Dictionary of Terms

Adapted Plant – A plant that is not native to a particular geographic area, but thrives in the ecological and climatic conditions of that area without being harmful (i.e. is not invasive).  Adapted plants are highly effective on green roofs.  For a list of adapted plants that work well on green roofs, check out our online catalog.

Annual – A relative term that describes a plant that produces vegetation, flowers, sets seeds, and dies in one growing season.  Annuals can be a good choice as accent plants on a green roof, however they may not reappear after several growing seasons due to the lack of open ground for the seed to establish in.  Plants that are considered perennials in warmer climates may be considered annuals in cooler climates.

Biennial – A plant that produces vegetation the first growing season, and flowers, sets seeds, and dies during the second growing season.  Biennials are not a good choice for green roofs because they leave gaps in the plant cover after the second season.

Confirmed Order – An order is confirmed when Emory Knoll Farms receives either a completed Purchase Order or a down payment.

Cultivar – A plant variety that has been created through intentional breeding and that has been deliberately selected specific desirable characteristics.  A variety of plant selected for distribution.

Cuttings – Un-rooted pieces literally cut from mature green roof plants.  Sedums and some other species are able to be grown on green roofs from cuttings, which are spread directly on top of the growing medium.  Spreading cuttings around plugs is a technique sometimes used to decrease the amount of time it takes to achieve a fully covered green roof.

Deciduous – A plant that drops its foliage during some portion of the year, between growing seasons.  Deciduous plants add organic material to the green roof environment.  Over time, an abundance of deciduous plants on a green roof may change the balance of the composition of the medium thereby affecting the plants or the functioning of the green roof.

Drainage / Positive Drainage – Similar to directing water away from the foundation or basement of your home, water on a roof must have a way to drain away from the roots of the plants and also must be provided with a way to leave the roof.  The structure of a green roof can be thought of as a large-scale container; as with any planted container (aside from ponds), the water must have a way out.  There are several construction techniques that can achieve proper drainage for a green roof; these should be employed according to the specifications and needs of each project.

Drainage Layer – A middle layer of green roof construction, above the root barrier and below the filter layer.  Its purpose is to provide positive drainage on the roof and prevent an excess of water retention.

Drought Tolerant – A plant that will tolerate periods of low rainfall or drier climates.

Evergreen – see Winter Interest

Extensive Green roof – A description based on depth of the growing medium on the green roof.  Extensive green roofs typically have growing medium at a depth of 4 to 6 inches.  Extensive green roofs may be retrofitted onto an existing building or planned as part of new construction.  All green roofs must be engineered to support the increased weight of plants, growing medium, and other green roof components.

Filter Fabric (Filter Cloth) – A middle layer of green roof construction, above the drainage layer and below the green roof medium.  Its purpose is to filter small particles and prevent the drainage layer from clogging.

Flat – A plastic container comprised of many individual cells in which plants are grown.  Plants at Emory Knoll Farms are grown one species per 72 cell flat (i.e. 1 flat = 72 plants).

Grass – An herbaceous plant with narrow leaves generally growing outward in an array from the base.  Sedges, rushes, bamboo, and turf grass are all examples of grasses.  There are both native species and cultivated varieties of grasses.  On green roofs, grasses typically need irrigation and at least 6 inches of media to thrive.  Additional ongoing maintenance is usually required of these plants.

Green Roof – A roof system designed to support the installation and growth of plants.  Green roofs are an attractive and energy-saving alternative to conventional roof construction.  In addition to their environmental and aesthetic advantages, green roofs help insulate buildings, benefit the energy systems of buildings, and can extend the life of a roof.  For more on green roofs, please see the Green roof Technology section of our website.

Groundcover – A plant with significant horizontal growth that can be planted to cover large areas of soil.  Groundcovers work exceptionally well on green roofs as they cover the growing media fairly quickly, limiting maintenance due to weeding.

Growing Medium – A specially engineered, often lightweight and extremely porous material used for green roof applications.  Extensive green roof medium is typically made up of 75 – 90+% inorganic materials mixed with a small amount of organic material (preferably compost).  The exact mix used in the medium should be designed on a per project basis depending on the plant palette and environmental factors specific to each project.

Hardiness Zone – A number assigned to a geographical location by the U.S. Department of Agriculture, based on average and low temperatures found in that area at ground level.  Using the same numbers, Hardiness Ratings are assigned to plants based on their ability to survive in the geographic area of that temperature range.  The zone number is a guideline, and complying with the zone may not ensure the success of a particular plant on a roof, which may experience different conditions than ground level.  Look up the Hardiness Zone of your project in our Catalog.

Heat Island – A spot that heats up quickly during the day, reaches high or extremely high temperatures, and retains that heat a long time, releasing it gradually and thereby increasing the overall temperature of that area compared to adjacent areas.  Traditional dark colored rooftops and asphalt parking lots are examples of man-made heat islands.

