Mulch research article examines relative benefits of gravel and topsoil in prairie plantings

A new research paper, To mulch or not to mulch? - effects of gravel mulch toppings on plant establishment and development in ornamental prairie plantings, has been published.

Authors Anja Schmithals and Norbert Kühn said: "In recent years, North American prairie vegetation has served as a design model for highly attractive, low-cost and low-maintenance plantings in German urban green spaces.

"Where mixed-planting techniques, gravel mulch toppings and non-selective maintenance techniques such as mowing are used, prairie plantings are considered to be cost-effective alternative design concepts for public green space management.

In their study, they investigated the establishment success of different mixtures of prairie species plantings on two sites with different soil conditions: topsoil and topsoil with graywacke gravel topping.

The authors found:

  • Significantly higher average mortality rates on gravel mulch sites in the first year after establishment.
  • Further development of mortality was not significantly different between sites.
  • Weed species were always more numerous on topsoil sites and had an obvious effect on the visual impact of the plantings. The mulch created an effective barrier for wind-dispersed germinators.
  • Soil temperatures down to 30 cm were significantly higher on gravel mulch sites throughout the year, stimulating more vital plant growth and a prolonged growing season.

The authors conclude: "Our results emphasise the importance of considering these kinds of practical issues during the planning process as they are critical to the success or failure of the design."

The study focused on the effect of different soil treatments on the establishment success of prairie mixed plantings in a Berlin urban context and produced valuable insight on the suitability of the employed species and on establishment techniques.

  • The establishment process itself proved to be less successful on gravel mulch plots than on topsoil.
  • The lack of material capable of storing nutrients and water caused significantly higher mortality rates in gravel mulch sites during the growing-in phase.
  • A gravel mulch layer on top of in-situ soil caused highly elevated surface temperatures in high radiation summer weather and higher soil temperatures across all depth layers throughout the year. This warming process resulted in significantly increased biological activity and enhanced individual development of target plants during the summer once they were successfully established.
  • Winter site conditions were shifted towards significantly warmer soil temperatures above the frost level on mulched sites, which caused species with a preference for well-drained soils to suffer regularly from temporary water-logging (e.g. BcurtipendulaAtuberosa).
  • The presence of seed-propagated weedy species was significantly reduced on mulched plots, while no effect was found for rhizomatous weeds.

The authors said: "In the mulched plantings, a generally enhanced visual impact, the success of individual target plants and the obvious reduction in weeds need to be weighed against the time and cost of installing the mulch layer and the need for more intense and longer lasting establishment care to compensate for the harsher site conditions".

The authors concluded: "Our findings will hopefully support the planning process of future projects. Further analysis of how management and species mixture affect the development of the experiment site will be of great interest. Ongoing investigation into the medium- and long-term development of the plantings will be necessary to answer questions on community processes. Successful establishment of sustainable planting designs depends on long-term rejuvenating processes within the plant community, and thus the possibility that the mulch layer will inhibit seed propagation needs to be carefully considered."

Schmithals A, Kühn N (2017) To mulch or not to mulch? Effects of gravel mulch toppings on plant establishment and development in ornamental prairie plantings. PLoS ONE 12(2): e0171533. doi:10.1371/journal.pone.0171533

Editor: Jian Liu, Shandong University, China.

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