8. Planning revegetation work

This section covers the technical aspects of a revegetation project, including revegetation area and density, plant sizes, plant selection and estimated cost of planting stock.

When planning revegetation work, it is essential to take into consideration the three vegetation strata:

  • herbaceous and perennial species
  • shrubs
  • trees

8.1 Revegetation approaches

The first step in project design and budget planning is to determine the type of revegetation that should be carried out. The type of revegetation required is determined by evaluating the existing condition of the target riparian ecosystem. This involves determining the most appropriate revegetation approach and then the type, size and quantity of planting stock and other materials.

There are two possible options. The first is a densification operation, which is recommended in cases where the herbaceous stratum is well established and the goal is to accelerate plant succession on the riparian strip by planting trees and shrubs. The second option would be complete revegetation of the riparian strip, which applies to bare areas or areas colonized by undesirable species that need to be removed.

Projects involving densification would not need to include revegetation of the herbaceous stratum. If this applies in your case, make sure to specify this in your funding application. However, if the characterization of the riparian strip indicates there is no herbaceous layer or it needs to be replaced, you will need to plan for seeding as part of your revegetation work. Seeding produces quick, effective results in the control of soil erosion.

8.2 Revegetation area and densification

To calculate the area to be revegetated, two parameters, i.e., the width and length of revegetation areas, have to be measured. This involves identifying and measuring (or estimating as accurately as possible) all bare surfaces to be revegetated. Then a third parameter, planting density, needs to be addressed.

In general, the proposed width of the riparian strip in your project should be equivalent to that recommended in Quebec’s Protection Policy for Lakeshores, Riverbanks, Littoral Zones and Floodplains (PPRLPI), specifically a minimum of 3 m in agricultural areas, or 10 or 15 m elsewhere. This may of course vary locally owing to barriers, relief, and water filtration requirements. Also, a wider riparian strip should be developed in areas where, for example, runoff is concentrated. An average width may be established for all planned revegetation work.

The length of the riparian strip to be revegetated represents the sum of the lengths of all the revegetation sites. This measurement must be accurate to ensure an accurate assessment of planting stock requirements.

The plant density should aim to reproduce the natural environment. In ecology, competition among species is what determines the space they occupy. To ensure leaf cover of the soil surface using a minimum number of plants, use a quincunx (or staggered) pattern (Figure 1). To quickly obtain complete coverage with no partial openings between plants, a 0.76 ratio should be used instead of a 0.87 ratio, thereby increasing the number of plants per square metre.

Figure 1. Quincunx or staggered planting pattern

Quincunx or staggered planting pattern
Quincunx or staggered planting pattern

To calculate the number of plants required for a given area, you will need to calculate the number of plants per row and the number of rows, taking into consideration that spacing between the rows is 87% (or 76%) of the distance between the plants. In the case of densification projects, the density you use should be lower than recognized standards to take into account the presence of existing vegetation.

For freshwater riparian strips, the MDDEFP recommends a revegetation distance of 0.5 to 1 m centre to centre for shrubs and 2 to 5 m centre to centre for trees (MDDEFP 2009a). Of course, some species will require more or less space to reach their full potential. The optimal spacing will depend on the species selected (8.6).

Now that you have specified these three parameters, you can calculate the number of plants you will need for your project (Section 8.3). If you opt for a different planning model, you need to justify your choice. A simple way of determining the right number of plants required is to prepare a scale drawing for each revegetation area.

8.3 Estimating plant quantities

Your funding application must specify the required quantities of native shrubs and trees, indicating the number of plants along with container sizes. Quantities may be specified by species of the same size. This is not a requirement because, barring exceptions, there is generally little variation in price among plant species of the same size.

Final revegetation plans, which are generally developed during the course of the project, will specify the final selection of species in terms of local criteria, which can only be determined on site. Nevertheless, a specific quantity of planting stock needs to be indicated in the funding application, along with the final budget.

For herbaceous and perennial species, seed requirements need to be determined for the area to be covered. Seed suppliers generally specify kilogram per hectare rates.

In short, you need to specify the following three quantities:

  1. Number of trees (by species, if you wish) of each size
  2. Number of shrubs (by species, if you wish) of each size
  3. Kilograms of herbaceous and perennial seed to cover bare areas

Appendix 4 presents an established calculation method for quincunx (or staggered) planting.

