palmate - shaped like the open palm of a hand having four or more lobes (fingers) or leaflets radiating from a single point.
panicle - flowers are borne on a structure called the inflorescence.  The peduncle is the main stem of the inflorescence. When this stem has branches, the structure is called a panicle.

The species, Hosta tibiae have its flowers borne on panicles.

parallel - lines running in the same direction and equal distance from each other at all points. Plants in the monocotyledon (grasses, hostas, etc) category have parallel veins. See dictotyledon.
parent material - the rock layer from which a mineral soil originated.
pathogen - a disease-causing agent. In plants the vast majority of diseases are caused by fungi while a few are caused by virus, bacteria, mycoplasma and others.
peat moss - from the day they are formed, mother nature wants to fill in lakes and ponds. Over thousands of years, the water plants die and sink to the bottom of the lake and eventually fill it with decomposed organic matter called peat moss.

Depending on the type of plants that grew in the bog, the peat moss will have varying textures. Sphagnum peat tends to be coarse textured while so-called Michigan peat is very fine textured.

Peat moss is used as a soil amendment in the garden but is most often used as part of soilless media used in pots and containers. It is combined with perlite and vermiculite to form a light, well-drained media that holds onto nutrients.

pedicel - the "stalk" that attaches an individual flower to the peduncle which is main stem of the inflorescence.
peduncle - flowers are borne on a structure called an inflorescence. The peduncle is the main stem of an inflorescence of flowers.
percolation - the slow movement of water through the pores in soil or permeable rock layer.
perennial - one of the natural life cycles of plants where the same plant lives for more than two years. The number of years a plant lives may vary widely.

Within perennials there are two subgroups:
1) Woody perennials would include trees and shrubs that form solid, woody tissue that persists from year to year. Trees, of course, may live hundreds or, in a few cases, thousands of years.

2) Herbaceous perennials form soft, non-woody tissue that may persist in tropical or subtropical climates or dies to the ground in temperate regions. Herbaceous perennials may live decades (peonies, hostas) or just a few years (delphinium, gaillardia).

See Annual and Biennial  Images of Perennials.

perennial ryegrass - a common turfgrass species for northern lawns. It is noted for rapid establishment and tolerance of wear. Ryegrass is generally a finer textured grass and it is relatively shade tolerant.

Perennial ryegrass is commonly combined with Kentucky bluegrass and fescue in cool-season grass seed mixtures for the home landscape in temperate regions.

perfect flower - a flower that has both male (stamen) and female (pistil) parts in the same blossom. It is also called a complete flower.
perianth - collectively, the petals and sepals of a flower taken together. In most flowers, the sepals are green and a different color from the petals. In some plants, however, the sepals are the same color as the petals.
pericarp - this is the wall of a fruit that encases the seeds.
periderm - secondary protective tissue that replaces the epidermis when it is destroyed during secondary growth; includes cork and cambium.
perlite - a material made from volcanic rock (lava) which is very light and porous. It is formed in a very hot process at 1,400 degrees F so is completely sterile. Perlite helps to provide more air and water pore spaces in artificial growing media when combined with peat moss.

Care should be taken with using this product in the garden because it is very light and may be blown around in the wind. Also, all the white specks mixed into the soil makes it look unnatural.

permeability - the capacity of a porous rock or soil to permit the flow of water through its pore spaces. Clay has low permeability while sand is highly permeable.
permanent wilting point - as soils dry out, they eventually reach a point where the water is held so tightly to the soil particles that it is no longer available to the roots of plants. The cells of the plants will begin to collapse and the addition of water to the soil at this point will not revive those cells.
pesticide - the suffix, icide, means "to kill". Pesticides are therefore, substances that are meant to kill pests. This is a general term that encompasses a wide variety of materials including insecticides, herbicides, fungicides, miticides, rodenticides and others.
petal - generally the colorful part of a flower that is located between the outer (often green) sepals and the stamen and/or pistil on the inside of the flower.

The petals of hosta generally range in color from pure white to dark purple. Violet and lavender are also terms used to describe hosta flowers. Some cultivars have dark stripes on lighter colored petals.

petaloide - petal-like in shape, texture and/or color but not actually a petal.
petiole - the stem or stalk that supports the leaf blade.

When deer eat hostas, they generally eat the blade but leave the petiole looking like celery stalks. Most petioles are the same basic color as the blade but some hosta cultivars have purple or speckled ones.

pH - a chemical measure of acidity or alkalinity using a 0 to 14 scale. Substances with a pH of 7.0 are neutral.  Those below 7.0 are said to be acid and those above 7.0 are called alkaline or base. The scale is logarithmic meaning that each unit is ten times greater than the previous unit.

For example, a pH of 6.0 is ten times as acid as a pH of 7.0. A pH of 5.0 is 100 times more acid than one of 7.0. Dropping to a pH of 4.0 is 1,000 times as acid as a pH of 7.0.

