“C” horizon

- this is a soil transition layer below the subsoil but above the rock that is the parent material. This zone consists of rock in various forms of transition to soil due to leaching, freezing and thawing and other geological actions. 
calcareous - refers to a soil rich in calcium carbonate (lime) which will tend to have an alkaline pH.
callus - a type of plant tissue that grows over an open wound or graft union on woody plants which protects it against drying out or other injury.
calyx - the outermost whorl of organs of a flower, the outer set of perianth segments or floral envelope of a flower, usually green in color and smaller than the inner set.
cambium - one of the "living" layers of plants capable of producing new cells (i.e. meristematic tissue) and thus, growth. On dicotyledon trees, it is the layer just beneath the bark which produces new xylem and phloem cells resulting in the annual rings.

If the cambium layer is damaged or killed and can no longer produce new cells, that part of the plant above the damage will die. If it is interrupted around the entire circumference of a tree, it is called girdling and results in the death of the tree.  

campanulate - usually refers to bell shaped flowers such as those produced by members of the genus, Campanula.
cane - term used to describe woody stems of many plants including roses, grapes and brambles. The multiple shoots produced by certain ornamental shrubs such as forsythia or lilacs may also be called canes.
canker - generally a fungal or bacterial disease that causes the death of the bark, cambium and sapwood on a tree or shrub. It results in a sunken, open wound in the bark.

Cankers are common on certain species such as silver maples. They are also caused by damage to bark caused by lawn mowers or weed whips.

More on Cankers

capillary mat - type of bottom watering system sometimes used beneath flats and pots in greenhouses. It often consists of a fiber mat which will distribute water to the bottoms of pots. The water will then work its way up through the drain hole and into the pot by capillary action.
capillary water - openings in the soil or growing medium are called capillaries. Water moves through these spaces by the forces of adhesion and cohesion. This water is generally available for use by plant roots.
capsule - a dry, dehiscent fruit on certain plants which develops from two or more carpels.
carbohydrate - perhaps the key output of the process of photosynthesis are chemical compounds called carbohydrates. In the chlorophyll, the plant captures the energy of the sun and combines molecules of carbon, hydrogen and oxygen to produce carbohydrates which are later converted to sugars and starches. These compounds are then used for the growth and metabolism of the plant.
carotene - an orange colored pigment that gives color to many plant organs such as carrots, oranges and flowers
carpel - the bearing structure of a flower; each carpel encloses one or more ovules
carpet bedding (carpet bed) - plants that, due to their evenness of growth, symmetry, and/or lateral spreading qualities, produce a flat-topped or carpet appearance when displayed in a bed or border. They usually require close planting.

See Bedding Plants

cation - a positively charged atom (+). From a gardening standpoint, it comes into play when talking about the ability of a soil to hold onto nutrients and then make them available to the plant roots. This is expressed by a term of Cation Exchange Capacity which is reported on every soil test.
cation exchange capacity (CEC) - measure of a soil’s ability to retain nutrients and represents the sum total of exchangeable cations that a soil can absorb. Clay soils and organic matter have a high CEC and, therefore, hold onto nutrient such as phosphorus and potassium tightly. Sand has a low CEC and nutrients tend to wash or leach through them quickly making them unavailable to plants.

More on Cation Exchange Capacity

catkin - a spike like inflorescence of either male or female flowers without petals found only in woody plants such as willow or birch.
cellulose - a carbohydrate that makes up the primary substance of solid plant cell walls
cell-packs - a plastic container of transplants in which each seedling has its own root area. Annuals are commonly sold in cell packs that fit into larger flats.

