Floaters vs Sinkers

I've read that acacia seeds should sink in water if they are viable.

I have an acacia tree with seed pods in the garden, the pods are splitting and dropping seeds.
I gathered seeds from pods that had split, scarified them - some with sandpaper some with blade, gave them boiling water and let them soak for a day.
I didn't know about the floating vs sinking thing at the time.

A month after sowing with temps ranging from 15 - 35C, I have ZERO germination.
I've grown many trees from seed including ebay-sourced acacia and other leguminous trees, with some success, so although I've got a lot to learn, I don't think user error alone accounts for 0% germination.

This time round I gathered 100 seeds from the same tree, and gave them the float/sink test.
29 sank, a couple couldn't make up their mind. The rest floated.
So I'm planting 29 floaters and 29 sinkers and hopefully will get some kind of indication of whether this is a good test to filter out unviable seeds.

After some further reading I've decided not to scarify. Enough bloggers report success just with the boiling water + soaking that it should work for me if the seeds are good. I'll soak till the seeds show an approx. doubling in size then plant in two identically prepared pots and as far as possible take identical steps to coax them into germination.

Any tips or comments gratefully received as I'd really like some man-height acacias within a couple of years.

Even if it comes highly recommended, I would not cook any seed that I wanted to germinate. My experience with gardening and the Internet, is that there are a lot of rumors that get repeated so often, by people that have never planted a seed in their life, that it seems like gospel truth... If I were to give acacia seeds a boiling water treatment, it would be for a few seconds. For example: http://www.fao.org/docrep/006/q2190e/q2190e07.htm states that "A. acuminata and A. pycnantha will withstand 100°C for a maximum for 5 seconds (Harding 1940) and A. terminalis for a maximum of 30 seconds (Clemens et al. 1977). For many acacias immersion in boiling water for more than 30 seconds is detrimental."

The sink/float test works ok for some species of seed under some circumstances. There are so many variables involved that sometimes the float/soak test may not be definitive. Looking forward to a report on how it works for you with your seeds.

really hot water is better than using boiling water, for anything that you think needs that kind of hot water treatment. like the hottest water that comes out of the tap, IMO, that is safer. though there are some seeds that people say need this kind of treatment, i wouldnt ever use boiling water. i have frequently used really really hot but not quite boiling water, usually only if the seed gets funky, and throw in a bit of H2O2. i have tried to sprout a few seed that require heat treatment, here there are some fire dependant species, they usually require some kind of heat treatment, but still do under boiling temps.

i have no experience with acacia, but almost all trees, and many perennials, are harder to start, get lower germination because the trees produce more "duds" naturally. every tree i have ever started requires cold stratification. so either put them in the fridge in moss, or sand or paper towels/etc, for 2-4 months, or start them in early winter and let nature do the stratification naturally outside. that is if you get enough cold. we dont always get reliable cold here, so i do sometimes start them in the fridge...actually i usually do both, just in case one works better than the other. both methods have worked for me, even when we dont get those cold winter temps.

another thing i could add- is that sometimes it takes a while for the seed to sink. for the first few hours they float, if you leave it overnight you find the next day they sank. i plant them all anyway, i do think sometimes theres a floater that will sprout. but it indicates if the seed is mature enough usually, the floaters float because they werent quite done and ready. so its a good test, say if you get seed from a questionable source, you can do this with a few to test the seed lot. if most float and stay floating, that seed source is not very good probably.

I'm fairly comfortable with the boiling water. I have always done it with acacias and some of them have grown. Here on the island we have frequent wildfires and I think a few seconds of sub 100C water is nothing compared to the roasting they get in a forest fire. It never freezes here so I'm not tempted to use cold strat, but previous successes have involved sandpaper scarification and recent failure included some blade scarification. I'm thinking that might be a factor, and another factor might be I didn't soak the seeds long enough.

Could also be that boiling water after blade scarification killed the seed?

Anyway, let's see how this experiment goes and maybe next one I will do a side by side of boiling water vs hot tapwater

I've just checked on the seeds, which are still soaking.

All the floaters are still floating.
About half of the sinkers are now floating too.

The floaters have dyed their water an attractive pee yellow.
The sinkers have dyed their water about half as much.
Identical cups, same number of seeds, water quantities about equal.

Quite noticeable swelling, nearly doubled. I will plant about 12 hrs from now.

The 42 extra floaters from the original 100 also got boiling water and about 20 hrs soaking, and have just been scattered all over the garden, where I've got stuff that gets watered.

Sinkers on the left, floaters on the right. After this foto, they got a covering of fine soil just enough that you can't see the seeds anymore.

It's unseasonably cold last few days, about 18C at night and only getting into low 20s during the day so I'll leave them in full sun till it warms up then move them to part shade.

