I have also been thinking that those heaps of weeds could be put to good use. I've already tossed some sprouting old potatoes into the nettles. It'll be interesting to see if there is any crop next year. I have been collecting seeds for years now. I will definately start planting leftovers where ever I go :-)
The map idea would be great!!

We have all seen honesty boxes along roadsides. Containing farm produce, used books and stuff. Pick out what you want, stick a few bob in a tin. So here's a thought, that I sadly don't have the know how to make real. Or don't even know if it's a good idea or not. But definatly worth mentioning.

So, what if; some one would set up a page helping CC'ers, moorers without land, or just anyone on the move around the canals to find, harvest and grow vegetables they can not fit on their roof, mooring or lives for some reason? This could be done by some kind of semi organized gerilla gardening, seed bombing open source like "honesty lot" system. A google map called "The British Waterways Open Gardens" or something. Find a pin on the map along your route, marking where people have planted something before. Pick something, do a bit of weeding, plant something, tell everyone what, when and where you planted it by by adding a comment to an exixsting pin. Not necessarily on the same spot. If you choose to plant your "dept" in a new spot and adding a lot to the system. Mark it with a new pin and a picture, so that others can find it and plant new things. I'm sure that there are pieces of wasteland along the canals that could be planted full with crops to be shared that way. I have seen gardens work the same way on waste land in cities. So why not along the towpaths?

The map could also be used to share the whereabouts of that huge plum tree you never could pick clean, sloe bushes spotted to early in the year, or any other forage spots. Ok, I planted the first seed. Maybe it will grow into something.....

Some things I have tried that may be of interest.

Seeds - my first point for bought seed is Garden Organic. But check out other commercial suppliers too: most of the big name suppliers have organic options listed now.

Collect your own seeds - I collected my own seed for the first time last year (2008) by letting one or two plants that were running to seed run on. Onion and Leek seeds that I harvested have germinated OK this year, also Broad Beans.

Share your seeds - Most seeds have a limited shelf life and between myself and my friends and neighbours we always end up cultivating more plants than we need or have space for. Unless you have access to an allotment space, for most things when you buy seeds you will get more seeds than you can use. I offered my excess seeds up on Freecycle last year and plenty of takers. If you ask around you will also find other growers with excess plants and seeds and most people are happy to exchange or donate excess to another gardener.

Cultivate from existing plants - I'm experimenting with cuttings and things that I have got to take so far include lavender, horseradish and raspberry's. Most mints and lemon balm are notorious with gardeners as spreading and root cuttings and splittings usually take - I grow mine in pots. Hoping to try more drying and storage of herbs this year. Do check if you are taking seed and cutting from the wild that its legal cos a few things are protected. I always try to propogate strawberry runners. I pin the base of the buds on the runner into a pot of compost and keep watering. As the year goes by the runner will either wither naturally or when you are confident the bud has set roots you can just cut.

Check out dwarf varieties - This year I've tried some dwarf variety green beans, runner beans and tomatoes and the plants are more compact and space efficient so far. They are going well in pots but really benefit from a boost from good quality commercial organic feeds. I have also gone strictly for the stubby varties of carrots and these go much better in my tubs and pots than traditional varieties that seem to do better with a good depth of soil.

I always source a few things that need an early start from the garden centre in pots in the Spring. Now sure you have to balance the impact that these have by being cultivated in a commercial heated green house before they come to you versus whether you have somewhere warm enough to start them early. However good commercially cultivated plants seem to give better yields that what I have started from seed. So if you have limited space think about it?

I've not been very successful at growing food on my roof so far :(
It seems a lot easier if you're living on a fixed mooring and can have lots of tallish stuff sprouting happily up there. I found it really limiting as I move around the country and have to get through tunnels and low bridges etc. Most I've managed is herbs and a few tumbling tomatoes. I want a moveable allotment- wail!!!