Compost activators can be any organic matter that has active bacteria or high nitrogen in it. There is no need to add activators to a compost pile, although they are fun for experiments.
Every piece of organic material you put in your pile will have bacteria on it that is able to compost it, unless it has been sterilized. Always read the ingredient list of packaged activators to ensure you understand what you are buying.
You can always create a high nitrogen-to-carbon ratio by adding high-nitrogen compost inputs. The higher the nitrogen content in a pile, the faster it composts (and the more likely it is to stink). For instance, feed-grade cornmeal can speed up decomposition if you spread 40-pounds of it between the layers of a hot batch pile and keep it moist. That is an expensive way to generate compost, but you can do it if you like.
The least expensive alternative in activators is to grow your own. The herb comfrey, of which there are many varieties, is high in nitrogen and will act as an activator when added to a hot batch compost pile.
Comfrey is a perennial that can be harvested up to eight times per year from spring to fall because the plants regenerate quickly. They remain green through fall, and can provide much-needed nitrogen when building piles for autumn leaves. Just use the comfrey to increase the depth of your nitrogen layers. (Warning: Don’t make a pile entirely of comfrey. This would make the nitrogen content too high and produce a foul odor.)
Minerals such as potassium, calcium, magnesium, iron, phosphorus and Vitamins A, C, and B-12 are pulled up into the leaves from comfrey’s long tap root. The root can reach down into the soil as far as 10 feet. Comfrey concentrates nitrogen and potassium, and has been shown to contain 2 to 3 times more potassium than farmyard manure. Comfrey has been used to clean and extract nutrients from stagnant water.
Comfrey is widely used as a surround for gardens in Great Britain because it is said to block grass invasion, particularly twitch, sorrel, and buttercups, and act as a slug trap. Garden lore says the spent leaves attract slugs that stay in the comfrey rather than the garden. (I have not seen a slug in my comfrey in the 7 years I’ve grown it, so I cannot attest to this statement but there are a lot of claims that it is true. I don’t generally have a problem with slugs, so my experience may not be relevant as related to slug-proofness.)
Comfrey grows well in USDA Zones 3 to 9, in sun to partial shade. It is supposed to do well in normal to moist soil, but we have drought here every year and it continues to thrive.
There are different types of comfrey with varying characteristics. Bocking No. 14 Russian Comfrey is the best variety for the home composter. This is the kind I use. It s the least invasive Comfrey, and also is disease-resistant with a non-invasive root system. It is almost sterile so you don’t have to worry about the seeds, but easily propagates by root cuttings or crown division. Space plants 3 feet apart.
Common Comfrey is the most widely used Comfrey, but it is very, very invasive. It spreads both by seed and by creeping root. Ornamental Comfries include Creeping Comfrey, Goldsmith Comfrey, and Red Comfrey. All are small plants so would not be particularly attractive for planting as compost activators.
If you decide to plant comfrey, be sure to plant it where you want it. It is hard to kill once it has been established because the tap root is long and able to regenerate a new plant.
Mary Tynes, Master Composter, www.mastercomposter.com
Image Credit (top left): http://www.flickr.com/photos/smoo/471627880/
Image Credit (bottom right): http://www.flickr.com/photos/rightee/253118115/