Hello,
Just wondering what the effect of 4foot depth of HPB Gravel with only 6 inches of topsoil on it for sod will have on the rest of my garden and future garden plans? In central Etobicoke there is currently a major utility project involving trenching in backyards and on the boulevards (or hellstrips). Over 50 square feet (over 200 cubic feet) of my rich dark earth was removed and replaced with the gravel and sprinkling of topsoil, sod to be replaced later. Aside from the obvious fact that I am now unable to use my ground as I choose, unable to put a shovel in the compacted gravel (feels like concrete) and the soil depth is very limiting to many plants/vegetables that I might want to grow, does the gravel leach anything harmful, change existing drainage patterns or stress existing trees or plants? These trenches running in the hellstrips are extremely close to many city trees also.
Appreciate any advice about this issue. There are many, many properties that have been recently done, really concerned about the broader impact in the community with the loss of gardening options.
Thank you!
Thank you for contacting Toronto Master Gardeners with this difficult issue regarding trenching. HPB (high performance bedding) is a self-compacting gravel, often used as a base for patios, walkways and other landscaping hardscape projects. It is usually a 1/4 inch limestone chip, though composition can vary ( rocks and gravel ). As you have noted, it may not make a very useful “underlay” for traditional soil-based lawns, gardens and borders. It does, however, provide excellent drainage, even when compacted.
You have expressed several concerns. Let me try to tackle them one by one.
- Soil depth and sod replacement. The normal guidelines for soil depth under grass (whether sod or seeded) is 4 to 6 inches, the deeper, the better. You mention that 6 inches of topsoil is planned. Given that it will be on top of gravel rather than subsoil, 6 inches may be a little shallow for the growth of good, strong roots. The gravel may be a help if you have areas that suffer from poor drainage. I was surprised to read about a garden centre, Joy Creek , that recommends top-dressing lawns with gravel in fall or winter in order to improve drainage and reduce problems caused by waterlogged soil (admittedly in an area with milder winters than we experience).
- Soil depth and other planting options. Again, a 6 inch soil depth will be adequate for some plants and not really sufficient for others. It will depend on the root structure of the plants and their natural habitats. Gravel and scree gardens are becoming very popular, environmentally sound, options these days. You may wish to look into that as an option to make the most of the gravel, especially if you have a sunny situation. Take a look at these DIY ideas if you are interested: Gardenista article on gravel gardening, Peterborough Master Gardeners on scree gardening, and Creating an alpine scree garden.
- Gravel leaching harmful chemicals. Unless the gravel was treated, it should not be leaching harmful chemicals. If it is composed of limestone chips (which is typical with HPB), the challenge may be finding plants that are happy in an alkaline environment. Many ornamental and vegetable garden plants prefer neutral to slightly acidic soil. Adding a substantial layer of topsoil mixed with compost on top of the “provided” topsoil will help. You could even consider raised beds in order to reduce the pH problem while substantially increasing soil depth for planting. If you decide to try the gravel or scree garden approach, you can add some differing types and sizes of stone to the garden bed, as well as “planting pockets” of soil mixed with compost. Again, your best options will depend on the location and conditions of the proposed planting area, as well as your personal preferences.
- Possible changes in drainage patterns. There may well be some changes in drainage patterns because of the trenching and the introduction of the gravel. HPB drains very well, even when compacted. This is usually seen as an advantage, but it will depend on what the soil composition and drainage patterns were like prior to the introduction of the gravel. Again, a gravel garden, dry stream bed, or rain garden might be a viable option in this new situation. You will want to select plants that appreciate excellent drainage. You may wish to have a look at these resources on rain gardens and dry stream beds, Toronto Region Conservation on Rain Gardens, Fine Gardening on Dry stream beds, to add to the ones related to scree gardens and gravel gardens, above.
- Possible harm to existing trees and plants, including city trees. It is difficult to specify the risks to existing plants without knowing which plants specifically, how mature they are, whether they have suffered any damage due to the digging and laying down of the gravel, their proximity to the gravel trenches, and the location/conditions of the specific area.
- Loss of gardening options across the community. You are justified in your concern that this level of disruption, together with the challenges of planting in and around the gravel-filled trenches, will mean fewer viable plantings and possibly an increase in eyesores in your community. Perhaps you can mitigate this negative situation a bit by speaking with your neighbours about any of the options you try that turn out successfully…
Best of luck gardening in and around the trenches!