Herb – A seed-producing annual, biennial, or perennial that does not produce a woody stem, but dies back at the end of the growing season.  Herbs require growing medium of at least 6 inches in depth and irrigation to survive on extensive green roofs; these plants are generally short-lived in a green roof environment.

Herbaceous Perennial – A plant emerging from the ground at the beginning of each growing season which dies back to the roots at the onset of winter.  Perennials offer a vast range of colors, textures, and seasonal variability; however, few perennials are able to withstand the low maintenance and harsh conditions of a green roof environment.  Those that have proven to do well on green roofs generally require a layer of growing media layer of at least 6 inches in depth, and must have some irrigation to continue to thrive.  Perennials should be carefully chosen for use on green roofs based on the project location, environmental factors, and maintenance budget.

Impervious – On sites, impervious areas are those that do not allow the passage of water into the soil.  Parking lots, sidewalks, patios and the area within the building footprint are all examples of site elements that are traditionally constructed with solid materials that do not allow water to pass through to the soil.  Lawn, turf or sod areas are minimally impervious.  Under the LEED rating system, points may be earned for achieving a particular percentage of pervious area on a site.  For more information on LEED and green roofs, look at our Online Resources.

Inorganic Material– Typically refers to stone, rock, and other inorganic materials included in the green roof growing medium.  May include expanded slate, expanded shale, expanded clay, baked clay, volcanic rock, or sand.  See Growing Medium.

Intensive Green roof – A description based on depth of the growing medium on the green roof.  Intensive green roofs typically have growing medium at a depth of 12 in. or more.  Intensive green roofs more closely resemble ground level gardens and must be engineered to support the immense weights.

Invasive Plant – A plant that has thrives and spreads aggressively in locations outside its native range of habitat.

Irrigation – A system of watering plants on an occasional or daily basis; systems may be built-in or temporary.  Irrigation may be necessary for plant establishment or for the long-term success of the green roof, depending on the geographical location of the project, the plant palette, and the time of year the plants are installed.

Light Requirement – The amount of light a plant needs to thrive; typically broken into the following levels: sun, partial sun/ partial shade, or shade.

Live Load Refers to a roof that must be engineered for movement on the roof.  Examples include roof installation equipment, having maintenance people on the roof regularly, an observation deck on the roof, and wind, snow, ice or retained rainwater.

Membrane – A waterproofing layer; often the lowest layer of a green roof which comes directly in contact with the roof structure.  Made of flexible material, its primary function is the exclusion of water (from the building).

Microclimate – An area occurring on a roof that differs in some significant way from the environmental conditions of the surrounding area.  For example, vents, walls, heating and air conditioning units, and drains are some examples of fixtures that can create a microclimate on a green roof.  Architects and landscape architects may need to take these microclimates into consideration when designing a planting plan.

Module (Green Roof Module) – A pre-planted or pre-grown (vegetated) green roof container holding green roof media and plant material.  Modules are generally a rectangular shape covering 2 – 8 sf of roof area.  Containers are placed side by side on the roof in a grid system.  Modular systems can work well on sloped roofs, however they restrict the lateral mobility of the plant roots, moisture, and nutrients.  As with any green roof system, the advantages and disadvantages of the products used should be weighed according to the parameters and goals of each project.  Manufacturers should be contacted directly for specific product information.

Moisture Tolerant – A plant that will tolerate some moisture around its roots, including occasional irrigation and wetter climates.

Native Plant – A plant that has evolved in response to the ecological and climatic conditions of a particular geographical location, allowing it to thrive in that location.  Native plants are typically resistant to disease, insects, and animals found in the indigenous environment of an area, and are highly adapted to the soil conditions and nutrient levels found at ground level.  However, almost never are the insects, animals, and soil conditions occurring naturally at ground level found on the roof of a building; exposure, load bearing considerations, and access are all factors that may limit the re-creation of these indigenous conditions on the roof.  Because of this, native plants are not generally well suited to the conditions found in an extensive green roof environment.

Organic Material – Typically refers to the amount of compost or other forms of solid nutrients included in the green roof growing medium.  See Growing Medium.

Pervious – On sites, pervious areas are those that allow the passage of water into the soil.  Meadows and gardens are site elements that are traditionally constructed to allow water to pass through to the soil.  New materials are now available to create pervious hardscapes (driveways, parking lots, patios etc.)  Lawn, turf or sod areas are not always 100% pervious.  Under the LEED rating system, points may be earned for achieving a particular percentage of pervious area on a site.  For more information on LEED and green roofs, look at our Online Resources.

Plant Establishment – The time necessary to acclimate the plants to their new environment after installation; plants are generally considered established once they have rooted into the growing medium and are producing new growth.  Generally, much care must be taken during this time, including proper watering to promote root growth, and weeding to eliminate root competition.

