September 19, 2008

Last year there was considerable interest in double cropping sunflower behind wheat. In one instance on June 30 south of Lexington the combine, sprayer and planter were all in the same field together. This year the wheat harvest and everything else was 10-14 days behind normal. The following information is based on a producer question and my response regarding potential frost damage to sunflower.

Q

Given a late wheat harvest and cool fall, the second crop flowers are behind. They were in full bloom as of last week. I know that eight days of 80°F will help, but there are several questions: How cold does it have to get to kill the pollen if not all pollinated? How cold does it have to get and for how long to kill the sunflower plant?If we are below freezing, how does it affect the sunflower seed? How many days do we need to achieve full maturity after full bloom or GDUs?

A

I was just talking to a producer up here — we had some ice and frost reported Saturday night. We are at or close to our average frost date but September has been cool without many growing degree days.

The thought was that frost would inhibit the pollen due that next day — it could be like irrigating during pollination and washing off the exposed pollen. If the frost went further and stopped pollination, your flowers should be nearly finished with just the middle left — that would be a small percentage to lose.

The damage will truly be a function of temperature. It is thought that a frost or temperatures down to 30°F would impact the exposed pollen that would come out that day (like any other environmental or manmade pollination inhibitor).

Pollination is a ring process that starts on the outside and works in. Filling of the seed starts immediately after pollination so the entire process is concurrent. A pollination interruption could produce a ring of empty shells that would not be filled. Sprinkler irrigation can be a factor here — we usually recommend not irrigating in the morning since 80-90% of pollination occurs by noon. If the pollination interruption is more severe, such as from colder temperatures, more rings could be involved. If this happens and the inside rings are not filled, a high percentage of the seeds are pollinated and will fill.

Stay in contact with your contracting agent so they know what is going on. A freeze can produce discolored confection seed — the bottom part turns brown with little meat in that part. While there are differences among hybrids, generally a killing freeze is described as 24°F for two hours. What we want, then, is warm Indian Summer weather. Sunflower has a temperature range of about 10° below and above those for corn.

Information from North Dakota State University states: "Once pollination is completed and 10-14 days after petal drying occurs, the sunflower plants can withstand frost temperatures as low as 25°F and have only minor damage. Twenty-five degrees Fahrenheit at the bud stage will often damage the stalk below the bud and seeds will not develop. If hard frosts do occur, many times only the seed in the center of the head (the last to pollinate) will be affected."

Frost is more severe for confection sunflower since discoloration can occur and some are heavy enough to stay in the combine. The meat formation or filling in these seeds is just partially complete. A visual problem is a great concern for the confection processor.

Oil sunflower producers will just deal with lighter test weight and less oil percentage. The oil is the last thing to set in the filling process. The incomplete seeds will contribute to the lighter test weight and lower oil percentage. Bird seed processors take lower oil seeds but they must have enough test weight to fit into their specific sized bags.

Bill Booker
Extension Educator, Alliance