Obviously, the nature of perennials is to be planted once and then harvested for many years. This is a good return on the gardener's investment of labor. Perennials range from fruit and nut trees, to flowers, to culinary and medicinal herbs, to sundry types of berries, and also include a few vegetables. I see some perennials as a sort of "natural" season extension. Because perennials store a lot of their energy in root systems, some of them are among the first plants to emerge in the spring, without the help of cold frames or hoop houses. The downsides to perennials include that many are purchased as rootstock or seedlings, and these can be quite a bit more expensive than seeds for annual garden plants, that most do not give yields immediately, and that you must be more deliberative in choosing a site for them, since they will remain there for many years. Granted, fruit and nut trees (as well as berries) take up quite a bit more space than a small patch of lettuce. They also create shade as they grow. But there are plenty of perennial food plants with small footprints on the ground. Grapes and other vines, for instance, take well to trellising and can occupy very little horizontal space if given nutrient-dense soil to grow in. Though most nut trees become very large as they reach maturity, hazelnuts and filberts (and hybrids of these two species) remain small enough for a pair to fit on a subacre lot without shading most of the property.
For the space devoted to a dwarf pear or dwarf apple tree, you could expect to harvest one hundred pounds of fruit in a good year, once the tree is mature. Permaculture also includes many techniques for maximizing the production of useful plants under and around fruit and nut trees.
On the balance the benefits seemed to us to outweigh the downsides. In 2008 and 2009 we planted many types of perennials on our property, and we'll put in a few more this year. Below I've listed the perennials we either inherited with the property, have already put in, or plan to put in this year.
Trees and shrubs
- Cherry - standard may bear for 15-20 years, dwarf varieties probably less, best to have two types for pollination
- Pear - reaches full bearing capacity in 8-10 years, may bear for 50-75 years, best to have two types for pollination
- Fig - will be container grown in my zone (6a) and kept in unheated garage during winter, should bear for 15-20 years, new trees can be grown from cuttings
- Lemon - will be container grown in my zone and kept indoors during winter, susceptible to scale and other citrus diseases
- Lime - container grown, 3-year-old plant has not yet borne, kept indoors during winter, susceptible to scale and other citrus diseases
- Apple - consider a very late season apple if yellow jackets on groundfalls are a concern, dwarf varieties may bear for 30 years, standard for 50-100 years
- Hazelbert - (hazelnut/filbert cross) most growth in first three years is in root system, 8 years to maximum production, can bear for 30-50 years
- Black Currant - best to have two types for pollination, can tolerate significant but not total shade, produces best fruit in full sun, bears for 10-20 years
- Elderberry - two types needed for good pollination, (eventually) grows large enough for a privacy screen, reaches full bearing capacity in 3-4 years
- Various thymes
- Various sages
- Chinese chives
- Oregano - can be invasive, but can also be controlled by pruning or grown in containers
- Lemon balm - both culinary and medicinal, can be invasive, but can also be controlled by timely pruning, important nectar source for honey bees
- Various comfreys - multi-purpose, grows exuberantly but spreads only a certain amount unless roots are disturbed
- Echinacea - medicinal, grows slowly, can be divided every three years
- Spearmint - culinary, extremely invasive, best planted in a container
- Anise hyssop - lovely sweet taste, pretty blooms are a favorite of the honey bee
- Yarrow - multi-purpose, fast growing and spreads indefinitely - somewhat invasive
- Ramps - onion relative, very shade tolerant, emerges very early, propagate by seed or division
- Asparagus - needs 2-3 years to reach maximum production, can produce for up to 20 years, heavy feeder needing good soil
- Ostrich ferns - emerge very early as edible fiddleheads, tolerant of significant shade
- Jerusalem artichoke - edible tubers harvested fall/winter, spreads easily, difficult to eradicate once established, late season food source for bees
- wineberries - wild volunteers in our area, appearance/flavor resembling raspberries, lovely sweet-tart taste, plants are of no interest to Japanese beetles
- raspberries - purchased rootstock will bear by the 2nd year at latest, reaches full bearing capacity in 3-4 years, may bear for 10 years
- blueberries - can bear lightly in "1st" year if 3-year-old plants are used, pinching off blossoms in first few years helps the plant grow larger sooner, reaches full bearing capacity in 8 years, may bear for 50+ years
- blackberries - purchased rootstock will bear by the 2nd year at latest, reaches full bearing capacity in 3-4 years, may bear for 25 years
- grapes - purchased rootstock may bear in 2nd or 3rd year, may bear for 50 years
I have listed here only the perennials that we already have on our property or will plant this year. There are many other perennials to choose among, especially if you live in a warmer zone than I do. We are right on the line of feasibility for peach trees. Just a few miles north of here, and there's no chance of a peach harvest without the protection of a good microclimate. In our area peaches succeed in some years and are wiped out by frost in others. We like peaches, but not those odds, so peach trees were off our list.
Many reseeding or "self-sowing" annuals function like perennial plants in some ways. These plants grow, reproduce, and die every year, but their offspring grow in the same place without human intervention. Consider this sub-set of annual plants if you can't find a true perennial for the purpose you desire.
Much of the information listed above comes from my own observation. I've also drawn on charts in Edible Forest Gardensby Dave Jacke, a dense but invaluable reference book on perennial plants of use in permaculture design.