A horizon

- this is the top layer of soil that contains both mineral elements and organic matter. It is usually darker colored than other horizons and is often called top soil. Plants grow primarily in the topsoil.

abiotic (plant problems)

- the term applies to any non-living cause for a plant problem. Animals, insects, spider mites, fungi, bacteria, etc. would be considered biotic causes of plant damage. Nutrient deficiency, physical damage, weather impacts, poor drainage and other non-living factors would be called abiotic causes of plant problems. More...

abscise - (n. abscission) One of the mechanisms that plants have developed in order to survive cold or extremely dry periods is to drop their leaves. These tender tissues will then die and be replace when better weather appears again.

Deciduous trees form a zone between the stem and the base of the leaf stalk (petiole) called the abscission layer. This is encouraged by the production of a hormone called abscisic acid. Once the layer is complete, it severs the connection with the tree and the leaf drops to the ground without leaving a open wound.

abscisic acid

- this plant hormone is involved in bud dormancy, seed dormancy and helps regulate the opening and closing of stomata on leaves. It is also vital to the formation of the abscission zone at the base of leaves in the autumn which allows them to fall off without the loss of sap through the resulting wound.

abscission zone - in autumn, deciduous plants form a layer of cells at the base of a leaf petiole, flower or fruit stalk in response to a build up of abscisic acid. This layer forms a weakening of the tissue which causes the organ to separate from the plant and fall off.
absorption - when water is taken into the soil or into the root of the plant, it is absorbed. Compare with adsorption.
acaricide - aka miticide - any chemical or material capable of killing spider mites. Remember that spider mites are NOT insects since they have 8 legs and not 6. Therefore, not all insecticides will affect them.
accent plant - in the landscape, this is a plant used to call attention to a particular feature of an area, for example, an attractive shrub planted near the front door of a house.
acclimatization - (aka acclimation) the process of plants adapting to some element of the climate or environment, such as to winter cold or summer heat. The gradual preparation of seedlings before being introduced to a change in environment from a protected place such as a greenhouse or basement. It helps them to adjust from optimum growing conditions to potentially cruel weather outdoors. See hardening off.
achene - small, dry, indehiscent, one-seeded fruit with a tight, thin outer wall. Examples: sunflower and buttercups. Achenes with wings are called samara which are found on maples (Acer).
acid-forming - a material that helps to acidify the soil or acidifies the soil as it decomposes. For example, peat moss, garden sulfur, and oak leaves. Some fertilizers such as Urea or ammonium sulfate also help to make the soil more acid.
acid rain - precipitation containing certain acids and acid-forming substances and falling on soils, plants, and open waters. This is generally produced by industrial discharges into the air. Pollution control devices added to smoke stacks has helped to reduce (but not eliminate) this problem.
acid or acid soil - for gardeners, acid is defined as any substance which has a pH  lower than 7.0 (which is neutral). Most landscape plants grow best with a slightly acid soil with a pH in the range of 6.0 to 7.0 although a few species require a even lower pH (more acid) soil for proper growth. There are a handful of plants that prefer an alkaline soil above 7.0.

The pH of the soil is important in that it regulates the intake of nutrients into the plant. If the pH is in the wrong range for a particular species, it may show nutrient deficiencies even though there is plenty of that nutrient in the soil nearby.

active ingredient - the component of a pesticide product that actually kills the pest. Active ingredients (a.i.) are normally mixed with inert or inactive ingredients in the formulation process.  The percentage of active ingredient in a pesticide must be listed on the label. For example in the very common herbicide, RoundUp, the active ingredient is glyphosate.
actual nutrient (as in actual nitrogen). - the portion or percentage of a fertilizer that supplies nitrogen.

All fertilizer containers must by law list the percentage of N (nitrogen as elemental nitrogen - N), P (phosphorus in the form of phosphate - P2O5) and K (potassium in the form of potash - K2O).