As I was handling the seeds, I noticed the sinkers felt solid and the floaters felt hollow and mushy. Not in every case, but a fair generalisation.

Ten days after planting seeds...

Three germinations became visible today, all from seeds that were sinkers.

Follow up experiment underway.
A picture is worth a thousand words.
If you want words as well, more detail here: http://www.facebook.com/TFSForest

These 80 seeds soaked for about 50 hrs and are now out in pots. The floater vs sinker experiment saw 3 germinations on the 10th day, 3 more on the 11th day, and none since.
So let's revisit this one in ten days or so.

Am already thinking up my next follow up experiment for when the results of this hot stratification test are in.

First seedlings sprouted on 8th day and more until 14th day. Germination rates for each temp..
20C: 15%
40C: 20%
60C: 15%
95C: 35%

So 95C looking good, but with such low germination rate overall I'm thinking the seed batch wasn't great.
By the way 95C was a guess. I boiled the kettle till it cut out automatically and poured the water into a cup of seeds. Coulda been 90 when it hit the seeds, coulda been 99.

I chose seeds from pods that were still hanging on the tree but had partially split open. Thinking being they are ready and fresh as possible.
The only caveat is the tree was quite small and young, guessing just 2 or 3 yrs old. and a little over man height.

Am going to try seeds from a different tree a few km away next time.

That looks like a low germination rate, Steve.

Would generally expect higher with many Australian acacia species if the seed was good. Some acacia seeds can last for decades. Many are pioneer species that have to wait for an opportunity to get going after a fire or other event that opens up the canopy.

I've been running a small backyard trial with Acacia penninervis, decurrens, dealbata, mearnsii and elata. All native to SE Australia. I poured boiling water over them and picked out the seeds obviously swollen a day or so later. I poured boiling water over the remainder and after a day or two planted all of the remainder whether swollen or not. The dealbata were collected on our block. The other seeds were from Forest Seeds Australia in Bacchus Marsh. I removed any floaters. There were a a few in the seeds we had collected - first time we'd done any tree seed collecting - but very few floaters in the commercial seeds.

I planted the swollen seeds in Hiko cells with a commercial seed raising mix and the rest in seedling trays. I started the experiment in April which is generally after the brutally hot days we can get in Melbourne. I've had the trays outside since I planted them ie no period indoors. I had high germination rates from all these species. I would like to do this a bit more scientifically in the future and keep some records.

A few points that may be of interest.

a/ I transplanted seedlings from the seedling trays into Hiko cells. I did this over a period of a month or so which gave me a chance to have a look at the root development. They develop a lot of roots quite quickly.

b/ I had trouble with what I think was damping off with some of the penninervis. The others didn't appear to suffer this problem.

c/ A forester I know who used to work for Greening Australia says that some native reveg folks use a tool that scarifies seeds in bulk. He says this gives excellent results with acacias. I have about 600 acacia seedlings in the backyard at the moment so I didn't have the time to individually scarify.

d/ We had a frost last night. Down to -1C! Which isn't much for higher latitudes in the northern hemisphere but is cold for a Melbourne suburb. The acacia seedlings were well covered with frost. Some frost tolerant trees - which these species should be - are frost tender as seedlings. The acacia seedlings look OK so far but we'll see how they look in a day or two. We had some self-seeding spuds growing that are already looking a bit sad post-frost.

The real mark of success is if I can get them through to harvest down the track at our block. But it's been an interesting and educational experiment so far.

Regards

David

Interesting info David, and yes my germination rates are much lower than I expected. DO you know approx. age of the trees for the seeds you collected? Did you leave them to germinate in full sun or in the shade?

I could be more scientific with the quality of the soil. Also I could be more selective like you were by rejecting seeds that hadn't swollen. I noticed maybe just over half were swollen but I planted them all (after initially rejecting the floaters). If my successful seeds were all from the swollen ones then my germination rate after filtering non swollen seeds out might be well above 50%. I'll do that in my next test.

Another test I want to run is heating the seeds in fire rather than boiling water, as that more closely resembles nature. Maybe also planting in soil with high ash rate to further simulate forest fire.

For the mechanical scarification you mentioned, was that in stead of or as well as soaking in water, and if they were soaked was it with boiling water?

Finally, interested to note you are getting a light frost. How does your germination rate go with temps that cold? For my last test, temps were well above 30C for some of the time I was awaiting germination and the pots were out in hot sun so the soil temp probably exceeded 40C at times.

Steve Farmer wrote:Interesting info David, and yes my germination rates are much lower than I expected. DO you know approx. age of the trees for the seeds you collected? Did you leave them to germinate in full sun or in the shade?