For the gardener, pH of the soil is important because it has an impact on how effectively the plant can use available nutrients from the soil. The vast majority of plants grown in the temperate zones perform best in a slightly acid soil somewhere between 6.0 and 7.0.

A few plants, most notably those in the Ericaceae Family, prefer a highly acid soil in the range of 4.0 to 5.5. In the landscape, this would include rhododendrons and azaleas, heaths, heathers, boxwood and pin oak among others.

If the soil pH is outside the preferred range for a plant, it will be unable to absorb nutrients even if they are in the root zone. A classic example would occur when rhododendron leaves turn yellow but still have dark green veins. This indicates an iron deficiency but a soil test might actually show plenty of iron in the soil. The problem is usually that the pH of the soil is neutral (7.0) or alkaline and the plant is just not able to take up the available iron.

phenotype - the physical appearance of the plant which may be observed visually. The phenotype may or may not be directly related to the genetic makeup (genotype) of the plant. For instance a plant resulting from the cross of a red flowered plant and one with white blooms may have pink blossoms. However, genetically, it would carry some genes for red and white flowers.
pheromone - chemicals secreted by insects (or other animals including humans) that provide a type of communication between individuals of the species. The most common pheromones of interest to gardeners are insect sex attractants which are excreted by females which may draw males to them from miles away.

For example, gypsy moth females do not fly. To attract the male moth which can fly, they excrete a strong pheromone. Male gypsy moths will be drawn to them from as much as a mile away.

Another use for pheromones is to draw certain insects into traps so that their populations may be monitored. Commercially produced pheromones are used to monitor gypsy moth build up in an area.

Pheromones are also used as an organic way to control certain pests of fruit trees. Tags containing pheromones are attached to branches in large numbers. There is so much pheromone saturating the area that the males become confused and few of them actually find the females. Studies have shown that this can help to reduce certain pest species without having to spray pesticides.

phloem - a liquid conducting tissue in plants, the chief function of which is moving carbohydrates and sugars from the leaves downward toward the roots. See xylem.
phloem necrosis  - a fatal disease also known as elm yellows.
phosphate - a chemical compound with the formula P2O5. In fertilizers, this is the form of phosphate that is actually in the bag i.e. 10-15-10 has 15% phosphorus in the form of phosphate.

Phosphorus (P) is one of the macro-nutrients (i.e. needed in relatively large amounts). Generally, it is used in production of flowers and roots.

A fertilizer bag with the numbers 15-10-20 on it  would include 15% nitrogen (N), 10% phosphorus (P) in the form of phosphate (P2O5) and 20% potassium (K) in the form of potash (K2O).

Phosphorus, like potassium,  has a strong electrical attraction to the clay particles in soil. Therefore, in many gardens, it may build up to optimum levels over years of application. In these cases, you may only need to apply nitrogen in subsequent years.

As always, the only way to know how much of each element you need for your particular crop (lawn, flowers, fruit, vegetables, trees, etc.) is to complete a soil test.

photosynthesis - Plants are different from animals and the process called photosynthesis is probably the key factor that separates the two life forms. Green plants are the organisms that capture energy from the sun and incorporate it into carbohydrates and sugars. This supports all life on the planet earth (science has recently discovered some very, very minor exceptions)

Green plants take the energy of the sun i.e. light, in the presence of chlorophyll and combines it with water, carbon dioxide (CO2) and mineral elements to form carbohydrates and sugars used for plant growth. It releases excess oxygen from the water into the air which we animals can breath in before we exhale carbon dioxide.

Each plant has a minimum requirement for light, nutrients, chlorophyll, water and temperature for photosynthesis to take place. If any one of these factors is below the minimum for that species of plant, photosynthesis will be at a low rate or will not occur. For instance, If the plant is not getting the light that it needs, adding fertilizer will not help.

Shade plants are those which have a low minimum light requirement. They are NOT shade LOVING since they cannot grow in the dark but are actually shade TOLERANT and are adapted to low light environments.

phototropism - basically, this is the attraction of plants toward the light. It is why they usually (but not always) grow upward.
physiological functions - this is a catch-all term for all of the processes that go on within a plant during its life. This would include photosynthesis, building carbohydrates and sugars, creating new plant tissues, respiration, absorption of water and nutrients, exchange of gases with the atmosphere, etc. etc.

