Germinating American Linden??

Does anyone have experience stratifying and germinating American Linden seeds?

After much reading I am beginning to think no one on the internet has had much success reducing it to a science.  And  lot of the information I am finding is questionable in nature.  Most of the people doing it have no demonstrated long term success.  And other information is questionable.  I bought 50 American Linden seeds.  But before I end up ruining those I gathered a bunch of Little leaf Linden(LLL from here on) seeds figuring I could learn with them without running the desired seeds.  

One of the things I found said if the seeds floated they would be hollow and no good as they didn't have a seed inside.  Only work with seeds that sank.  Well I dumped all my LLL seeds on the water and they all floated and were still floating hours later.  They are all no good?  Well I decided to cut a few open and see.  The pericarp is made up of a thin layer of fruit around a really tough husk/shell that looks like a miniature coconut after you skin the fruit off.  I opened 11 seeds.  2 were hollow inside with one showing signs of a worm carcass.  7 had a single seed looking piece inside and 2 had 2 flat one side seed back to back inside the shell with a thin septum between the seeds.  Does this simply separate the seeds from the shell/husk or am I damaging something so they can't possibly germinate?  I had 2 of them that showed very minor knife damage but the rest I don't think I hurt the seed looking part.  At any rate those are now in the fridge in a hydrogen peroxide paper towel in a sandwich bag beginning stratification.  Best way found so far for opening the husk was to gently pinch it with its axis parallel to the jaws of the pliers.  This would generate a crack at one end.  Insert the knife blade in that crack and using a cutting prying motion split the shell.  Guessing I could do 2 a minute with some practice so doable for hundreds but probably not practical by the thousands.

Beyond that the online directions talk about soaking the seeds in various acids to help the moisture get thru so the seed and germinate and force its way out.  They also talk about putting the seeds in a food processor and pulsing for a couple seconds several times to do damage.  I assume by cutting what I will call the seed out I bypassed this?

Here is a picture of what I learned so far on the LLL seeds?  Seeds? on the left and a few half husk/shells on the right.  These on the right are about 3/16" to 1/4" in diameter.

Continued.

The online stuff talks about some trees having a little worm that enters at blossom time and bores out late season.  In those areas 95% to 99% of seeds are destroyed or sterile.  Sounds like general germination is about 50% typically with seeds taking 1 to 4 years to stratify and germinate.  With the cold stratification time being listed as 90 to 120 days.  If nothing happens you are supposed to warm stratify and watch for germination. (problem here is they don't say how long this time needs to be)(guessing also probably at least 90 days?  If nothing then back to cold stratification.  Rinse and repeat at least 4 full cycles.  These are apparently a hard seed to germinate because of this.

What else to I need to know?

Okay opened the fridge yesterday to notice the ziplock bag with green in it.  On 10-18-21 I placed roughly 3 dozen little leaf linden seeds in this bag.  The tough shell was removed mechanically by crushing and prying apart.  The seeds were abraded on fine sand paper a bit till thru the dark part of the seed.  They were then placed between layers of paper towel soaked in hydrogen peroxide in ziplock freezer bags and refrigerated to stratify.  In the spring of 22  a bit over half the seeds were planted and the rest were put back in the fridge to continue stratification.  Nothing grew from the planted seeds.  The plan was to take half the seeds after the second stratification and try again.  But I forgot about that bunch of Lindens this spring as they were on a different shelf from the rest of my seed starts.  Which leads to yesterday.  Aug 14 2023.  2 of them had germinated and I had a roots about 6 inches long with a tiny green poof at one end(they had pushed right up in the corner of the bag and were thus barely visible above the other baggies.  One interesting thing to note is there were 2 black mold spots on the paper towel right with where those seeds were.  None of the other seeds showed mold having been thru the H2O2 bath and soaked paper towel.  So an early marker for growth might be mold appearing on the paper towel when the seed breaks and exposes it. They obviously were in the fridge too long and that may end up killing them via exhausting their resources but they are potted.

Now the bags with intact shell, cracked but not removed shell, soaked  bare seeds done all in both plain water and in hyrdogen peroxide show no growth along with the baggy of bare abraded seeds done with water.  Putting them back in the fridge for another year to hopefully remember better next spring.  The ones done in plain water the paper towel is almost black with mold.  Does this help or hurt??  

I have read that American Linden is notoriously difficult to germinate. Many people propagate it from suckers, and that seems to be a primary mode of dispersal in the wild. Of course some seeds must germinate or it would have gone extinct.  Let us know if you find a solution

With tree seeds, I pot them up, leave them outside and water them every now and again if the soil dries out completely.

Have a bunch of pots right now, some Poncirus trifoliata - I've seen some sprouts, Kentucky Coffee Tree seedling, one so far.

Plus some Pawpaw seeds, some were cold stratified when I bought them.

Suppose there's some Sabal minor seeds as well.


Now, with American Linden. It's similar to some other species, in that it does this really funny thing.

The really funny thing, is that it builds a dry / rough coating outside of its seed once it dries.

This reduces the germination down to around 30%.

If these are planted before this period - it's maybe at around 80% germination and they germinate almost right away.


The 30% seems to be more of a case of where the seeds will eventually sprout, and it's probably higher than 30%. It'll just take a few years.



I've also read that American Linden attracts Japanese beetles, and can be completely defoliated.

Yeah.


It may be worth growing Tilia cordata or other Tilia species to try and obtain a resistant hybrid at some point.

They're prone to disease and dieback in Europe.

So, the trees here will probably be infected by some virus eventually.



Good luck germinating these. I've found that easing some seeds into a fridge after the freezer works well.

I've been wanting to buy a mini fridge with adjustable temperatures for this reason.