I am interested in this question because I have never ever seen a self sown linden and have always assumed they must be quite tricky, at least in my climate. I googled and found the linked information. It seems that Tilia species can be 'deeply dormant'. There's a useful little guide book to download at the link too on growing trees from seed. I have tried layering a linden and had zero success.

I looked at that site and did some reading. I think I will go ahead and use the boiling water method. When I did the KCT, I poured the water over the seeds to help soften the coat. I think I will do the same for the linden seeds. Maybe that will soften the coat enough to speed up germination? It's worth a try. Marg

INDEN... SOAK SEED IN ACID FOR 10-15 MIN THEN IN WATER FOR 1-2 DAYS THEN STORE AT 40f FOR 3 MONTHS..... LIGHTLY COVER THE SEED SOIL TEMP 57-78f TAKES 14 TO 35 DAYS TO GERMINATE MAY TAKE LONGER
kct ... SOAK IN ACID 2-4 HOURS THEN LIGHTLY COVER SEED SOIL TEMP 65-70f TAKING 30-90 DAYS TO GERMINATE
i USE BATTERY ACID

I've had success with Kentucky Coffee Tree germination by scarifying the seed coat with a Dremel. I planted 3 non-scarified and 3 Dremel scarified seeds and the 3 scarified seeds all germinated while none of the non-scarified ones did. This will be the first winter they go through, so we shall see if they make it!