8.4 Plant size

The size of the plants (trees and shrubs) is one of the key elements for ensuring the success of your revegetation initiative. There will be competition between the stock you plant and the existing plants, specifically herbaceous species. If you opt for small trees and shrubs (in cell packs less than 300 cm3), they will have to compete more with the herbaceous species on site. In addition, weeding will be more difficult because the planted specimens will be less visible. By contrast, large plants cost more, but the survival rate will be higher and maintenance less onerous.

For successful revegetation, you need an effective strategy. The better developed the existing plants are, the larger the planting stock should be (e.g. plants in one-gallon containers) to ensure they have an advantage over existing vegetation. If there are practically no existing plants, it is recommended that you use a mix of small, medium and large plants. In addition, never underestimate the benefits of herbaceous and perennial species when it comes to effective and rapid soil erosion control. Section 9.2 outlines other strategies for overcoming competition from existing vegetation, and Section 9.3 describes plant maintenance and protection measures to ensure a successful outcome.

8.5 Estimating planting stock costs

To establish a budget for plant purchases, the unit cost per size must be estimated. Barring exceptions, unit prices vary with the size of the plants, but remain relatively similar for different species of the same size. To come up with a realistic estimate, follow the steps below:

  1. Calculate the total area to be revegetated.
  2. Evaluate the proportion of the total area to be planted with trees and shrubs.
  3. Determine the fraction of this area to be planted with the different sizes of trees and shrubs (Section 9.2).
  4. Convert each of these areas to plant and seed quantities using the proposed formula (Appendix 4).
  5. Determine the average cost per plant size (Appendix 5). Pay special attention to unusual plants that may cost far more and could lead to a substantial error in the estimate.
  6. Multiply the average price by the number of plants of each size or the quantity of seed required.
  7. Add up the individual amounts to determine the total cost for the budget.

These are estimated amounts. The quantities may vary slightly once the project is underway and according to what is available at the time. You will nonetheless have to work within the overall budget specified in your application; hence the need to be as accurate as possible in the planning stage.

8.6 Selecting plants

The plants you select should be native species suited to the environmental conditions in the riparian strip to be restored. If you are not knowledgeable in this regard, you would do well to consult a botanist. You are advised to follow the steps below: 

  • Identify species in non-degraded riparian strip zones near the area to be revegetated.
  • Check whether the species observed appear on the MDDEFP’s recommended plant list (FIHOQ 2008) for shoreline revegetation by consulting its online listing (http://fihoq.qc.ca/recherche-plantes.html).
  • Ensure the selected plants are suitable for the soil, relative humidity, shade and other conditions.
  • Vines growing over walls and riprap could increase the albedo effect and hence reduce warming of the water.
  • Consider plant species that attract fauna such as butterflies, bees or birds.
  • Give priority to trees, especially on the top of slopes. Some species have a root system that provides excellent soil stabilization and good surface water filtration.
  • For bare soil areas, plan to seed native herbaceous and perennial species adapted to riparian environments.
  • Give preference to plants from local nurseries to avoid significant genetic and climatic variations.

Extensive documentation exists on plants available for revegetation purposes. A selection of references is provided to facilitate your research (Section 9.2).

Some species, such as willow (Salix) and dogwood (Cornus), are easily propagated by cuttings. To ensure success, however, you need to start the process at the right time. You should also remove only a fraction of the branches. Don’t forget to obtain prior authorization from the landowner.

When the riparian strip has to be redeveloped, transplanting plants to the redesigned area is a potential strategy. For successful transplanting, you need to remove plants along with a large root ball, heel them in temporarily and water generously until they are transplanted.

In coastal environments, some groups recommend transplanting species such as Sand Ryegrass (Elymus arenarius) and Beachgrass (Ammophila breviligulata). In such cases, the clumps from which you take specimens must be large and very dense.

Note that Renaud (2005) describes an interesting technique for selecting plant species. She recommends selecting plants in terms of the ecological niche of the area to be revegetated. This approach is a sure way of increasing the survival rate of planting stock.

Tips and tricks: If you opt to collect cuttings, make sure you obtain prior permission from the landowner and inform the relevant authorities (MDDEFP and municipality) about this initiative. Prepare a cutting protocol to present to them.