Plugs – Sometimes referred to as landscape plugs; all of our plants are sold as plugs in trays of 72 plants per tray.  Our plugs have a root mass that is approximately 1 inch across at the top, and 3 inches deep tapering slightly towards the bottom. Through testing, we have found this to be the optimal size for growing plants on a green roof.  Even larger sized plants will not outperform plants started from plugs over the course of a growing season.

Protected or Preserved Habitat – A natural area on a developed site that protects the existing ecosystem so that it may continue to function as habitat in conjunction with the developed portions of the site.   Protected habitat should be able to support the variety of organisms native to the area.  Under the LEED rating system, points may be earned for protecting habitat on a site.  Protected habitat is not possible to achieve on green roofs due to their location on the roof of the building. Also see Restored Habitat and Native Plants.

Rain Garden A planted area, typically at ground level or in a low-lying area, which makes use of plants that tolerate flooding.  Rain gardens use drainage pipes, reservoirs, landforms, and other storm water management techniques to manage rainfalls varying from small amounts to major storm events.  Extensive green roofs do not typically support the installation of rain gardens due to the types of plants, soils depths and substructures required.

Restored Habitat – An area on a developed site that is environmentally similar to the type of ecosystem that would have been present prior to the site being developed.  Restored habitat should be able to support the variety of organisms native to the area.  Under the LEED rating system, points may be earned for restoring habitat on a site.  Restored habitat is difficult if not impossible to achieve on green roofs due to exposure, load bearing considerations, and access.  Also see Native Plants.   For more information on LEED and green roofs, look at our Online Resources.

Roof Garden – An intensive green roof; a ground level style garden planted on a roof.  Some people may consider a roof garden any plants grown above street level, whether those plants are grown in several small pots or are grown directly on the roof surface in an area filled with growing medium.  See Intensive Green roof.

Root Barrier – A middle layer of green roof construction, above the waterproof membrane and below the drainage layer.  Its purpose is to prevent the roots of the plants from damaging the roof membrane.

Sedum Mat / Sedum Carpet – A vegetated mat comprised entirely of Sedums.  Sedum mats are available at Emory Knoll Farms.  See Vegetated Mat.

Self-sowing – A plant that reproduces from viable seed; self-sowing plants will spread across a green roof as seasonal conditions favor that particular plant.  Over time, self-sowing plants may impact the planting design or may reproduce at the ground level around the building from seed carried by the wind or by birds.

Species – Biologically, a group of plants capable of interbreeding and producing fertile offspring, and separated from other plant groups with which breeding does not normally occur.  Species boundaries are constantly changing with increased scientific knowledge (i.e. botanical names often change as plants are re-classified).  Sedums are one of the most commonly found species on green roofs.

Storm Water Management System – An overall plan comprised of smaller systems that are designed to work together to capture, contain, transport, or disperse precipitation occurring on a site.  With proper engineering and planning, a green roof can be one component of a storm water management system.

Urban Heat Island Effect – There is a concentration of materials in cities that create heat islands (e.g. brick faced buildings, asphalt parking lots and roads, dark rooftops, concrete sidewalks and patios, etc.).  The combined affect of these increases the average temperature of the urban area compared to surrounding rural areas.  Research has found that the higher urban temperature increases energy demand, air pollution and greenhouse gas emissions, and heat-related illness and mortality.  Higher temperatures can also adversely affect water quality.

Vegetated Mat – A pre-grown green roof plant product consisting of a 2-3 in. layer of plant material rooted into a 1-2 in. layer of mesh and growing medium.  Vegetated mats are typically cut into rectangular sections and rolled up for transportation to the site.  This product can work well for sloped roof applications, but may require a significant lead time when ordering, require special shipping arrangements, and take extra care to establish.

Weed – A plant that is not intentionally planted and is not desired in the place it is found growing.  Weeds are generally plants that spread by producing vast amounts of seeds during a growing season.  They may be invasive plants, or simply a plant from someone else’s garden that grows from a transported seed.  Weeding is the most important maintenance procedure on a green roof during establishment; weeding should be done according to the seed cycles of weeds common to your project area.  Note that some green roof plants may be grown from seed or may emerge as seedlings on a green roof; having personnel that are able to identify these plants is key to a successful maintenance program.

Winter Interest – Many green roof plants have interesting attributes in colder or winter months.  Some plants retain their green color through the winter months (some varieties of Jovibarba and sempervivum).  More often, plant leaves change to a different color (for example, Sedum hybridum ‘Immergrunchen turns orange in the fall and holds much of it’s foliage throughout the winter).  Several plants drop most of their foliage but the plant stems or rosettes provide interest (for example, most varieties of Sedum spurium). Other plants die back to the roots completely, but leave behind a structure that lasts through the winter (grasses, for example, leave behind light brown stalks that some people consider sculptural).  For more information on plants that have visual interest in the winter months, please see our Plant Catalog.

Xeric – A term describing a plant that has adapted a biological process for survival during periods of extremely low amounts of water, as in the desert.  These plants typically have shallow root systems and can survive extreme temperatures (both hot and cold).

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