See Bedding Plants

centered-out - occasionally, when hostas, ornamental grasses and some other perennials get "old", the center of the clump i.e. the oldest part of the plant, will die out. Often, these plants may be rejuvenated by dividing the clump into smaller pieces. Cutting into the crowns will also stimulate new, vigorous growth.
certified plants - nursery stock that is produced according to requirements of a program having the objective of producing pathogen-free plants.
chelate - chemical compounds that are formulated to not react with the soil or potting media. Iron chelates are commonly used to supplement plants. They can be mixed with water and sprayed onto the leaves thus avoiding the possibility that the iron would be tied strongly to the soil particles and made not available to the plant.
chilling requirement - many plants require exposure to cold temperatures during their yearly cycle before they will produce flowers and fruit. Normally, there is a specific low temperature requirement and a specific time period of exposure needed. For example, tulips need around 13 weeks of exposure to temperatures below 40 degrees F in order to produce flowers.

Other examples of plants with a chilling requirement are daffodils, apples, cherries, lilacs, and peonies.

chimera - cells of more than one genotype (genetic makeup) sometimes occur adjacent to each other in plant tissue. The most common chimeras exhibit themselves as variegation in certain plants. The cells in a variegated leaf all originated in the same apical meristem of the shoot, but some cells lack the ability to synthesize chlorophyll. These cells appear white rather than green even though they are components of the same tissue system.
chimeril rearrangement - at times, the pigments of plant tissues may change causing a change in color. An example would be a gold margined hosta producing a gold centered or all gold division.

When the color changes from a variegated pattern back to the original base leaf color, it is referred to as a "reversion".

cholinesterase - this enzyme is produced by animals to ensure proper nerve function. Organophosphate or carbamate insecticides inhibit or damage the production of the enzyme. A person's level of exposure to these poisonous chemicals can be determined by measuring the cholinesterase in his or her blood.
chlorophyll - this molecule absorbs sunlight and uses the radiant energy to synthesize carbohydrates from carbon dioxide, nutrients and water. This process is known as photosynthesis and is the basis for sustaining the life processes of all plants...and by extension all life on earth. It reflects the green light spectrum resulting in the typical color of most plants.
chloroplast - disk-like structure containing chlorophyll inside a plant cell.
chlorosis - (adj. chlorotic) the loss of chlorophyll which causes plant tissue to appear yellow when it should be green. These tissues are said to be chlorotic. See necrosis
chronic toxicity - often used in relation to the impact of pesticides on animals (including humans) where the exposure to small amounts over a long period of time causes a problem later in life.

See acute toxicity.

clay - the smallest of the three soil components (sand, silt & clay) with generally tiny (0.0002 to 0.002 millimeters), flat, plate-like particles. Soils high in clay have small pore spaces and, thus, suffer from low levels of oxygen and poor drainage. These conditions are generally detrimental to most plant life. Clay soils also tend to have a high CEC which allows them to hold onto nutrients tightly...sometimes too tightly.
cleistogamous - production of seeds by self-pollination in the unopened flower. Tomatoes fall into this category.
clone - a group of genetically identical individual plants produced by asexual propagation such as division, cuttings, grafting or tissue culture.

In Hosta, the most common techniques for producing hosta clones are division and tissue culture.

colchicine - this is a chemical that is extracted from the bulb plant, Colchicum. It is a medicine for certain human ailments and is also used by plant hybridizers. Flowers treated with colchicine during pollination may produce double the numbers of chromosomes (tetraploids) in the resulting seedlings. This technique is often used by daylily plant breeders.
cold frames - garden structures with glass or plastic tops similar to hotbeds but usually without a source of supplemental heat. They are used to provide frost protection for tender plants.
cole crops - plants of the genus Brassica, includes broccoli, Brussels sprouts, cabbage, cauliflower and kohlrabi.
columnar - the height of the plant is several times greater than its width and the plant’s width is fairly uniform. Most branches have a vertical orientation.
common name - the non-Latin or non-scientific name or names by which plants, animals, and insects are known, for example, pine, oak, marigolds, horse, ladybug, etc.. Often more than one common name is used around the country and the world for the same plant or animal which may be confusing. See binomial nomenclature.

