I am concerned about heat delay.

Can you please define what temps would cause heat delay. (high temps is a relative term)

How many days (or nights) would be needed to cause it.

What days if any, of the crop cycle should be considered?

Thanks!

This is a timely question. Rebecca Schnelle is the graduate student who is doing our heat delay research. She is looking at Prestige meristems today and tomorrow to see how much delay this recent hot period has caused compared to past years. She will post some results in a couple of days. Also, she can provide some new information on the critical times in development when poinsettias are most sensitive. Jim

I will be looking at meristems a later today, but I can answer your questions based on the last few years' research.

I've found that daily average temperatures of 80F or above cause heat delay. So if your greenhouses are typically 85 or above during the day and 72 or above overnight you will see heat delay in your crop.

You should consider the time when the plants are sensing short days and initiating. So if you are expecting an initiation date of October 1, high temperatures between Sept 15 and Oct 15 could cause heat delay.

In general the amount of heat delay you can expect is about half of the time the plants are exposed to heat delay temperatures. So if you have high temperatures for two weeks you may see one week of delay from the expected schedule.

**Of course some cultivars delay more than others so this is only a general guideline.

I'll be back with the update on this year's Prestige crop

Rebecca

Rebecca I have a fairly good microscope. Assumming that is what is needed to look at the meristem. Would you be willing to share some inforamtion on how and what I would be looking for to determine the development of our crop?

Looking at meristems to determine initiation requires some practice. It took quite a few experiments to get it down. Also the process is pretty time consuming, so for most growers waiting to see first color or visible bud is more practical.

That said, if you do want to check for initiation you certainly can if you have a dissecting microscope with 30X or more magnification.

What I do is harvest a lateral branch and remove the last 1cm of the shoot tip and preserve it in a fixative. Then when I have all the samples I want, I remove the unexpanded leaves under the microscope until the meristem is exposed. (This is the tricky part, you'll end up destroying some meristems)

The vegetative meristem is very small with a round, domed shape. It can’t be clearly seen with 30X magnification. Just before initiation, the meristem becomes flattened and large enough to be seen at 30X. At initiation, the meristem becomes larger and has a pyramidal shape. That is the change I look for in my experiments.

I collect 6 shoot tips per cultivar every other day for my experiments which equals a whole lot of meristems, but if you wanted to get an estimate taking sample from a few plants once or twice a week would work. When you're learning what to look for, it's easier to start with shoot tips closer to visible bud and work your way backward in time to initiation.

Prestige Update:

In FL we've had above average temperatures in the past few weeks and as expected Prestige initiation is a bit delayed.

My research is in a fan & pad cooled glasshouse. In this environment Prestige initiation was about 4 days later than last year (October 12th this year vs Oct 8th last year). I also checked a crop grown in an open-sided house. These plants did not show any initiation as of Friday October 12th. Of course I didn't check this last year and my research plants usually do finish slightly earlier than those in the naturally ventilated house. So nothing to get too worried about yet. Also this weekend's cold front should help things along.

I'll take a look again this thursday the 18th.