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I realize that sunflowers can grow easily from direct-seeding, but to get some earlier sunflowers, I thought I'd try starting them early (since I've heard that works well, too).

I've heard that sunflowers can be sown up to two weeks before the last frost (although they recommend after the last frost if your season is long enough), but what about transplants? Is the same true for them, or do you definitely want to wait until after the last frost to transplant them if you want the plants to survive? I'm just wondering if the young plants can take a light frost.

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    What's a light frost in terms of temperatures? – Graham Chiu Mar 12 '18 at 22:47
  • @GrahamChiu Between about 27° and 41° F. (temperatures you're likely to see near the last frost). It can frost when it's above freezing since they take the temperature higher up where it's potentially warmer. – Shule Mar 13 '18 at 8:53
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Sunflower plants are not frost tolerant, but the seedlings are, they will tolerate down to -3deg C at the cotyledon stage for a brief period; that is why the suggestion to sow 2 weeks before the last frost date is made, because it's unlikely the seedlings will be any size at all if there is a frost and will therefore survive. But once they become small or large plants, they will not tolerate any frost at all, which means your starts should wait until one or two weeks past the last possible frost date. See here https://www.growveg.com/plants/us-and-canada/how-to-grow-sunflowers/

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Shule, you are getting spring fever about now, huh! I've always planted my sunflowers out of doors but only at date of the so called last frost. Here it is Mother's Day, last year it was June.

Are you thinking of starting sunflowers in your grow room and then transplanting?

perennial sunflowers

The only thing I could find was sunflowers do best planted as seed right in the garden. That is all I've known. But it takes a week to germinate and if there is a freeze, that will kill those little plants. They say 2 weeks before frost date but that doesn't make sense to me. I've never tried starts in the garden but I would imagine sunflowers as tough as they are would do just fine as long as you acclimated them from their grow room to the out of doors or green house.

I wouldn't plant until the frost date. I hate losing plants to a surprise freeze. Sunflowers are very tough but baby sunflowers not so much. Or use row cloth on top of your baby plants? That should work just fine.

Sunflower cultivation

Did you know THIS about sunflowers?

Sunflower seeds, leaves and stems emit substances that inhibit the growth of certain other plants. They should be separated from potatoes and pole beans. Where sunflower seeds are regularly used as bird feed, toxins from the accumulated seed hulls eventually kill the grass below. Harmless to animals or people, the toxins eventually biodegrade in the soil.

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From the range of temperatures you posted of 27 - 41 F, then it looks like they'll survive if you avoid frost forming on the plants. Maybe use floating row covers and keep them suspended off the plants.

Sunflowers are most susceptible at flowering and during pollination. In North Dakota, more than 50 percent of the sunflower acreage was in the flowering stages of R5.1 to R5.9. Temperatures of 31 F or 32 F can result in sterile sections or rings in the flowering head, stated Berglund. Once it warms up, the remaining portion of the head will pollinate normally. The sunflowers still in the bud stage, such as R4, are fairly tolerant to frost at down to 26 F before injury. After pollination and petal wilting and drying (late R-6), sunflowers can withstand temperatures as low as 25 degrees with only minor damage. If 25 F temperatures occur at the bud stage, this often will damage stalk tissue below the bud and seeds will not develop.

https://www.ag.ndsu.edu/winterstorm/winter-storm-information-farm-and-ranch-information/farm-and-ranch-crops-general/crops-vary-in-their-tolerance-to-frost

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