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One of the most attractive of all walkways that a designer can introduce into a landscape or garden layout is a natural stone path. Stone is one of the fundamental elements of nature, and when surrounded by filler plants, becomes an enduring part of the natural outdoor environment.
Aesthetically, a fine stonework path will seem to “always have been there“, blending in exquisitely with the other elements of a naturally-themed garden or landscape. Nothing looks more welcoming to the inquisitive garden explorer, inviting them to look further.
Stonework paths are usually comprised of random shapes of naturally flat cuts of stone such as shale, granite, sandstone, or slate, arranged into a fitted pattern that accommodates the natural pacing of human traffic. They are usually laid atop or set slightly into the ground and bordered by a low carpet of plants that trail about them, anchoring the composition into the natural setting.
Flagstones are the stone of choice for a truly natural look, but almost any natural stone or even constructed brick with a flat surface and adequate surface area for a human foot will do just fine. Many pre-cast alternatives are now being sold with designer patterns and repeating tiling, opening up a world of possibilities to the creative designer!
The proper construction of a stone walkway is the topic of other discussions; here I want to focus on the plants that are used to fill the spaces in between the rocks or bricks. You see, not all plants are ideal candidates for such an application; in fact, only a very select few are.
Besides being an excellent groundcover, it takes a certain set of attributes to make the perfect filler plant. But be not afraid, for choices abound, and with the right selection, your stone walkway will make you the envy of the neighborhood “garden path” club!
The Makings of a Filler Plant
Ideal candidates for filler plants should meet certain criteria, given the specifics of the intended application. For one thing, they should be low-growing. Give or take a little flexibility for scale, they should range in height from virtually nothing at all to no more than about 1-2″.
Anything taller will start to make the stones look like they’ve sunk into the ground and will make the surface seem uneven and feel a little awkward to the pedestrian. Generally, the larger the scale, the taller the filler plants can be.
They need to be vigorous and dense enough to carpet the area in between the stones, keeping out competition from weeds and quickly blending into a uniform contiguous surface for maximum visual appeal. They certainly need to be tough and durable enough to take the occasional trampling from an errant foot without suffering any ill effects or turning into a pulpy residue that sticks to the soles of one’s shoes.
And they need to be able to tolerate the inevitable coating of dirt, grime, and possibly even a little salt that will get tracked in from dirty shoes passing overhead.
Another important criterion for this application is the ease of maintenance. Ideally, groundcover plants for filling between walkway stones should require as little maintenance as possible, once appropriately planted and fully established.
That also means they should possess a habit of growth that spreads outwards naturally, hugging whatever surface the plant trails across, with negligible vertical growth or “mounding”, yet without eventually consuming the stones entirely.
And finally, the ornamental and landscape attributes of the plants should at least be considered and evaluated in the context of the overall design. These include foliage color, texture, and form, flower and fruit color if any, as well as seasonal colorations. These should complement the colors and textures of the stones rather than competing with them, and be congruent with the theme of the landscape.
Site Preparation and Planting
The key to success with filler plants in a walkway application goes further back in the process than most would expect, all the way back to the point where the stones themselves are installed. In the preparation of an ideal base for the stonework, it is important to anticipate the optimal growing conditions for the filler plants that will ultimately be growing in between.
The ideal base for a natural stone path is typically comprised of a base of at least a few inches of crushed rock or coarse gravel for drainage, over which a layer of sand has been applied on which the stones are firmly seated. Unfortunately, such a substrate is less than optimal for all but a few groundcover plants; most tend to prefer a soil substrate with good organic content.
For that reason, it might be a good idea to only apply the gravel or crushed rock as a base immediately underneath where the stones will sit. In between, a blend of gravel, sand, quality garden topsoil, and organic compost can be used to maintain a degree of drainage while offering a better substrate for the plants to grow in.
Likewise at the surface, sand should be used to settle the stones into their final seating, while at least one or two inches of good topsoil will make an optimal planting medium for the plants.
When installing the path, the stones should be completely set first, then the planting base laid, following which soil can be used to fill in the areas between. The plants can then be planted as they would be in any garden.
As with all plants, first-year care is critical; proper weeding, watering, fertilizing, and “keeping off the walk” will ensure that they are allowed to firmly set root and establish themselves.
Most vigorous groundcover plants will only require about one season of special care to take, and the path can then be used the year following with no special concerns for trampling the plants. A little patience will go a long way!
And with that, here are a few of the most popular options for filler plants between walkway stones;
Thyme (Thymus spp.)
