Wild blue phlox, Phlox divaricata. Picture: Christine Chin.

By Philip Fry

In my last column, I discussed the soil profile of an undisturbed site. The top layer, generally labelled “O,” is composed of organic materials called humus that have been decomposed over time. The process of decomposition takes place as organ litter fallen on the site is broken down by insects, bacteria and fungi. In undisturbed sites, this takes place on the site itself. In the vegetable and flower borders of our settler gardening tradition, the organic litter produced each year is most often “cleaned up,” exposing the “O” layer to the elements. This means that to protect the “O” layer, the garden has to be supplemented annually with compost prepared off-site and/or with chemical fertilizers. Moreover, to protect the “O” layer during the growing season and shelter perennial plants during the winter, a suitable top dressing called a mulch is required.

With some notable exceptions – sand dunes, eroded hillsides – undisturbed soils are usually covered with site-specific mixtures of dead plant and animal materials. These are important factors in the recycling of a particular blend of nutrients, the conditioning of soil acidity, and the protection of the soil from climatic extremes. Three main types of mulches can be distinguished: 

Mor mulches: these are composed of evergreen needles and branches or accumulations of peaty materials such as dead sphagnum moss. These materials break down very slowly, forming rather thick mats, and are quite acidic. The acids (hydrogen ions) react with the nutrients in the soil, which helps determine which nutrients are available in solution for plant use. Some plants, for example blueberries, are adapted to absorb nutrients under acidic conditions. The underside of mor mulches frequently exhibits a white mould or fungal layer which enters into a reciprocal and vital relationship called mycorrhiza with evergreen trees and some species of plants. 

Deciduous or “mul” mulches: these are formed by the leaves of deciduous trees, small branches and rotting hardwood. They are less acidic than mor mulches. The materials composing deciduous mulches break down much more rapidly than those of mor mulches, and can create a nutrient rich and biologically active “O” layer in a just few years. Most of the “spring ephemerals” in our deciduous woods require a good mul layer to thrive well. One of these is Dutchman’s Breeches, which likes almost neutral mulch conditions: a little limestone dust powdered around the plant stimulates flowering.

Off-site Mulches: often prepared commercially, these mulches are brought from elsewhere and applied as a top-dressing in the garden. Some, when applied with care, for example partially rotted leaves, wood, and livestock bedding, do a good job at mimicking the vital functions of litter. Others, such as fresh or coloured wood chips, while they can help to control “weeds” and moderate soil moisture and temperature, create a hard barrier that blocks the development of many native plant species. Although covering the intervening spaces between plants with a fairly impenetrable material can be said to have an aesthetic value, the impulse behind the practice seems to be more about imposing a human plan on the site than working with the plants to develop a collaborative habitat. 

Woodland phlox exemplifies how mor mulch works to the mutual benefit of the gardener and the plant. This plant’s pale blue flowers appear during May in partially sunlit glades were there is dappled shade. The flowers, held at the top of a fertile stem, will eventually produce seed. But the plant has another strategy for spreading and reproducing: it has non-flowering stems that lean down to the ground and send roots into the surrounding litter. From this conjunction new stems grow, eventually producing a clump. The same principle holds true for Wild ginger, which spreads by sending out stolons between the litter and the “O” layer, and Foam flower which has whip-like leaders that root in the litter at some distance from the main crown. The flourishing and  progress of these species depend on how hospitable the mulch is to their needs. Gardeners have a choice…  

The Provincial election is coming. As you go to vote, remember how the present government has gutted the environmental protections and initiatives formerly set down in law.

If you wish, contact me at: [email protected].


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