Answer: a shingle path.
You would think that a shingle path would be a barren desert, as far as seedlings are concerned, wouldn't you? However, nothing is further from the truth, as a shingle path seems to provide a perfect germination and nursery bed for many plants, including trees!
Above are three photos which I've taken in the last fortnight, each showing the large handful of tiny seedlings which have been weeded out from three different Clients and their shingle paths.
The first handful are Field Maple, from a shingled parking courtyard: the second are common Ash trees from a shingle path around the house, and the third are Lime seedlings, from a decorative shingle-and-rubble seating area (don't worry, it looks prettier than it sounds!) around the base of an old Cherry tree, and at least a hundred yards away from the Lime Walk, which presumably provided the seeds.
Each handful contains over fifty seedlings … ah, if only I had a couple of acres, what a wonderful tree nursery I could have.
Why are they so successful? Well, shingle paths are a matrix of small stones, which water clings between: even on a hot day, if you scrape back a couple of inches of shingle, you will find that the lower stones are damp. But at the same time, this matrix allows for really good drainage, so shingle virtually never becomes waterlogged. This combination provides just the right amount of water that the seeds need, plus protection from the hungry mouths of small mammals who would otherwise scoff the lot. Seeds bring along their own store of nutrients to get them started, of course, but even paths with good quality membranes below them will accumulate a surprising amount of organic material, from “bits” that drop down from above, from weeds that grow and die, from the dead bodies of worms, small invertebrates etc, and from the very dirt which is washed down with the rain.
This is enough to let a seedling get a good start in life, and it doesn't end when they are barely a couple of inches high: I have weeded out young trees well over a foot high, with roots which stretched to 18” or more through the shingle.
Removing them - for weeding, or for potting on - is very simple, all you have to do is to take a small hand tool (I use my daisy grubber, but a hand fork works just as well), push it into the shingle close to the seedlings, and wiggle it around to loosen the shingle, while you hold the plant by a leaf. As the shingle is loosened, the seedling will come free.
Last year I was walking past a local factory when I noticed a lot of tiny, dark red seedlings in the rough gravel path. Inside their compound was a rather lovely purple Norway Maple, so I carefully lifted every seedling I could find, brought them home and potted them up. Now I have a dozen small purple trees, free of charge.
So, if you want some trees for free, keep a close eye on your nearest shingle paths!