D-Horizon -this is the lowest i.e. deepest, layer in the soil profile. It consists of the rock that served as the parent material for the soils that formed above it.   
damping-off - a fungal disease that attacks seeds and seedlings. It is especially prominent in cool, damp conditions. Generally when starting seeds indoors, use a sterile seeding media like perlite or vermiculite to avoid this problem. Also, sterilize previously used equipment like flats, seed trays or pots by washing them with a one part bleach and nine parts water solution.
day-neutral plant - a plant in which flower formation is not controlled by photoperiod (day or night length)

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

deciduous - plants that normally drop their leaves in autumn as a result of shortening day lengths. Generally, these are woody plants with simple or compound leaves such as oaks, maples and walnuts. However, there are at least three trees that drop their needles every fall including dawn redwood, bald cypress and larch.
decline - a general term for a plant that is losing its vigor. Often, it is applied in situations where a specific disease or insect pest cannot be identified but the tree is still suffering. For example, a syndrome called shade tree decline is used for trees with thinning foliage, dead branches and early fall color. Often it is caused by soil compaction in the root zone.

- leaf bases that extend downward below the point of insertion.

deer Deer have become a huge problem for landscape ornamental plantings across much of America. Even in urban and suburban areas, populations are expanding and they love to nibble on many types of plants including hostas and roses.

Lists of plants that are supposedly "deer resistant" are available from many sources but these must be used with caution.  What deer eat will depend on how many of them wander through your yard and how hungry they are at the time. For most backyard landscapes, the only effective way to minimize damage is to use a repellant. Commercial products work fine but they must be reapplied frequently since they wash off with rain.

- Deer and slugs are the two greatest animal pests of hostas. Deer generally eat the leaf blade and avoid the petiole resulting in hostas that look like stalks of celery. Slugs chew tiny to larger holes in the blade of the leaf and may also chew at the bottom of the petioles where they tend to hide during the heat of the day.

dehiscence - some plants have the ability to "launch" their seeds. When the seed is ripe, the seedpod builds up pressure so that when it opens, the seeds are thrown away from the plant. (adj. dehiscent)
deltoid - shaped like the Greek letter delta i.e. triangular.
dentate - leaves with with coarse, sharp teeth set perpendicular to the margins
desiccation - drying out, such as from leaves losing moisture from excess temperature, wind or droughty conditions.
design - the process of arranging plants in the landscape to achieve certain aesthetic or cultural goals. It involves aspects of both Art and Plant Science. Two professions are involved, Landscape Architecture and Landscape Design.
determinate - usually refers to vining plants that reach a certain height and, then, stop growing in response to the opening of early flowers on the vine.

See indeterminate.

diatomaceous earth - a whitish powder prepared from deposits formed by the ancient  skeletons of tiny organisms called diatoms. It is used as an organic pesticide for certain insects. The material has a sharp edge to the particles which irritates slugs, snails and other soft bodied pests as they crawl over it.

Note: Care should be taken in the use of diatomaceous earth since the powder-like particles may be breathed in to the lungs.

dicotyledon - a cotyledon is a "seed leaf" and seed bearing plants can be divided generally into two groups.
  1. Monocotyledons (generally grasses and grass-like plants) and;
  2. Dicotyledons (all other seed bearing plants).
Monocotyledon Dicotyledon
Embryo with single cotyledon Embryo with two cotyledons
Pollen with single furrow or pore Pollen with three furrows or pores
Flower parts in multiples of three Flower parts in multiples of four or five
Major leaf veins parallel Major leaf veins reticulated (webbed)
Stem vascular bundles scattered in stems Stem vascular bundles in a ring inside the bark
Roots are adventitious Roots develop from radicle
Secondary growth absent Secondary growth often present
dieback - a condition where shoots or branches die from the tips downward. This often indicates some sort of water problem such as too much or too little water, root problems or something blocking the vascular system of the plant. It is a common symptom of shade tree decline.
differentiation - plant cells, tissues and organs change from one state to another during their movement from juvenile to mature i.e. reproductive, states. 
digitate - shaped like a hand with the fingers outstretched
dioecious - plants that have separate male (staminate) and female (pistillate) plants. An example is holly (Ilex) which must have both a male and female plant near each other in order to produce berries on the female plant.

Examples include Kentucky Coffee Tree (Gymnocladus dioicus), Holly (llex), Spice Bush (Lindera) and Katsura Tree (Cercidiphyllum).

See monoecious and polygamodioecious

diploid - most plants have two sets of chromosomes (diploid) and get one set from each parent during sexual reproduction.

See haploid, diploid, triploid and tetraploid.

disease - any disturbance of the plant that interferes with its normal structure, function, or economic value. Diseases are generally caused by pathogens, including bacteria, fungi, viruses, and other related organisms.