- this term as it relates to horticulture is generally applied to the soil in the root zone of the plants. Clay soils already have very few pore spaces but under the pressure of foot traffic or equipment movement will "compact" further. Such soils lack the air and water spaces necessary for plant root growth.

complete fertilizer - a fertilizer containing all three of the major plant nutrients: nitrogen (N), phosphorus (F), and potassium (K).
Composite Family (Compositae) - Those plants whose compound flowers are daisy or aster-like. Each “petal” of a daisy is actually an entire tiny flower, called a ray flower. Tiny flowers cluster to form the compact center of the bloom and are called disc flowers. Examples of composites are chamomile, dahlias, marguerites, marigolds, and sunflowers.

Chamomile is a composite that often produces only disc flowers in a button-like head.

Note: This plant family has changed name to Asteraceae.

compost - everything that is alive will one day die and decompose back into the basic elements of life. Composting in the backyard consists of accumulating a pile of dead plants and plant parts and allowing them to decompose. When you cannot tell what it was before, it is compost.

Compost is not a fertilizer. It is generally low in nutrients needed by plants. However, it is a wonderful soil amendment which helps clay soils have better drainage and sandy soils to hold water and nutrients better. It also encourages earthworms and other micro-organisms that contribute to the "health" of the soil. It is GREAT STUFF as a soil conditioner!

More on Composting.

compound leaf - a minority of plant species have compound leaves which consist of a petiole and two or more leaflets. Examples would include roses, walnut, ash, and honeylocust.

There are several types of compound leaves including:

  • ternately compound - leaflets are in 3's
  • palmately compound - 3 or more leaflets that arise from a common point and resemble the palm of your had with the leaflets spreading out like fingers.
  • pinnately compound - 3 or more leaflets along a common stem (rachis), or if only 3 are present, the terminal leaflet has a petiole
  • odd pinnate compound - the terminal leaflet is present and the total number of leaflets is odd
  • even pinnate compound - no terminal leaflet is present and the total number of leaflets is even

See simple leaf.

concentric - pertaining to circles with a common center but different diameters. In horticulture, this is often used to describe the symptom of a disease such as bacterial canker.
conifer - plants such as juniper, cypress, fir, and pine that are sometimes called evergreens. Some species are not evergreen, but all produce seeds in a cone-like structure.

Four types of conifers are deciduous and drop their needles each fall including Bald Cypress (Taxodium), Dawn Redwood (Metasequoia), Larch (Larix) and Golden Larch (Pseudolarix).

contact pesticide - this is a chemical that kills its intended target upon contact with the skin or epidermis. It does not have to be ingested or breathed in to be effective. These are the opposite of systemic pesticides.
container-grown - herbaceous plant material grown from seedling to the point of sale in a container. Trees and shrubs are generally consider container stock if they have been grown in the container for at least one year (and not necessarily from seed).

More on Types of Nursery Stock

controlled-release fertilizer - also called slow release fertilizer. These products are formulated with some sort of hard covering to the individual granules which controls the rate of release of the nutrients inside. The cover may need to be broken down by microorganisms or by repeated exposures to water over time.
cordate - heart shaped, generally referring to leaf bases or blades.
cordon - used to describe a type of espalier where trees are trained along a horizontal wire capable of bearing arms, spurs and canes.
core aerification - a procedure using a power machine which inserts tubes into the soil under pressure and removes a plug of soil which is the  deposited on the ground surface. Aerification helps to loosen compacted soils in lawns and is also used to help decompose thatch layers. To be effective, there should be a hole punched every 3 inches in all directions to a depth of 2 inches or more.
corm - underground storage organ similar to a bulb but consisting of a swollen stem tissue. Examples include gladiolus and windflowers (Anemone blanda).
cormel - corms multiply by developing new, little corms on buds around their circumference.
corolla - this is the collective term for the set of petals and sepals that are sometimes fused together into tepals in plants such as hostas.

When the parts are separate and distinct, the parts are called petals and the corolla is called polypatalous.

