The Field

Polyculture Maintenance and Plant Palettes

This post is about the maintenance decisions that can have a profound effect on the range of plants useful for an aesthetically qualified urban polyculture. Some of the issues are addressed in the spreadsheet that was presented in part 8 of this series. For example, relative aggressiveness will help determine if plants play well together or if one plant is almost sure to dominate. However, the discussion that follows is on factors affecting plant palette decisions that go beyond the intrinsic characteristics of each plant that is considered.

Pruning

Polycultures of herbaceous perennial plants and grasses are low maintenance but will frequently be more useful for aesthetically qualified native urban polycultures if they are pruned two or three times a year. Just because a plant is native does not mean that it must be allowed to express only its non-maintained form. This is especially true when soil amendments and irrigation are used. Water, fertilizer, and soils that are richer than what the plant would normally grow in without human intervention tend to make the plants taller, fuller, and more aggressive than otherwise, and may even cause them to flop over, particularly when they are blooming. Selective pruning may actually bring their appearance and stature back closer to a “natural” state.

Another big advantage to selective pruning is that it broadens the range of plants that can fit the aesthetic criteria of a particular polyculture. For example, one of the best native plants we have for shade conditions in North Texas is Inland Sea Oats (Chasmanthium latifolium). It is tolerant of both drought and seasonal inundation, stays attractive throughout the year, and establishes and spreads very easily. However, with irrigation it can easily get 3-4 feet tall, which may not be a desirable trait in an urban polyculture where other lower plants could have a seasonal focus. By cutting Sea Oats in half early in the season, it can easily be maintained at 18 inches tall. Some of the plants can also be left taller as “scatter plants,” which is how we are maintaining the UT-Arlington polyculture featured in part 7 of this series.

In North Texas, we have a large palette of native plants that function much better in metropolitan settings with a 4th of July mid-season pruning. Some examples include Maximillian Sunflower (Helianthus maximiliani), Mexican Bush Sage (Salvia leucantha), Blue Mist Flower (Conoclinium coelestinum), and Turk’s Cap (Malvaviscus arboreus var. drummondii). This pruning can be done very carefully with a line trimmer if the operator REALLY knows what they are doing and does not cut back other plants by accident. The line trimmer will, of course, not leave as clean a cut as a hedge trimmer or the more time consuming but much better looking use of pruning shears. A good compromise is to use the pruning shears where viewers are closest and follow up with the line trimmer or hedge trimmer further away from walkways. This midseason pruning can help determine how the plants are used in the polyculture and which polycultures they are appropriate for.

Another important consideration with pruning is the seasonal character of many native plants, particularly in non-temperate areas. Even with some irrigation, there are plants that need “freshening up” after a long, hot summer. Plants such as Horseherb (Calyptocarpus vialis) and Dwarf Water Clover (Marsilea macropoda) will benefit from careful trimming with a line trimmer around the other plants in a polyculture. A complete mowing is possible, but the yearly growth cycle of the other plants in the polyculture must be carefully considered.

The edges of the polycultures must also be maintained in order for the design to exhibit intentionality to a wide audience. Figure 2 shows the edge of a coarse-textured polyculture with Turk’s Cap, Heart leaf Skullcap, Lyre Leaf Sage, Branched Foldwing, and White Avens. By mid-summer, the edge under the Turk’s Cap is mostly the spring blooming Lyre Leaf Sage. The regularly trimmed edge keeps it from looking thin, stressed, and overgrown.

The same issues apply to the Frogfruit/Sensitive plant combination featured in part 9 of this series. Coralberry is another edge condition that would not be possible without pruning 2 or 3 times a year. The natural form of the Coralberry would be an irregular mass that spreads indefinitely. I was able to make use of this beautiful, drought tolerant, shrub as an edger only by including its pruning in the polyculture maintenance plan. Careful consideration of the height of plants through the season and the edge condition can make the difference between acceptance of a polyculture by the client or a mandated reversion to a more traditional monoculture groundcover. It opens up new avenues for more varied plant combinations and personal garden expressions.

Weeding

The discussion that follows is about removing undesirable plants, not changing the balance of species in what Piet Oudolf refers to as “dynamic planting.” Key to keeping weeds under control is a dense enough spacing when they are first planted and the use of mulch. These are the same issues that apply to any planting but they are more crucial here as it will be more difficult to separate the polyculture seedlings from the undesirables than it would be with a monoculture planting. If the weed seeds are kept from germinating until the other polyculture plants are established, the balance of propagules going forward will be in favor of the intended species.

My experience with weeding shows that it is better to remove all weeds from an area at the same time, rather than working species by species. Otherwise, the bare ground that results from pulling a plant can be an invitation to the other weed species that surround it to drop their seeds and increase their numbers. Mulch can be placed on any bare ground created by pulling the weeds to prevent their reestablishment. It is critical to immediately place weeds into a container that will keep them from dropping seeds as you go. The weeds should then be either carefully composted or placed in the trash. Some weeds, such as the highly invasive Hairy Crabweed (Fatoua villosa), can produce seeds when they are only a few inches tall despite their mature height, which can be several feet, so they must be removed when very small. Don’t worry about the biomass lost as the majority of the biomass is underground in the root zone.