A 100 pound bag of 27-10-15 fertilizer would contain 27 pounds of nitrogen, 10 pounds of phosphate and 15 pounds of potassium.

acuminate - usually referring to a leaf that has a pointed tip or one that is tapering to a point
acute toxicity - an impact that occurs quickly after an exposure. If someone spraying a pesticide does not use the proper protective equipment and then becomes ill immediately after spraying, that would be acute toxicity.
See chronic toxicity.
Aden, Paul Early hosta hybridizer and author of The Hosta Book, published in 1988 by Timber Press in 1988 (ISBN 0-88192-087-8)
adjuvant - an ingredient that improves the properties of a pesticide formulation. Includes wetting agents, sticker/spreaders, emulsifiers, dispersing agents, foam suppressants, penetrants, and correctives which tend to make the pesticide more effective.
adsorption - the binding of a chemical to surfaces of mineral or soil particles. Often nutrients in the soil are adsorbed to soil particles and then are available to nearby plant roots. Compare with absorption.
adventitious - referring to a plant structure arising from an unusual place, such as buds at other places than leaf axils or roots growing from stems or leaves. Sometimes these may be thought of as "buds in waiting" which may not become active until the plant needs them.
aeration - it is the process of opening up the soil to mix in more air such as core aeration in lawns or double digging of flower beds. Soils need to have about 25% of their volume in air/oxygen for healthy root growth on most plants. When the air has been squeezed out of the soil, it is called compaction.
air layering - a means of asexual propagation that involves rooting the stem of a plant without completely detaching it from its original root system. Certain shrubs and vines may be propagated this way. Normally, a small branch is cut just through the bark but left attached to the mother plant. A rooting compound is applied to the wound. Then, the entire stem around the wound is encased in wet sphagnum moss covered by aluminum foil or plastic wrap. Over time, new roots develop at the wound. The stem may then be cut below the roots and the resulting rooted cutting may be planted on its own.
air prune - this term applies to the practice of allowing the roots of plants to grow out the drainage holes in pots. Then, they are allowed to dry out and the roots exposed to the air will die back.
albescent - or albescence -  Some hosta leaves change color as the summer season progresses. Leaves that start out yellow or green in the spring and slowly turn white as the season progresses are called albescent. Here is a list of albescent hostas.
alkaline soil - soil with an alkaline or basic reaction and has a pH  higher than 7.0 (which is neutral). Most landscape plants grow best with a slightly acid soil with a pH in the range of 6.0 to 7.0. Only a few prefer a soil with an alkaline pH.

The pH of the soil is important in that it regulates the intake of nutrients into the plant. If the pH is in the wrong range for a particular species, it may show nutrient deficiencies even though there is plenty of that nutrient in the soil nearby.

allelopathy - some plants have developed the ability to excrete a substance from their roots that is poisonous to other plants. This helps them to compete favorable in the environment. Perhaps the most common example of this is the Juglone which is produced by the roots of the black walnut tree (Juglans nigra).
alpine - in the strict sense, a plant that grows naturally on mountains above the tree line, but used generally for any plant that is small and suitable for use in a rock garden. True alpine plants usually have a short growing season so they come up quickly as soon as the snow melts, grow, set flowers and then go dormant.
alternate - the bud or leaf arrangement characterized by a single bud or leaf per node alternating on different sides of a stem. Most common tree species have alternate bud arrangements. Examples include Birch (Betula), Beech (Fagus), Oak (Quercus), Elm (Ulmus), Walnut (Juglans).

See opposite

alternate host - certain fungal diseases (many of the rust diseases for example) survive part of the year on one type of plant and then jump to a second type of plant for the remainder of the year. For the disease to be active, both hosts must be present in relatively close proximity.

An example is cedar-hawthorn rust which spends the winter on the cedar (Juniperus virginiana) and then jumps to the new leaves of the hawthorn (Crataegus spp.) in the spring and back to the cedar in the fall.

ammonium sulfate - a single source fertilizer that contains 27 percent nitrogen and has an analysis of 27-0-0. It helps acidify the soil and should be used for plants in the family Ericacae such as rhododendrons, azaleas, boxwood and other acid loving plants such as blueberries and pin oak.
angiosperm - the major division of plants that produce seeds that have a seed coat. See gymnosperm
anion - a negatively charged ion (-)
annual - an herbaceous plant which completes its life cycle within a single growing season and then dies after producing seed. There is some confusion in this matter since many of the plants we purchase as "annuals" do not fit this definition. Most of these plants are actually herbaceous perennials which would normally live more than two years but cannot survive the winter temperatures. Many of these come from tropical or at least warmer climates.

