Mmm, we need more plants, but how do we do it?
Better use of space, resources, water, management, and concepts of what's acceptable (or edible) and what's not. I cover this in a garden setting in my latest blog post:
Gardening like the Wild - Science Forums
Where I am trying to create what's known as a "forest garden":
Forest gardening - Wikipedia, the free encyclopedia
One of the goals is to get more edible, useful greenery in the same space as a traditional garden. A similar "forest" or agroforestry concept can be applied to other areas or spaces, IMO, and it can be done with other things like herb, flower, or native plant gardens. Another thing that I am trying to do with certain plants is to create or force them to stack--that is, to take up vertical space or make better use of 3D space rather than letting them be simply 2D or horizontal. I think plants which are very flat or low-growing may not make efficient use of space in most gardens. They're not that much different from lawn grass. In nature, many such plants actually inhabit the forest floor, small nooks and crannies, shady areas, or climb or grow on top of other plants, which justifies their mode of existence. Plants which I think work well are vines, peas and beans, strawberries, etc. I've run across some specialty pots for growing strawberries that use many small pots or openings at different levels or other interesting ideas such as from this hydroponics grower:
TenGreenThumbs Blog Archive Strawberry Tree
TenGreenThumbs Blog Archive Vertical Container Gardening
Also, if you're doing something like a garden pond or water garden, please keep in mind that little green things, like algae, pond scum, and duckweed, do their share to keeping water clean and purifying the air, so long as they don't become invasive or out of control with respect to other plants in the garden.
Edited: Want to recommend that if you like English ivy, like I do, keep them inside and in pots. Can be a very invasive plant, but it won't get outside if it spends its life in a house or a windowsill. In fact, do this with any plant you suspect may outgrow, outcompete, or spread wildly and prolifically out of place. Don't let little things like English ivies run wild in the wild.
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Logic
The art of thinking and reasoning in strict accordance with the limitations and incapacities of the human misunderstanding.
--Ambrose Bierce,
The Devil's Dictionary