Thyme has been considered one of the finest groundcovers for filling in between flagstones since time immemorial (pun intended). It meets virtually all of the criteria of a good filler; it spreads vigorously to form a thick carpet without being invasive; it trails nicely over rock and stone, hugging it as it goes; it tolerates a high degree of trampling without suffering visible damage or any ill effects; and, it is ornamentally attractive.
Various species and cultivars provide a range of foliage colors from gray to green to chartreuse, textures from soft and fuzzy to spiky, and flower colors of pink, purple, and white. And of course, there’s the spicy fragrance it releases when crushed. No wonder it’s the designer’s premiere choice!
One of the underlying reasons behind the popularity of thyme is that it is frighteningly easy to grow and maintain. It needs adequate sunlight, relatively good soil, and regular moisture, all of which can be provided with some good foresight in planning and a minimum maintenance effort.
In fact, this one plant is about as close to zero maintenance as is possible to achieve in the plant world; the grass in your lawn can only dream of being this easy to care for. One thing to note, it is an evergreen and may suffer some early spring damage if exposed to the winter elements without snow cover, although it will almost always recover quite quickly.
Irish Moss (Sagina subulata)
Long a favorite pathway filler in Europe, this “moss” is in fact not really a true moss at all, but is rather an herbaceous perennial, albeit one of the tiniest and finest of all perennials on this earth – you’d be hard-pressed to believe that this fuzzy green rug is actually comprised of the leaves of a plant.
Here you have a plant that grows quickly into an impenetrable carpeting of an almost uniform texture, forming slight mounds which undulate around and over the stones of your walk. The dark green leaves set off the delicate and tiny white star-flowers in late spring.
We can also include in here Scotch moss (Sagina subulata ‘Aureus’), which is a golden-foliage sister to Irish moss (actually more chartreuse, but who’s keeping score?), but otherwise identical in all respects.
As with thyme above, Irish and Scotch moss are evergreen and may burn without some cover in winter. Unlike thyme, this species is a little more susceptible to winter damage, and may take longer to recover, or may even be killed in a particularly nasty zone 2 or 3 winters.
For that reason, they are best planted in partial shade, or in a location that receives morning sun and afternoon shade, especially in late winter. The risk is worth taking, in my opinion, as the plants are easy to propagate and will grow back in rather quickly. The textural effect is magnificent, and the plants require virtually no maintenance at all!
Lawn Grass
Yes, the very turf grass in your lawn makes an excellent filler in between your pathway stones, especially if they are on the larger side, are being used in a greater lawn context, and are set a fair distance apart.
You see, the grass is much coarser than either thyme or Irish moss, therefore requiring use in a larger context. If the stones are too close together, the individual blades of grass will stand out, detracting from the intricate appeal of the stonework.
As you’re probably more than enough aware, grass requires a fair amount of regular maintenance. Whether used in a lawn or in between walkway stones, it requires regular mowing, fertilizing, and watering, and needs to be planted in good soil.
However, with good design and proper height placement of the flagstones, you can literally mow right over them as if they weren’t there. Also, don’t use grass anywhere near a garden, as it will almost assuredly cross its boundaries in short order!
Lichens And Mosses
Speaking of scale, at the other end of the size spectrum, natural mosses and lichens can make wonderful fillers. These are so fine and delicate in appearance that they make Irish moss look coarse! Yet they are tough, durable, and oddly easy to grow under the right conditions.
The key with these is to use them in a small and intimate context, where their extremely fine appearance will be noticed against the contrasting stones. I’d say to use them only with walkway stones or bricks that are a foot or less along their widest dimension, and which have only a minimal spacing in between.
What are the ideal conditions for growing moss? It requires a fair deal of shade at all hours of the day, and a considerable amount of moisture without being submerged in standing water. This may require a little effort on your part to maintain unless you happen upon the ideal circumstances by chance.
Moss is also regionally sensitive, so it would be a good idea to obtain a specific type of moss that’s known to be successful in your particular area and conditions. Frankly, I find that simply by providing the right conditions in the right place, moss will actually find its way there, without any human introduction at all!
Creative Fillers
The above options are by no means the only ones out there, just some of the more popular alternatives used by today’s gardeners and landscape designers. By understanding and seeking out the appropriate attributes in the selection of a filler plant, the truly creative designer can make all kinds of plants work in a number of unique situations.
I have seen bearberry (Arctostaphylos uva-ursi) used as a filler in more natural walkway applications set atop a particularly sandy or grave substrate. Certain of the low-growing veronicas are gaining popularity as colorful walkway groundcovers.
Many of the carpet sedums have been used as filler by more audacious designers, although their succulent leaves tend to leave behind a gooey residue when trampled upon. Even moss phlox (Phlox subulata cv.) can suffice as a colorful filler, despite its relatively coarse appearance and meandering ways.