When the parts are united either as a whole or in parts, the parts are teeth, lobes or divisions and the corolla is called gamopetalous.

corrugated See seersuckering
cortex - region of tissue between the epidermis and the phloem inside stems and roots
corymb - a nearly flat-topped inflorescence with the outer flowers on the structure opening first.
cotyledon - often called a seed leaf, this organ is a food storage structure that provides energy for germinating seeds until they can form true leaves and begin photosynthesis.
crawler - scale insects are generally protected beneath there hard shells. However, once a year, they reproduce and their young, called crawlers, move around the stems seeking a place to begin feeding. During this 2 or 3 week period, the crawlers are susceptible to control with insecticides or insecticidal soap. Each species of scale has its own timing for when crawlers are produced.
crenate - describes leaf margins with shallow, rounded teeth.
critical photoperiod - the daily ratio of light hours versus dark hours that are necessary to trigger certain responses in plants such as flower bud development or germination.

See Long-Day Plant, Short-Day Plant and Day Neutral Plant

crop rotation - Plant diseases and insects may overwinter in the debris (leaves, mulch, etc) from the garden. If the same family of vegetables (tomatoes, vine crops, cole crops, etc.) are grown in the same spot every year, the diseases and insects of those plants will buildup and increase. By moving the crops to different parts of the garden each year, this problem is minimized.

- in the landscape, this concept can also apply to the continued use of the same annual bedding plants in the same locations year after year.

cross-pollination - occurs when the pollen from one plant is used to pollinate the pistil of a different plant.

See self-pollination.

crown 1) Trees - point where a main stem (trunk) and root join at or just below the soil surface.

2) Trees - sometimes used to describe area of the tree that is covered with foliage

3) Herbaceous Perennials - generally, the fleshy structure beneath the ground which survives the winter each year and produces buds for the following year's growth.

cultivar - a contraction for the phrase "cultivated variety". Many people use the term "variety” interchangeably with cultivar. Technically, a variety is a naturally occurring variation of the species. A cultivar may arise naturally or through cross-pollination (with or without human assistance), mutation or sporting of the original plant.

A cultivar is a plant that is clearly distinguished by unique physical characteristics and maintains these characteristics through properly applied vegetative (asexual) propagation techniques. Most named cultivars do not reproduce their characteristics reliably from seed.

Cultivar names are surrounded by single parenthesis i.e. Hosta 'Sum and Substance'.

cultural control - used for ways of controlling or managing disease or insect problems through the use of cultural techniques such as proper watering, adequate nutrition, providing proper air movement, etc. These methods help to reduce or eliminate the need for chemical pesticides.
cuneate - wedge-shaped leaves that are triangular at the base and taper to a point.
cupped - a leaf blade having the center of the leaf lower than the margin, concave and looks like a cup. Often caused by the tissue at the edge of the leaf growing slower than that in the center. Also called the drawstring effect.
cuticle - a thin, waxy layer formed of cutin and wax that covers the epidermis of aboveground plant parts including leaves, herbaceous stems and fruit. If insecticidal soaps are improperly applied, they may melt the cuticle and the leaf tissue will dehydrate.
cutting - a propagation technique in which a part of the shoot, leaf or, in some species, root is removed from the plant and placed in a rooting medium such as perlite or vermiculite. It will then develop roots and become a new plant with exactly the same genotype of the parent i.e. a clone.

This technique is based on the fact that certain plant tissues have adventitious buds which can be induced to become roots under the proper conditions.

cutting garden - a bed, border or part of the vegetable garden devoted to growing plants that will be used as cut flowers. The plants may also be grown as part of the landscape design or in rows.

More on Cut Flowers

cyme - a rounded or flat-topped inflorescence in which the inner or topmost flowers open first.
cytokinin - a naturally occurring plant hormone that promotes cell division, stem elongation, bud formation, breaking dormancy, and other plant processes.
cytoplasm - inner substance of a cell exclusive of the nucleus.


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