Propagating perrenials is easy
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Perhaps the easiest way of getting free plants, apart from being given them, is by dividing herbaceous perennial plants. A herbaceous perennial is a plant that doesn't have a woody structure and survives from year to year. They can be evergreen like many Agapanthus and Hellebores or they can die back to the ground and be deciduous like Delphiniums or Phlox. Mosts are incredibly easy to propagate by division and within a few years of planting a 'parent' plant it is capable of being able to spawn tens of extra plants using this method of propagation.
For propagation purposes many herbaceous plants can be divided every year which is a financial bonus if you are looking for plants in large numbers. Fast spreading plants like Geranium, Anemone, Rudbeckia and Lysimachia can be divided annually for control purposes as well as for propagation. Plants that are slow to spread are best left for five years before you atempt to split them. Agapanthus, Zantedeschia and Paeonia would be examples of these. Constant division of these plant can interfere with their flowering quality. You can divide at virtually any time of year, even summer if you water frequently after replanting and some spring flowering plants like Irises prefer it. If you summer divide the chances are the plants foliage will die back to the ground and reshoot from there. That said the dormant period is generally the main time to undertake the task.
To split or divide your perennials you usually start by lifting the completle plant carefully and gently from the soil with a garden fork. If the plant has an old crown work from the centre outwards lifting the fresher growth away from the old growth. You can then discard the old centre core. If the plant is younger it is possible to take some outside growth while leaving the bulk of the original plant in situ and undisturbed. Generally it is best to shake off excess soil so you can see what is required to complete the division once lifted.
Plants like Ajuga will produce little plantlets that can be teased off the main growth and potted up if small or just replanted. Running plants like Vincas will usually root where a running stem touches the ground. These can be lifted independent of the main plant which makes sense as these type of plants are used for ground cover to hide or cover difficult ground where it is not desirable to lift complete plants and expose the area again.
Other small fibrous rooted plants like Geranium and Epimedium can often be pulled apart by hand once lifted. Fleshy rooted plants like Agapanthus and Kniphofia can be broken apart be using two garden forks back to back inserted into the root mass and then pushing the two fork handle together to lever the root system open. Old clumps of this type of plant can often be too tough to divide this way so some brute force may be called for. A knife, lawn edging iron, spade or even an axe may be required to cut through the root systems and generally the division will still be sucessful. Hostas can also fall into this category. Some plants like Helleborus and Bergenia that have strong woody runners many become straggly away from the main root mass and when lifted. This often leaves you with a root attached to a lonely group of foliage two feet away. Here you are best to cut back the about six inches of the main root and allow the plant to shoot again. Crocosmia and Dierama grow from little corms that are easily seen once lifted. These can be broken into individual bulblets or groups and they will still grow on. While winter is the main time for dividing it is worth leaving very fleshy rooted perennials like Agapanthus, Crinium and Paeonia until early spring as this discourages rotting at a time when the plant is begining to grow again.
Potting up divided plants has the added benefit that they can be swapped or given away as plants in the summer or you supply them for a summer charity fete garden stall.