Gravel Gardening: The Planting Formula to Success
In our first discussion of gravel gardens, we discussed a small planting around the Entrance Rock. This planting simply consists of tough plants in gravel, planted deep enough to reach the soil underneath. The inspiration came from the designer, Cassien Schmidt, a German garden designer who has developed naturalistic methods of planting in gravel. Schmidt came to the Scott Arboretum to hold a workshop and also consulted on the traffic circle planting on Chester road. Cassien has developed his way of planting as an easy and efficient formula mostly for municipal plantings and larger, low-maintenance plantings, but it can also be used for the home gardener.
After seeing so much success with the traffic circle in just under 2 years after initial planting, the College and Arboretum decided to plant more areas using gravel culture. In the last few weeks we have installed a gravel garden around the bus stop in front of 101 Chester Ave., on Fieldhouse Lane, and replanted a gravel garden in front of the PNC bank (a replanting due to construction). We used Cassien’s formula to create all of these gardens.
First we laid out a grid. Within the grid there is a list of plants and within that list there is a percentage of each type of plant. Different types of plants contribute different benefits to a group planting and act as a community.
In this planting there are 5% structural plants, 35% companion plants, 50% ground layer plants, and 10% filler plants. The plants chosen are mostly native and are selected for their ability to live through drought and full exposure to the sun. All of these are perennials but it is possible to incorporate woodies, as we did in the traffic circle.
Within each group of plants there are between 4 and 6 different plants. For example: within the ground layer grouping there are four selections; Geranium cantabrigense ‘Karmina’, Ruellia humilis, Eragrostis spectabilis, and Asclepias tuberosa. In one block, if you need four ground layer plants, you can use one of each.
The plants for each block were grouped in crates off-site, then moved to the site and placed in each block of the grid. This made it easy for anyone to take plants out and plant randomly, and random is key. It’s hard sometimes to not think about where you’re placing plants, especially if you’re used to grouping or making swaths. In this planting design, the plants are all meant to grow together and through each other like they would in nature. It’s important to plant densely to cover the ground and inhibit weeds. Although the idea is to plant randomly, it is helpful along paths to plant things that won’t get big and flop over. If you have a narrow strip or an area that someone in a car needs to see over, it is best to leave the taller, structural plants out.
Planting in gravel culture is different. It is important that the roots reach the soil level because they cannot grow out into the gravel. The nutrients or water that most plants need are not present in gravel. In our gravel beds there is 6 inches of gravel above 4 inches of gravel-soil mix. To get to the soil, it takes a bit of digging and it’s important not to get soil in the gravel because that’s where the weed seeds can grow.
I’ve found that a short-handled spade is best for digging. We dug deep, wide holes down to the soil, pulling the gravel layer out wide. It sounds harsh, but each plant gets the soil removed from its roots before it’s planted. Having a bucket close by for soil removal is important to keep soil from falling into the gravel. Removing the soil encourages the plant to go farther down with root growth and discourages weed seeds from growing out of the nursery mix that the plant was in.
After filling in with the gravel that has been dug out, it seems that you have just dug its grave because it is so deep, but this has been proven to work. We have had some casualties, but the majority of plants have thrived in this situation.
Low maintenance is a major benefit of gravel culture. With the new plantings, we will water them as needed. The schedule is more frequently immediately after planting, and after a year, plants should be established enough not to need any watering except during an extreme drought. No fertilizers are added. Ideally, the planting should only need a few visits a year for possible weeding, and a cut back in the fall or winter to prevent debris from breaking down into soil for weed seeds to grow in.
Give gravel gardening a try in your sunny spot.