I could be more scientific with the quality of the soil. Also I could be more selective like you were by rejecting seeds that hadn't swollen. I noticed maybe just over half were swollen but I planted them all (after initially rejecting the floaters). If my successful seeds were all from the swollen ones then my germination rate after filtering non swollen seeds out might be well above 50%. I'll do that in my next test.

Another test I want to run is heating the seeds in fire rather than boiling water, as that more closely resembles nature. Maybe also planting in soil with high ash rate to further simulate forest fire.

For the mechanical scarification you mentioned, was that in stead of or as well as soaking in water, and if they were soaked was it with boiling water?

Finally, interested to note you are getting a light frost. How does your germination rate go with temps that cold? For my last test, temps were well above 30C for some of the time I was awaiting germination and the pots were out in hot sun so the soil temp probably exceeded 40C at times.

Responses to above:

a/ Age of trees. For the dealbata seeds we collected we did what foresters call a "best tree" selection which means we picked trees of good size and form. Several were big old paddock silver wattles (perhaps 20+ years old) and the others were perhaps ten years old. Dealbata is a relatively short lived species so these trees should be well and truly sexually mature and capable of producing good seed. The other seeds were sold by a guy who has been doing this for ages so I would anticipate that what he collects and what he sells as collected by others would be from trees producing good seed. Certainly the germination we've seen from his seeds is very good.

b/ They were germinated in areas where they were shaded for part of the day and in full sun for part. As I mentioned that was in April (our autumn) so the worst of the summer was well past. From memory the hottest day they saw in the first few weeks would have been ~30C with max temp most days in the low to mid-20C's.

c/ I didn't reject seeds that hadn't swollen with the first application of hot water. I poured hot water over them again and planted them afterwards.

d/ I think you would have to very careful using fire as a technique to start germination. It would be very easy to kill the seed embryo. As for ash this might help but commercial nurseries report over 90% germination rates with many wattle species just with hot water or scarification. However, there's a lot of wattle species. Some might show a better germination response with the liquid smoke or other related approaches. Good opportunity for some citizen science.

e/ I think generally it's scarification or hot water but I'm not certain.

f/ I haven't tried germinating during the coldest part of winter. I wouldn't expect it to be successful

Regards

David

Looks like the acacia seedlings have survived the frost which is pleasing. Now we're past the winter solstice the days are getting longer and we've had some warmer weather this week. The seedlings are responding with more leaf and new branches. All going well I'll soon be planting them out at the block.

How tall are they when they are ready to plant out? How do you prepare the hole they go into?

I've got another batch germinating at the moment. 72 seeds, 11 have come up. 4 of those have been eaten by lizards. About ten days since planting so maybe I will get 4 or 5 more. Losses are high at every stage - rejecting floaters, non germinators, lizards - so I want to make sure trees get the best start after planting out, especially as it's a harsh environment (40C some days, 30mph winds often, no rain expected for at least 3 months).

On a positive note, I have a pretty much unlimited local supply of free seeds. I've got a target of how many trees I want, and if I have to start 10000 seeds to get there then I will.

Steve Farmer wrote:How tall are they when they are ready to plant out? How do you prepare the hole they go into?

On a positive note, I have a pretty much unlimited local supply of free seeds. I've got a target of how many trees I want, and if I have to start 10000 seeds to get there then I will.

I'm going to start popping a few out of the Hiko cells once they reach 10-15cm. I'll start planting them if the root system looks well developed.

We use a Finputki/Potputki planter. To riff on Samuel Jackson in Jackie Brown, "If you absolutely, positively want to cover that hillside with seedlings quickly and cheaply accept no substitute "

We're in a high rainfall area - by Australian standards at least - with good soil. And we're planting lots of trees. So there's an element of tough love in that we're picking species and sites that should be compatible after which it's up to the trees.

We're mostly planting into pasture. Pre-spraying the planting spot with Roundup is the common grass control technique but we would prefer to avoid that. So with a mattock we scrape an area free of sod for about 40cm square to reduce competition with the grass. We have deep enough topsoil to do that.

What acacia species are you working with?

Steve Farmer wrote:How tall are they when they are ready to plant out? How do you prepare the hole they go into?

... so I want to make sure trees get the best start after planting out, especially as it's a harsh environment (40C some days, 30mph winds often, no rain expected for at least 3 months).

That does sound like a difficult spot to get trees going. There's a load of arid climate acacia species in Australia. Why don't you contact one of the Landcare groups in an arid area to ask how they get trees going? They might be able to give you some good advice that's relevant to your site.

I don't know what species of acacia it is. I just got the seeds from the garden off a tree that's lived here longer than I have. It has no thorns.
I also have one small tree grown from ebay seed that is Acacia polyacantha ssp. campylacantha.