More on Plant Diseases.

disease cycle - the various steps involved in developing a disease in a plant. Generally, it includes three factors: 1) susceptible host, 2) presence of the disease organism, and 3) conditions favorable for the disease to develop.
disbudding - to encourage the development of larger blossoms on certain species such as dahlias or roses, it may be beneficial to remove smaller flower buds leaving only the central or largest bud to flower.
disk flower - flowers of plants in the Asteraceae Family (a.k.a Compositae) have composite flowers. The showy part on the outside of the bloom consists of organs called ray flowers which are just for "show". In the center of the bloom are the true flowers that will develop the seeds and these are called disk flowers.
division - one of the key ways to multiply (propagate) landscape ornamental plants is to divide them into smaller pieces. Generally, a division consists of a stem or bud attached to part of the underground crown along with some roots. The resulting plants will be identical to the original plant. This is the most common way that named cultivars are multiplied.

- a hosta division consists of part of the crown with roots and at least one bud or leaf attached. This subdivision of the original plant is capable of sustaining itself and producing another plant. If one of the parts is missing, the division will not grow. Each part of the division results in a clone that is identical to the original plant.

- plant propagation by dividing parts (crown, suckers, tubers) and planting segments capable of producing roots and shoots.

dormant spray or dormant oil or horticultural oil

- certain pests may be controlled by applying a pesticide while the plants are in a non-growing i.e. dormant, phase. Perhaps the most common example is the use of dormant or horticultural oils for the control of scale insects.

Generally, these oils have to be applied when the air temperature has been above 40 degrees for several days so that they will spread over the scale and smother them. Some horticultural oils are called summer oils and may be applied during the growing season at a more dilute rate. Always follow label instructions.

dormancy - generally, the dormant season is that time of the year when the plant is not in an active growth phase. Deciduous plants are considered dormant during the time when they have dropped their leaves. Herbaceous perennials are dormant when their foliage has been killed back to the ground by frost. Bulbs are also dormant when their foliage has died back and certain species such as tulips and lilies need exposure to cold temperatures to break dormancy and grow.

Some plants have a "rest" period where they appear to be dormant but will resume growth if given water and/or warm temperatures. True dormancy requires that the plant be exposed to some specific factor such as cold for a defined period before they will grow. Merely getting warm weather will not be enough to trigger new growth in these plants.

double digging - a labor intensive but valuable technique for preparing the soils of beds and borders in the landscape. Generally, it consists of removing the topsoil layer (dig one), then loosening the subsoil layer below (dig two) and finally replacing the topsoil mixed with a large amount of organic matter (compost).

double-potting - placing a small pot inside a larger pot and, perhaps, filling the space between the two pots with a moisture-holding material such as sphagnum moss, peat moss, or vermiculite. This is done when a plant will be in place only temporarily as with one that is to be moved shortly or when placing a non-hardy plant in the garden before moving it to a greenhouse for the winter.
drainage - the term describes how water passes through the soil. The soil is well-drained if water disappears from a water filled planting hole in a few minutes. If water remains in the hole after an hour, the soil is generally poorly drained. The water itself does not damage the plants, but standing water drives out oxygen from the soil so roots may suffocate or be attacked by moisture loving organisms (fungi) that cause rot.
drawstring effect - the drawstring effect happens when the outside cells of a leaf multiply at a slower rate that those in the center of the leaf. This results in a cupping effect similar to when you pull the drawstrings on a hooded sweatshirt.

Hosta 'Lunar Eclipse' exhibits the drawstring effect.

drift 1) an informal planting of a single species or cultivar of plants stretching through a bed, border or patch of lawn.

2) the movement of a pesticide or other spray carried by the wind from a target area to a non-target area. Avoid this by only spraying on calm, low-wind mornings or evenings.

drill-hole fertilization  - a method of fertilizing woody plants by drilling 12 inch deep holes in the soil at equal intervals around the plant and depositing fertilizer in the holes. This gets the fertilizer down to the tree roots so that it is not used by the shallower roots of grass plants.
drupe - simple, fleshy fruit having its single seed enclosed in a hard or stony endocarp, as in a peach.
dry stone wall - a wall made of loose stones that are not bonded together with cement or motar. If the wall is utilized as a retaining wall, plants may be placed on top or on the face of the wall between the cracks. These plants are usually trailing or vine-like and are often considered rock garden plants.
dry well - a hole in the ground filled with gravel or rubble to receive drainage water and allow it to percolate away into the surrounding subsoil.
drying flower - any plant that produces flowers, floral parts, stems, or leaves that are commonly preserved by air drying, chemical drying, or pressing.
dust - chemical product in the form of extremely fine textured powder, used to control insects or disease organisms. It is applied by blowing the powder onto the plants by means of a special applicator device. This will form a cloud of dust that settles on the surface of the plant. Since it requires no mixing or water, this formulation is convenient to use, however, you should never apply dusts in windy weather and always wear a respirator to avoid inhaling the pesticide.
dwarf - this term applies to plants that are smaller than the standard or typical species type. Be aware that a dwarf version of a tree that is normally 120 feet tall might still reach 20 feet in height.

- Dwarf was once the smallest size category of hostas with a mound less than 4 inches tall. The current size options by which a plant may be registered include only Giant, Large, Medium, Small and Miniature. Dwarf has been deleted from the list by The American Hosta Society.

dwarfing rootstock - the characteristics of a particular root system may regulate the ultimate size of a plant. In trees, especially fruits, it has long been a practice to graft a desirable fruiting variety onto a rootstock that will limit its ultimate size. This is how dwarf fruit trees are developed. This technique can also be used to limit the size of ornamental crabapples.


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