If you have established perennials, they are a readily available source of new plants for your garden.

Most herbaceous perennials can be propagated by division. In spring, for fall blooming perennials, and fall, for spring blooming perennials, dig up the clump, break it up into smaller sections, making sure that every section has a good amount of roots attached, and replant them immediately in their new locations. Perennials can be divided at any time during the growing season, if you really want to do it, just keep in mind that extreme temperatures will add to the stress the new plants are already subjected to, due to transplantation.

Woody perennials, like rosemary, roses, butterfly bush, sage, and hydrangeas, are easily propagated by stem cuttings. Cut a four to six inch piece of cane with growing nodes, from the midsection of a young, sturdy stem. Bruise the bottom end, dip it in root hormone, if you have any (it is available at plant nurseries), and plant it in a good growing medium, remembering to keep the soil and the plant evenly moist, so that it doesn’t dry out.

Some plants, like African violets, cacti and sedums, are easily propagated by leaf cuttings. This means exactly what it sounds like: take a leaf and stick its petiole into the ground. It will grow roots.

Most of the plants in the mint family will root in water, if you give them a week or two.

Raspberries and blackberries can be propagated by layering: bend a long, flexible cane and bury its midsection. After roots develop, you can cut it off from the mother plant. Strawberries, violets and mint and bugleweed propagate by runners. If you want to plant their offspring in other areas of the garden, all you have to do is dig them up and cut the string that connects them to the rest of the clump.

Keep in mind that many plants can be propagated by several different methods, not one, and many will do all the work themselves, very enthusiastically, if they like their spot.
Check your garden in spring for baby plants and colonies that have grown over the previous year, you’ll be sure to find enough material to populate a new flower bed.

Last, but not least, the obvious way to propagate plants - by seed. Every plant can be propagated by seed, why else would they expend energy to produce it? Some require very specific growing conditions, chilling, a particular type of medium, but all plants can be grown from seed. The rest of the methods are shortcuts for the gardener.

Author's Bio: 

Main Areas: Garden Writing; Sustainable Gardening; Homegrown Harvest
Published Books: “Terra Two”; “Generations”; "The Plant - A Steampunk Story"; "Letters to Lelia"; "Fair"; "Door Number Eight"; "A Year and A Day"; "Möbius' Code"; "Between Mirrors"; "The Blue Rose Manuscript"
Career Focus: Author; Consummate Gardener;
Affiliation: All Year Garden; The Weekly Gardener; Francis Rosenfeld's Blog

I started blogging in 2010, to share the joy of growing all things green and the beauty of the garden through the seasons. Two garden blogs were born: allyeargarden.com and theweeklygardener.com, a periodical that followed it one year later. I wanted to assemble an informal compendium of the things I learned from my grandfather, wonderful books, educational websites, and my own experience, in the hope that other people might use it in their own gardening practice.