If you can keep a plant over the winter indoors under lights, it is not a true (botanical) annual but is a tender perennial. Examples would be pelargoniums (sold as geraniums), coleus, impatiens and many others.

See Perennial and Biennial

annual ring - dicotyledon trees grow in girth (circumference) through the action of the cambium layer building new xylem and phloem cells. At the end of each growing season, these new cells are thicker than others developed earlier in the season and appear as a ring when looking at a cross section of a tree trunk or branch.

The age of a tree can be determined by counting the annual rings in the main trunk of the tree. Also, judgments may be made about the type of growing season based to a certain extent on the width of the various rings. 

annual ryegrass

- a temporary grass with rapid germination; often found in inexpensive grass mixes. Should not be used when establishing a permanent lawn. May be used when you need a "quick fix" such as a lawn wedding in two weeks but this grass will die in the fall.

anther - the male part of a flower is called the stamen. It consists of a filament or stem and a capsule like structure at the top called the anther which produces the yellow pollen. Hybridizers move the pollen from the male parent to the pistil of the female parent when cross-breeding hosta, Hemerocallis (daylilies) and other plants.
anthocyanin - this is a red pigment found in plants that produces red leaves, fruit or other plant parts. In the autumn, it is produced in certain deciduous trees if the weather is warm during the day but cool at night. The variation in temperature governs the type of fall color display.
anthracnose - disease caused by a fungi and characterized by sunken lesions and black blotches (necrosis) of the leaves. ash, white oak, sycamore and maples are trees that are susceptible along with many other plants. In bad years, sycamore trees may be totally defoliated by this disease. Fortunately, they have the ability to produce a whole new set of leaves by July.
apex or apical - (pl. apexes, adj. apical) tip of a root or shoot, containing the apical meristem
aphicide - an insecticide (i.e. poison) that kills aphids
apical dominance - influence exerted by a terminal (i.e. apical) bud on a stem which suppresses the growth of lateral buds on the stem below it. This results in the "A" frame form of certain plants. See pinching
apiculture - pertaining to the care and culture of honeybees. Honeybees, of course, are the agent for moving pollen from the stamen of flowers to the pistil resulting in seed formation. They are NOT required for a plant to merely produce flowers.
apomixes -some plants have the ability to produce seed without having to have the egg fertilized by pollen. This asexual process results in the seedlings being genetically identical to the mother plant since there is no mixing with a male flower.

In hostas, the species, H. ventricosa , is an example of a plant that can produce seed this way.

arboretum (arboreta) - a place where trees and shrubs are grown either alone or with other types of plants for scientific and educational purposes.
arboriculture - the branch of horticulture or forestry that deals with the care and maintenance of ornamental trees and shrubs.
soil mix
- a "soil" or growing mix called a medium which is composed  entirely of peat moss, vermiculite or perlite, and fertilizers, without any mineral loam or field soil. Leaf mold and bark are other materials used in artificial soil mixes.
asexual propagation - plants have the ability to reproduce more individuals without having to mix the genetic materials of two flowers (i.e. sexual reproduction). In nature, bulbs, corms, rhizomes, tuberous roots, tubers, offshoots and other structures provide the avenue for asexual propagation. New plants produced in this way are genetically identical to each other. This process is also called vegetative reproduction.

Humans have developed several asexual techniques for multiplying plants. These include grafting, budding, division, layering, cuttings and tissue culture.

More on Propagation

auricle - an earlobe-shaped appendage usually at the base of a leaf, petal, or bract that is used in the identification of species of grasses.
auxin - a plant hormone that controls cell elongation and also encourage the development of roots.

The most common auxins include indoleacetic acid (IAA), indolebutyric acid (IBA), and naphthaleneacetic acid (NAA). They may be purchased in various forms (liquid or powder) and strengths. Applied to the cut end of a cutting before putting (sticking) them into the media will encourage root formation.

available moisture - amount of moisture in soil between the field capacity and the permanent wilting point that is available for use by plant roots.
awl-shaped - tapering gradually to a stiff, fine point, as an awl-shaped leaf.
axil - the upper angle between a twig or leaf and the stem from which it grows.
axillary bud - bud that develops in the axil of a leaf i.e. where the leaf stem (petiole) joins with the twig or stem. See lateral bud

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