Figure 1 shows the bioswale planting at The Botanical Research Institute of Texas (BRIT) featured as the lead image in part 8 of this series as a Photoshop mockup. The plants were spaced much too far apart due to delays in propagating the species, most of which are not generally available in the horticulture trade. There was also too little weeding and bare ground was allowed to persist where mulch had washed away. The original design of the polyculture is almost imperceptible. Figure 1 shows the bioswales after several days of weeding and spreading mulch on areas that were left bare after the weeds were removed. The design intent is once more coming into focus and the swale is ready for a fall planting that will infill the bare areas with plants from the original plan, bringing the polyculture closer to the design intent. The plants can then fill in without competing with a mass of species that have not been through the qualification process described in parts 8 and 9 of this series.

The polycultures that I have been personally maintaining have proven to be very low maintenance as long as a careful eye is kept on them to keep things on track. Having carefully-executed Photoshop mockups will certainly help clients understand where the design is headed. Having a maintenance book is also crucial. Every plant in the polyculture should have at least 2 pictures: one when it is a seedling and another when it is larger showing mature leaves and form. A seasonal table for maintenance should also be developed that shows each species and how it should be maintained throughout the year. The booklet is especially important when using native plants because many homeowners and landscape professionals will be unfamiliar with maintaining them. Additionally, books on native plants rarely describe the maintenance that will keep them looking their best throughout the year, much less in a polyculture planting.

Watering

One other relevant issue that must be touched on in making plant palette decisions using native plants is watering. In many areas of the country, watering will not be a prime determinant of plant palettes, particularly if native plants are used in appropriate ways. In other areas, such as North Texas where I live, decisions about watering are one of the most important determinants for plant palette decisions. The imperative to save water has pushed many designers in our area to adopt non-native species from drier parts of the state and from other dry areas around the world. This trend is another manifestation of the ethos of favoring environment over ecology discussed in part 1 of this series.

In Texas, as in other non-temperate areas, rainfall amounts vary greatly by year and are expected to diverge even more as the climate changes. They range in the Dallas-Fort Worth area from a very dry low of about 18 inches a year to the record last year of 62.6 inches. We can have several months with no rainfall, either in summer or in winter, as well as months where it rains more than 16 inches as it did in May of 2015. Native plants tend to be adapted to these varied conditions but will not necessarily perform acceptably in metropolitan situations if left to the vagaries of natural rainfall or the quantity of water available from water harvesting strategies. Therefore, I advocate a regular program of irrigation for most planted areas (as opposed to “natural” areas). The watering will be very minimal in wetter years. By more carefully monitoring of soil conditions and rainfall, watering can be kept to a minimum in normal to dry years. The goal should be to artificially create a normal to wet year that will keep the plants thriving. Not overwatering will actually reduce maintenance by discouraging weed seedlings and lessening the need for trimming as described above.

A decision should be made at the start of the project on how often a site will be irrigated during droughts. A watering interval of one to two weeks after a rain provides flexibility in native plant palette decisions but will require careful consideration of lowland species to make sure they will not go dormant in the dry months. This is a significant reduction in water use from the regular twice a week watering that is the norm in North Texas. Watering once every week or two will assure that the plant patches provides as broad an array of both environmental and ecological services as possible, where and when they are needed most, in dense cultural environments surrounded by buildings and hardscape materials.

This series on future viable plant palettes for metropolitan areas has presented a rationale and a methodology for using complex intermingled native plant combinations to create a better balance of aesthetics, environmental services, and ecological services in planting designs by landscape architects and others. The next post will respond directly to objections to the use of native plants—objections that are currently a pervasive element of the culture and professional practice of landscape architecture in the United States.

Missed parts 1-9 of this series? See below for the links to previous installments, and stay tuned for the next post:

Future Viable Plant Palettes for Metropolitan Areas

Part 1: Aesthetics, Environment, and Ecology in the Creation of Plant Palettes
Part 2: Fine Gardening
Part 3: The National Green Industry ‘Utility’ Plant Palette
Part 4: Contemporary Native and Adapted Plant Palette
Part 5: Case Study—Lessons from the Bush Presidential Center
Part 6: Native Plant Turf Polycultures
Part 7: Beginning the Transition to Native Polycultures
Part 8: Case Study—Extracting native polycultures for bio-retention structures at The Botanical Research Institute of Texas
Part 9: Assembling Polycultures from a Qualified Palette

David Hopman, ASLA, PLA, is Interim Director of the Program in Landscape Architecture and an Associate Professor at The University of Texas at Arlington, a research associate at The Botanical Research Institute of Texas (BRIT), and co-chair of the ASLA Planting Design PPN.