I have written a couple of articles, now, about the Salix caprea "Kilmarnock": I'm not, generally speaking, a big fan of grafted trees, but this tree is a great way to get something small and wildlife-friendly for a small garden, and they are really quite easy to manage, once you get the hang of it.
I wrote about a particular little example of this tree which had been totally neglected, then brutally pruned by me, and which had subsequently been restored to full health and beauty, which was quite a result *punches air*.
The original post continues to get a lot of views, and the other day there was a question from Steve ("Hi, Steve!" *waves*) asking about why we can't grow them from seed.
There are three answers to this (very interesting!) question: the first is that the small weeping Kilmarnock is a grafted tree: it's about as un-natural a tree as you can get, because the top or "weeping" part is grafted onto a straight upright stem of a different type of willow. This, incidentally, is why these trees don't grow any "bigger" over time - they just grow wider (and more dense, if they are not pruned properly), which is why they are ideal for small gardens.
But a grafted tree is two different trees, artificially joined together to get the desired effect, which means that if you take seed from the top, the part above the graft, any seedlings won't look at all like the parent. And yes, if the bottom part of the tree is allowed to grow and flower (which is a bad idea for many reasons) any seedlings from that won't look anything at all like the parent either.
The second point is that all willows are industrially promiscuous, the little beasties, and they will pretty much all pollinate each other. This means that you will have no idea whatsoever as to which willow pollinated your Kilmarnock - it could have been from the nearest other willow, it could have been from one half a mile away - so the seedlings could be any sort of hybrid willow, but will most definitely not be a standard-with-weeping-top.
The third part of the answer lies in the fact that they are, as Steve says, dioecious. A dioecious tree is one where each individual tree will produce either male flowers, or female flowers, not both.
The clue, for all you budding botanists out there, is in the name - dioecious means literally "two houses" and dioecious trees have two types of tree, one of which produces only male flowers, one of which produces only female flowers. You will usually be completely unable to tell whether a given dioecious tree is a male or female - or, more properly, a male-flower-producing or female-flower-producing - until it actually flowers.
The opposite is monoecious ("di" always means two, "mono" always means one) which means, as the name suggests, one house, ie one individual tree will have both male and female flowers on it. The advantage is that they can, if desperate, self-pollinate. The disadvantage is that self-pollination uses all the same genetic material, so there is less chance of mutations. This might not sound like a disadvantage: surely you want all your little willow trees to look just like the big willow trees? But on an evolutionary timescale, changes are essential to allow the organism to cope with changes to the environment.
Holly, like willow, are dioecious, and you may already have noticed that not all holly trees produce berries: only the female ones will do so.
Birch and Alder, for your information, are examples of monoecious trees which produce both male and female flowers on the same tree.
Willow are dioecious, which means that even if your particular Salix caprea Kilmarnock is female and therefore produces seed, it will by definition have been fertilised by a different Salix - leading to the problem mentioned at Point Two, ie hybridisation: the seeds will not be the same as the original, as the pollen will have come from a different, male, tree, which might be a different species of Salix altogether.
Both male and female willows will produce the silky catkins but they are subtly different - the male flowers mature into yellow-ness, as the pollen becomes apparent, and the female ones mature from silky silver-grey into green.
So there you go, Steve, now you know everything there is to know about grafted trees, promiscuous willows, and catkins! (well, not quite everything.....)