A completely closed sunflower will often not open after cut. A good percentage of the petals must be starting to lift before cutting. Personally I prefer cutting when the sunflower is a tight cup shape, or that all the petals have lifted.

Closed sunflowers do open. Years ago, After a hurricane flattened a field of sunflowers, we went through and cut all the stems so they would not curve. 90 percent of those bloomed over the next week or so. We cut all of our Sunflowers tight. (not yet lifting of the center disk) Less damage to petals while being delivered to florists. They usually are fully open for me in three days.

Thank you to both for your responses.

I'm really puzzled why my cut sunflowers wouldn't open...????? The plants grew very well and bloomed plenty. Flowers had no problem opening fully on the plants.

I never cut any that were still tightly shut, only the ones about 70%-80% open, meaning the petals were already pointing outwards. Then I put them in the house which was VERY warm this past summer, which should prompt cut flowers to open(?).

Following the logic that perhaps the cut flowers needed more energy to open, I tried adding sugar and bleach to the water, but then I read here that sugar would over hydrate sunflowers so I stopped using sugar, and the sugar didn't make the flowers open anyways. I guess next year I will try the sugar method again and see what happens.

Happy TG everybody.

Same experience here: once cut, sunflowers failing to continue to open. Did you find a reason why?

Edit: Just read in The Flower Farmer by Lynn Byczynski, page 137, “I have found that some sunflower cultivars are less receptive than others to cutting early, so I recommend that you experiment with a dozen or so of each type that you grow in order to find out just how early you can cut.” She cuts some as early as before the flowers even unfurl. The lack of opening after cutting I have experienced was on a yellow variety we grew (forget which variety), but also saw it happen on a yellow sunflower my mother received in a gift bouquet, and a local florist who only gets a yellow variety says they never (or was it only sometimes?) open more for her if cut before open.

Lynn continues, but this is referring to flowers generally, not necessarily to sunflowers specifically, but it is in the paragraph following the quote above: “It’s better to cut unopened flowers in the evening, when their stems are full of starches and sugars that will help them continue to open. You also should use floral preservative [but maybe not for sunflowers, see note below*], which contains about 1 percent sugar. Some preservatives can be used at double strength to prompt buds to open; check the label. You can also make a bud opening solution that contains 2 percent sugar by adding 5 ounces of sugar to 2 gallons of water. Leave the flowers in this solution in a cool place out of the sun (but not in the cooler) until the flowers open.”

*On page 17 she writes: “Sunflowers will do fine if you put them in plain water—research shows that only a few cultivars do better in water with floral preservatives added than in tap water.” So, I’d choose to skip the floral preservative when trying to open sunflowers, and just try the “opening solution” she suggests above.