Fluctuating temps to promote germination?

Hello friends,

I’ve only been hybridizing for a few seasons, but have found this forum SO helpful when I encounter an obstacle. Thanks for your help!

My latest question: I have had several of my seeds from 2020 germinate in my fridge in plastic bags over the last few months, however most seeds have done nothing. I’m considering putting them out in a garage for a while to cycle them through a different temperature, but am unsure for how long I should do this. The spring weather in CO is also challenging, going from sunny and 70 degrees this past Mon to snowing and 29 degrees yesterday. I’ve heard that cycling seeds for a week at 50-55 degrees and then putting them back in the fridge could work. I’ve also heard that higher temps may arrest germination. Does anyone have any tips for me as I embark on this experiment? The week ahead doesn’t have any more freezing temps, but the high might be 80 here in Denver. Will my seeds be harmed if they reach 60+ shaded degrees for a few hours or days? Thank you in advance for any/all suggestions!


My experience in a significantly different climate (Sydney, Australia), I don’t stratify because I just make mold (and don’t really want mold in the fridge) and I don’t need to for whatever reason.

Heat doesn’t seem to be a problem other than germination doesn’t seem to happen over a certain temp. I’ve planted seed (outside in direct sun) in the beginning of summer (so have experienced temps up to 111F/44C during the following months) and nothing germinated until autumn when daily minimum temps started hitting 59F/15C and below (and pretty much straight away when temps reached that point). Potentially reduces some germination but wasn’t a noticeable reduction if any occurred (but again, I don’t stratify so my baseline may be different). Given the fairly consistent behaviour over the last several years of the nothing until 59F/15C and below, it seems to be the max temp germ happens under but gets better/increased rate at slightly lower temps (but could also just be duration being in cool damp)…either way I don’t fuss about it, lazy :smiley:

I have germinations happening every day at the moment outside (autumn here) with daily temps ranging between 55F/13c to 86F/30C so I don’t imagine your current temperatures being a problem as long as there isn’t any surprise freezes (no experience with freezes, so no idea if it’s problematic or to what extent).

I stored mine in the fridge until germination in the bag, then I took that bag out of the fridge and moved it to a cool room. Temperatures ranged from 50s through 60s. If germination slowed down after a bit, I put the bag back in the fridge. (I have done overnight, cycling like day and night temperatures, as well as anywhere from a couple days to a week back in the fridge.) Each time I got the bag back out of the fridge I would have more germinations over the next few days. I have worked to keep them below 70, so I can’t say what would happen at that point. Also, perhaps the genetics you are working with may be different than the ones I am using.

Thank you both so much for this helpful information! It looks like I’ll just need to start experimenting with temps above my refrigerator settings, yet below 70, and document my results. Hopefully I’ll see some movement in some of the other crosses I have seeds for. Two varieties were very successful with almost all of the stratified seeds sprouting, but most have continued to look the same in their bags for 3-5 months. Maybe I’m just too impatient! Thanks again. :slight_smile:

Something to consider for the future, perhaps not when you are first starting out, is embryo extraction. You can look it up on this forum and find a well written article for instruction. It does require time, so that may be a factor. I believe many people keep track of how the seeds germinate and choose seed parents that are easier. That may or may not fit with your goals.
I have been busy extracting seeds for the last.couple of weeks that have failed to germinate and have had some success from it.

Hi Duane,

It’s funny that you suggested the embryo extraction because I recently printed out and read all of the instructions here to do so, but was so overwhelmed that I just set them aside for the time being. I feel like I need to watch a video or see it happen in person to feel really confident, but practice makes perfect and it can’t hurt to try to extract from the seeds that aren’t germinating anyway, right? Thanks for the encouragement and congratulations on your success with it!



A fellow rose grower, Werner, posted some videos of his work with extraction of embryos from rose seeds. He uses a bonsai tool (expensive, but well worth it for this kind of work). It’s also good to get a binocular dissecting microscope so you can see what you’re doing and maybe avoid cutting off your finger tips or destroying seeds. You might be able to find both the bonsai tool and the microscope used. So much the better.

Here are several of his videos:

Good luck.


Thank you, Peter! I really appreciate those tips and will certainly check out Werner’s videos. Exciting!

Hi Sue!

Great discussion! I’ve found embryo rescue difficult, but I don’t think I’ve given it a fair chance yet, having only tried it a couple of times with one person’s technique. I’m planning on trying again, but in the meantime, I have a somewhat standard temperature fluctuation regimen that I put my seeds through. I did a two-year experiment with hundreds of seeds from several species roses where I gave them zero to 60 days of moist room temperature pre-treatment followed by 30 to 90 days of moist cold treatment (34 F), and then alternated between the two conditions for up to the entire two-year period. I don’t have a full evaluation of the experiment yet, but the trend I saw was that some period of warm pre-treatment shortened the total time to germination. This may not translate to garden roses fully because of the Rosa chinensis influence in many of them, but the seeds from my latest cross showed 50% germination after two months of room temperature and then less than three months of 34 F (they germinated just before I checked at the three-month mark). Slightly less than 150 days seems “quick”.

Here are a couple of papers that gave me the inspiration:

Germination requirements of Alaskan Rosa acicularis - https://www.frames.gov/catalog/3899

A Century of Rose Germination - A Century of Rose Germination | PDF | Seed | Germination

I hope this helps. Although it might help you to decide to practice embryo rescue… :slight_smile:


Thank you so much for sharing this experience, Tony. It motivates me to continue with my own trial and error, as well as eventually go after some embryos! Thanks also for attaching the paper & link for the Alaskan rose info.