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.

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). photo credit: S. Quinn

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.

Plants are planted within the grid. photo credit: S. Quinn

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.

Susan McGinley helping to plant the Chester 101 planting. photo credit: R. Rboert

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.

In this planting there are 5% structural plants, 35% companion plants, 50% ground layer plants, and 10% filler plants. photo credit: R. Robert

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. photo credit: R. Robert

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.

It is important that the roots reach the soil level because they cannot grow out into the gravel. photo credit: S. Quinn

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.

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. photo credit: S. Quinn

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.

Gravels covers the base of the plant. photo credit: S. Quinn

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.

It sounds harsh, but each plant gets the soil removed from its roots before it’s planted. photo credit: R. Robert

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.

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If you’ve passed the Swarthmore College Entrance Rock in the last couple of months you may have noticed something different. Where there once was a patch of grass hugging the rock, now there is gravel and a few small plants. This may seem strange to some because we have learned to plant in the soil and top-dress with mulch to give plants nutrients and retain moisture. Why would we grow plants in gravel?

The goal with gravel culture planting is to create a more sustainable garden requiring less maintenance and less water. photo credit: S. Quinn

The goal with gravel culture planting is to create a more sustainable garden requiring less maintenance and less water. This way of planting is being promoted and practiced globally through the ecologically focused  “New Perennials” movement in landscape design.

Low growing sedums were chosen for the front of the rock in order not to obstruct the night lighting. photo credit: R. Robert

One of the leading actors in this movement is Cassian Schmidt, a progressive landscape designer and the director of Hermannshoff Garden in Germany. In the fall of 2016 he taught a workshop at Scott Arboretum to share methods he developed for naturalistic gravel plantings. Examples of his methods are in Swarthmore Roundabout, and you can read more about the planting in the Swarthmorean. This planting method is so successful here that we are planting more gravel areas.

This technique only partially plants the roots in the gravel. The top 6 inches are pure gravel. Below the gravel is a 4-inch layer of mixed gravel and soil. The key to the planting is making sure the roots are in the soil. For small plants, it’s okay if some of the roots are in the soil/gravel layer.

The deepest digging was for Stachys byzantina ‘Helene von Stein’ (lamb’s-ears) and several other perennials. photo credit: R. Robert

The goal of this kind of gardening is to encourage the plant to send roots into the soil layer. There are many benefits including less watering by the gardeners. The soil retains moisture better when it is below the surface and protected. The gravel allows water to trickle down and stay for longer as opposed to a top layer of mulch where water evaporates quickly.

Inevitably some bold seeds make it but far fewer than those found in a bed of soil and mulch. Here is a weed sprouting underneath the sedum. photo credit: R. Robert

With a thick layer of gravel, wandering weed seeds are less likely to find a home to germinate, so gardeners will have less weeding to do. Inevitably some bold seeds make it but far fewer than those found in a bed of soil and mulch. So far in the Swarthmore Entrance Rock bed, the crabgrass has only come up along the edges where there was more soil left behind, and a few bits of pioneering nutsedge have come up in the middle.

The new Swarthmore Entrance Rock planting is mostly sedums which only needed to be tucked into the first layer of gravel. photo credit: S. Quinn

The new Swarthmore Entrance Rock planting is mostly sedums which only needed to be tucked into the first layer of gravel. The deepest digging was for Molinia caerulea ssp.  arundinacea ‘Skyracer’ (purple moor grass),  Stachys byzantina ‘Helene von Stein’ (lamb’s-ears), and Calamintha nepeta ssp. nepeta (calamint).

Thriving in the shade is Carex plantaginea (plantain-leaved sedge). photo credit: R. Robert

Having already chosen plants for tough conditions, the plants and design changed little when the Collections Committee decided to convert the bed to a gravel garden. Thriving in the shade are Helleborus x ballardiae ‘Coseh 800’ (HGC Snow Dance hellebore) and the Carex plantaginea (plantain-leaved sedge), while the rest have bright sun in an island bed surrounded by asphalt.

A taller sedum, S. SUNSPARKLER® ‘Blue Pearl’ (stonecrop), is for variation in height and color. photo credit: R. Robert

Low growing sedums were chosen for the front of the rock in order not to obstruct the night lighting. The mix of sedums includes Sedum hybridum ‘Immergrünchen’ (little evergreen sedum), S. album (white stonecrop), S.  kamtschaticum var. floriferum ‘Weihenstephaner Gold’ (orange stonecrop), and S. spurium ‘Ruby Mantle’ (creeping stonecrop).  A taller sedum, S. SUNSPARKLER® ‘Blue Pearl’ (stonecrop), is for variation in height and color. Plants like Molinia caerulea ssp.  arundinacea ‘Skyracer’, Helleborus x ballardiae ‘Coseh 800’, Stachys byzantina ‘Helene von Stein’, and Calamintha nepeta ssp. nepeta were chosen to compliment the surrounding beds.

The only issues so far are that the Stachys had its lower leaves burned by the reflection of the sun off the gravel. photo credit: S. Quinn

After regular watering in the first few weeks following installation, the plantings are doing well despite the persistent heat. The only issues so far are that the Stachys had its lower leaves burned by the reflection of the sun off the gravel. The leaves were quickly removed to keep them from decomposing on the gravel.

Our next venture in gravel gardening will be in the parking lot on Field House Lane. Follow us as we explore the techniques and tips in our Gravel Gardens series.

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