Here is an outline of my personal foraging rules, for picking edible plants in urban environments. 

Above all, be considerate, careful and moderate whether harvesting from your own plants or those growing in public spaces. Be:
Accurate
Light
Clean
Legal
Enriching
Safe

1. Accurate Harvest
100% CERTAINTY OF IDENTIFICATION
Know the plant, know the area, know look-a-likes, poisonous plants, local endangered species, grow the plants, get to know them intimately, learn all you can about them, what will you use the harvest for (no waste), which animals and other plants does it coexist with, how does it change the land it grows in (soil retention, nutrient mining, impairing growth of other plants etc.).

Start with herbs that you are very familiar with and use at least two good field guides to ensure correct identification. Foraging guides are often good for suggesting how to use the plants but should not be relied on for ID purposes. There are some guidebook suggestions on my books page. ID at the harvesting location and again back at home before preparing. Use a loop lens (jewellers) to help accurate ID.  Latin names change less frequently and are more reliable than common and regional names, so make the effort to learn them gradually. Look up all the ID features of the plant.  If in doubt, don’t pick or use a plant.

2. Light Harvest
Spread harvest, pick very sparingly (less than 10%), choose areas of abundance, overgrowth, I don’t harvest roots or bark (unless recently felled for the bark). Reduce any possible negative impact upon your body and the environment. I harvest just a pinch most days, over harvesting can and does lead to unnecessary rarity and extinction.

Pick sparsely to help conserve the health of the plant, its appearance and the creatures which it supports. Take time to do this, perhaps a year or more before becoming confident that it really is what you think. Never strip all the leaves, berries or whichever part you are interested in, from a plant, however tempting. Take only a little from each plant, leave plenty and avoid harming plants by rough picking. Likewise, flowers or seeds of annual and biennial plants, shouldn’t be picked; their seeds are needed for their survival.

Never pull up whole plants or harvest any part of a rare plant. I don’t harvest roots of wild plants. It is a certain way to destroy a plant, is time consuming and roots generally harbor more toxins than other plant parts.

3. Clean Harvest
Beware polluted soil, air and plants, some accumulate heavy metals, toxins more concentrated in the roots, least in nuts apparently, bug free environment is concerning, unusual growth, signs, be aware, manicured areas, pavement cracks, under power lines and use your instinct in addition to all of this. Then clean your harvest well, above dog-spray height when possible, avoid obvious areas of pollution (and old lead in soil). Most city councils now have policies of not using chemical fertilisers or plant pest control sprays but this is not always the case. It is wise to check the local policy and to find out the legal position on foraging from local public spaces. Council ecology teams are usually easy to contact and should be able to explain the local situation. Seek out the greenest and cleanest areas that you can find. There are unwelcome forms of pollution in both urban and rural areas; fertilisers, animal waste, chemicals, engine fumes and garbage being just a few. Avoid harvesting where pollution is highly likely, such as along busy roadsides, railway verges, building sites, non-organic farmland and industrial zones. Look out for clean, untreated planting areas, away from busy roads.

The best urban foraging grounds are usually within large green spaces and parks. It often helps to pick from as high as you can reach, this can minimise collecting harvests which have been soiled by passing people and animals, though it will still require proper cleaning. Avoid any material which looks dirty, unhealthy or unusual. When harvesting near clean free flowing water, only collect plant parts which have not been submerged. Harmful waterborne parasites can easily transfer to human when affected plants are eaten. Allow time and space for bugs to crawl out. Wash under running clean water. Suitable containers (tubs for berries, paper bags for flowers etc.).

4. Legal Harvesting
Foragers need to consider local laws, what is morally acceptable, leave flowers for insects and , no trespassing, stealing, ask and probably allowed, don’t pick what was deliberately planted, Never harvest plant parts from individuals without first seeking their permission. It can be tempting to pick ripe fruit and herbs whilst passing doorstep pots and private street gardens but it is so disheartening to the owners when they come to harvest their own tended fruit and find that a passer-by has beaten them to it. In the UK for instance, all wild flowers are protected.

5. Enriching Harvest
Leave the area better than you found it. Sow seeds, plant cuttings, grow Elder babies, bring on rare plants at home from ethically sourced seed then plant out in your own patch. Or plant them out in appropriate public spaces.

6. Safe Harvest
Try anything that is new to you, in very small quantities, whether as a food, tea, internal tonic or skin preparation. Here is a useful method to use to test your reaction to a new plant. About 20 minutes should be left between each step. Watch out for any signs that your body reacts badly to the plant. If this occurs – stop.
Smell, lips, gum, tongue, chew, cook…
Label at collection site, each plant in a different bag. Don’t store dubious or known poisonous plants with edibles, for fear you or your family will eat them. As with shop bought plant food, eat whilst in great condition.
Beware of local hazards such as Lyme’s disease and water borne parasites.