I did yesterday is put my seedling flats out in the sun for an hour - (it was calm and about 60 deg) - they then went back under the lights. Today I put them out for about 2 hours in the sun (same temp/still calm) - then back under lights. We may have a warm streak coming - eventually I will have them outside all day (I have very densely planted flats - at least 1000 tomato, pepper or eggplant seedlings, about 2 weeks post seeding - not yet at true leaf stage), probably within a week. Even if the cotyledon leaves get sunburned a bit, the plants will be fine - just be sure that they don't dry out.

[duajones]

I had a sungold that grew quicker than my other seedlings and before I knew it, it had grown into the bulbs, resulting in some burn on the leaves. once i noticed it and raised the light a bit, it grew on fine. I have since hardened it off and it is in the ground. The burns on the upper leaves are still visible but it looks fine otherwise. Should i be concerned or just let it grow?

[nctomatoman]

No worries. One year we had cold (35 degree), wind driven rain for 48 hours. Many of my seedlings looked half dead or more - leaves badly wind burned.....once things warmed up and they took off, they were fine.

Tomato seedlings are quite tough little buggers!

[melody]

I do what Craig does....I do the 'seedling flat dance' every spring. I have a south facing concrete area that is completely sheltered, so flats go out there on nice days for an hour or so.Then they are back under the lights. On nasty or cold days, they stay under the lights all day. It's amazing how sturdy and dark green they get from this short time outside. They also get a taste of actual breezes which strengthens the stems and makes them much easier to handle when i pot them up.

[carolyn137]

When I was growing hundreds and hundreds of plants I did not do the seedling tray dance.

My feeling is that you don't put them out until they are very close to being set out as transplants, so I'd put them outside only about a week before plant out time ( they had been grown cool in a farmer friend's greenhouse).

It seems to me that anything one might accomplish in terms of hardening off could be reversed by continually babying them taking them back inside. Such as toughening up the leaf epidermis, for instance.

Of course predicted frost or hard rains are sufficient reason for either covering them or moving them.

So I guess there's more than one way to look at this hardening off process and what it accomplishes and why so many folks do it, as do all of my commercial farmer friends, so it can't be all bad.

[nctomatoman]

My particular dance doesn't last long - just two or three times - then it is outside, all day, all the time. In fact, those seedlings that are a bit late to germinate and come up in the sun are just fine, since they never did get a taste of the coddled, grow light life. As I type, I think mine are about to head outside and start their full time sunbathing (65 lovely degrees here)

[melody]

I don't do the dance as long as I used to....several years ago it lasted for several weeks....now it's just a few times before they go out 24-7....about the time they get substantial first true leaves and before I pot them up with their second ones...I'll set them out.

Once they are potted up, they live outside full time...(although I've had to be creative about covering for frost) The only time I consider bringing them in is if tornadic thunderstorms with hail are looming on the horizon.

[feldon30]

I have burned several plants by pushing too hard.

1 hour the first day, 2 hours the next day, 3 hours the third day, and then 4 hours of mid-day sun when it was a surprising 83F. And then today they were out from 8am to noon and some more are burned. So I don't get it. I didn't lose a single plant last June.

[amideutch]

This was taken out of the book "Greenhouse Tomatoes, Lettuce & Cucumbers" by S.H. Wittwer & S. Honma where they recommend 'Cold Treatment' for hardening off tomato seedlings.
The cold treatment should be started just as the first true leaves emerge, whether the seedlings are still in seed rows or pricked-off. Air and soil temperatures should be lowered to 52 to 56 deg F for ten days to three weeks. A ten to twelve day cold treatment is adequate during periods of good sunlight. Three weeks are usually necessary in the fall and early winter when most of the days are cloudy and plant growth is slow. The amount of cold during the ten-day to three week period is more important than the time of day in which it is given. Cold exposure during either the day or night, or both, is effective. Night temperatures of 52 to 56 deg F are recommended when the days are sunny and partly cloudy.
Following the cold treatment, night temperatures should be raised to 58 to 62 deg F. Cool daytime temperatures (60 to 62 deg F) should be maintained in cloudy dull weather. On bright sunny or partly cloudy days, temperatures of 65 to 75 deg F accompanied by good ventilation are suggested.
Tomato plants properly exposed to a cold treatment develop large cotyledons and thick stems, with fewer leaves formed before the first flower cluster, up to double the number of flowers in the first, and often the second clusters, and higher early and total yields.

From what I have read this technique is still used today by commercial greenhouse growers and should work just as well for us non greenhouse growers. Ami

[feldon30]

Thank for the advice, however I have no way to provide 52-56 degree temperatures to my tomato plants for even a short period of time.

[amideutch]

Your right Feldon. This technique is not for everyone and not all Tvillians live in Texas. Ami

[carolyn137]

Tomato plants properly exposed to a cold treatment develop large cotyledons and thick stems, with fewer leaves formed before the first flower cluster, up to double the number of flowers in the first, and often the second clusters, and higher early and total yields.

From what I have read this technique is still used today by commercial greenhouse growers and should work just as well for us non greenhouse growers. Ami

*****

Ami, I first ran across that technique at a Canadian site many years ago.

Some commercial farmers who have the ability to shift temps around like that do use it, and I get the impression it's mainly those who are into hydroponic growing, as I think you might have been suggesting, but I don't know of any field oriented farmers in my area of the NE who use it.

The Cornell Coop Ext does an excellent job of keeping the farmers in this area up to date on all new and promising methods and they never suggested using it for the non-hydro folks.

But if you can do it, then I'd say go for it.

[amideutch]

Actually it was developed for greenhouse culture before hydroponics came onto the scene. And when they went from soil growing to hydroponically growing tomatoes the process followed. I learned about it from my past interest in growing hydroponically using the NFT method. More than one book mentioned this method of hardening off tomato seedlings. Ami, doin it on the other side of the pond.

[Ruth_10]

I think it would be hard to generalize between seedlings grown in a greenhouse and those under lights. Even on a cloudy day, the greenhouse seedlings are getting a lot more light than the shop light plants. There would therefore be much less hardening off to be done with greenhouse grown seedlings.