The herbaceous layer of a food forest is not just grass, though gardeners can leave the grass while they install the forest over a period of time. (Just don't allow grass within a two-foot circle of newly planted trees). The ground layer consists of carefully chosen plants that serve seven different functions, ideally with each plant serving two or more functions, for efficiency. In general gardening theory, maintenance is easier and appearances are cleaner when the garden contains fewer species planted in clumps and drifts, as opposed to a collector's garden with one of everything.
The plants in a food forest help the system take care of itself without needing to add fertilizers or pesticides. Some may still be necessary for the fussier fruit trees, but they should be considerably limited. The seven functions are as follows:
1) Nitrogen fixing. Nitrogen promotes green growth, and it is the limiting factor in all forests; the system can only grow as the available Nitrogen allows. Some plants capture Nitrogen from the air and store it in their roots, diffusing it to neighboring plants. Creating a Forest Garden by Martin Crawford contains excellent resources for all of these functions. Here I will just tell you my plant choices.
Lupine Lupinus perennis. This is our native blue Lupine, host to the Karner Blue butterfly, which has been driven nearly to extinction and is no longer found in CT.
New Jersey Tea Ceanothus americanus. This is a native shrub, whose leaves can be used for tea, or rubbed to make a foamy soap.
Bayberry Myrica pensylvanica. Another Latin name that indicates that it's native to this continent. You can use the leaves as a substitute for bay leaves.
2) Other nutrient fixers. Fruit trees need other nutrients in addition to Nitrogen, such as Phosphorus, Calcium, etc. Deep-rooted plants mine the soil for nutrients and bring it up to the surface for other plants to use. The nutrients become even more available when the leafy part of the plant is cut off and lain around the garden to decompose into the soil. I will be using:
Comfrey. Chop down once or twice a year and lay around fruit trees. A powerful nutrient accumulator. Only plant it where you're certain you want it, because it can become a nuisance. Typically I'm wary of plants like this, but I've grown it before and it's so useful.
Yarrow Achillea millefolium. I'm a little nervous about this one. Supposedly it's very aggressive, but it's native, so I'm a little more comfortable with that. And it serves so many functions. Watch how often it appears on this list.
3) Differing root zones. Trees have shallow roots that reside within the top two feet of soil, extending out as far as twice the width of the canopy. But there's so much space in the soil! You might as well plan to use it by planting things with differing root depths. This allows for maximum nutrient absorption and water capture which benefits the whole system.
New Jersey Tea. Deep roots.
Yarrow. Deep roots.
Comfrey. Deep roots.
Wild Strawberry Fragaria virginiana. Shallow roots.
Viola. Shallow roots. This is the only food source of the Fritillary butterfly, so I'm including it in the plan. It's always good to plant for specialist species like this, to ensure their survival.
4) Ground cover. When the soil is covered by plants you want, you'll have less appearance of plants you don't want, i.e. weeds. Ground covers also retain moisture in the soil and prevent erosion. In annual vegetable gardens, mulch serves this function, but in perennial gardens it can be accomplished with plants for reduced maintenance.
Wild Strawberry Fragaria virginiana. A very aggressive ground cover. Good lawn alternative. I may regret planting it if it takes over everything, but it's also a great wildlife plant.
Nasturtium. This is an annual, so I'll seed it each spring, until the ground is so covered that there's no space for it.
5) Bee attractant. You want your fruit pollinated and you want to feed these lovely creatures. There are a million bee plants out there, so the main challenge will be choosing which ones to use!
6) Beneficial insect attractant. These fall into a separate category from bees because they do different things. The most common ones are predatory wasps and flies, who lay their eggs in caterpillars, which in turn kills the caterpillar. Hopefully they mostly target fruit pest caterpillars and not our lovely native moths and butterflies, but you can't control it. The only thing you can do is provide lots of different native plants to attract a variety of insects and birds, and they'll balance each other out somehow.
Yarrow Achillea millefolium. Anything with an umbrella-like flower consisting of clusters of tiny flowers will attract beneficial insects.
Golden Alexander Zizia aurea. the original, native food of swallowtail butterflies. A somewhat traitorous plant, I suppose, as it also attracts the wasps who kill them.
7) Aromatic plants. Another layer of defense against fruit pests, these plants deter them from ripening fruit by confusing their senses.
I would argue that any garden, ornamental, vegetable, or food forest, would benefit from plants that serve these functions. Your typical nursery may not have all of these plants; Natureworks does, or it can order them if they're not in stock when you visit. You can also try your hand at growing them from seed, as I am doing with many of them, but a potted plant will give you quicker and more guaranteed results.