Plant problems that cannot be attributed to insects, diseases or physical environmental (abiotic) factors are often called physiological problems.

phytotoxic - anything that is harmful to plants but is usually used to describe the impact of chemicals on plants
picotee - originates from the French picoté, meaning "marked with points". In horticulture, it is used to describe a flower that has a different color on the edge of the petals. They usually have a pale ground color with a darker or brighter band around the edge of the petals.
piecrust - (in hosta) a consistent rippled or crimped appearance limited to the outer edge of a leaf. Looks like a piecrust at the edge of the pan.
pinching - removal of the growing tip (terminal bud) of a plant in order to allow the axillary (lateral) buds to grow. The result is a more bushy, compact plant with more stem tips which are usually sites for flowers.
pinnate - compound leaf resembling a feather with leaflets arranged on both sides of a common axis. Examples include Kentucky Coffeetree (Gymnocladus), Honey Locust (Gleditsia) in certain instances.
pistil - the female sexual reproductive organ of a plant. It consists of the stigma, style and ovary. Pollen lands on the stigma, forms a pollen tube down the style and into the ovary. The eggs are in the ovary and when the pollen reaches them, fertilization occurs and seeds result.
pith - this is the tissue in the center of a dicotyledon trunk or stem. In some species, this becomes hollow which may be an identifying characteristic. Forsythia types can be separated by the texture and arrangement of the pith. Several closely related Cornus species can be identified by pith color such as the Silky Dogwood (C. amomum) which has brown pith while the Bloodtwig Dogwood (C. sanguinea) which has white pith. 
plant classification - in the past, as more and more new types of plants were "discovered" it became obvious that a system had to be developed to give each one a unique name so that proper identification could take place. Early systems often resulted in names with 10 to 20 words to describe just one plant. This was way to complicated.

Carl Linnaeus from Sweden proposed a relatively simple system based on the number of stamens and/or pistils in a plant's flower. Also, each plant (or animal) would have a species name that consisted of only two parts, the genus and a specific epithet.

This classification system known as binomial nomenclature was finally adopted worldwide after decades of debate. Today, if you write about a plant called Monarda didyma, plant people all over the world will understand. If you used one of its many common names such as beebalm, Oswego tea, bergamot, red balm, crimson beebalm or horse mint, the chance for confusion is much greater.

pod - the name for the fruit of many plant species. After the ovaries are fertilized by the pollen, the petals drop off the flower and a pod develops which encompasses the seeds.

Hosta pods resemble very small cucumbers hanging on the peduncle.  They may be of different colors on the outside. The seeds, when ripe, are dark brown or black with a small wing.

pollarding - the practice of pruning tree branches back to the same, uniform length every year.  Although this is commonly practiced in Europe, it must be done properly and with the correct tree species to work. It is generally not a recommended practice for most types of landscape, ornamental trees.
pollen - 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 pollen which is equivalent to the sperm in animals.

Hybridizers physically move the pollen from the male parent to the pistil of the female parent when cross-breeding Hosta, Hemerocallis (daylilies) and other plants. In nature, this is done by bees and other insects that fly around from flower to flower. Bats and other animals also act as pollenizers in other parts of the world.

pollination - the transfer of pollen from the male reproductive organ (stamen) to the stigma at the top of the female reproductive organ (pistil).
poly - prefix meaning “many”
polygamo-dioecious - a dioecious plant has separate male (staminate) and female (pistillate) flowers on different plants.

A polygamo-dioecious plant will also have some perfect flowers (having both male and female parts) on each plant.

Examples include Honeylocust (Gleditsia), Ash (Fraxinus), Fringe Tree (Chionanthus), Osmanthus and Mulberry (Morus).

polygamo-monoecious - a monoecious plant has separate male (staminate) and female (pistillate) flowers on the same plant.

A polygamo-monoecious plant will also have some perfect flowers (having both male and female parts) on each plant.

pome - a fleshy fruit found only in members of the Rosaceae Family; e.g., apple, quince.
pomology - the division of horticulture that deals with the science of cultivating fruit. This includes both tree fruit (apples, peaches, pears, etc.) and small fruit (strawberries, raspberries, grapes, currents, etc.).
postemergent herbicide - a herbicide which kills plants that are emerged from the ground and actively growing. They will not kill seeds in the ground. See pre-emergence herbicide.
potash - a chemical compound with the formula K2O. In fertilizers, this is the form of potassium that is actually in the bag i.e. 10-15-20 has 20% potassium in the form of potash.

Potassium (K) is one of the macro-nutrients (i.e. needed in relatively large amounts) necessary for plant growth. Generally, it is used in "physiological functions" of the plant which means that it is needed for the plant's chemical processes somehow. It is also used in seed production.

A fertilizer bag with the numbers 15-10-20 on it  would include 15% nitrogen (N), 10% phosphorus (P) in the form of phosphate (P2O5) and 20% potassium (K) in the form of potash (K2O).

Potassium, like phosphorus,  has a strong electrical attraction to the clay particles in soil. Therefore, in many gardens, it may build up to optimum levels over years of application. In these cases, you may only need to apply nitrogen in subsequent years.

As always, the only way to know how much of each element you need for your particular crop (lawn, flowers, fruit, vegetables, trees, etc.) is to complete a soil test.

pre-emergent herbicide - a herbicide that prevents seed germination and/or seedling emergence. It has no effect on the plant once it has emerged from the ground. See postemergent herbicide.
primary infection - the first infection of a plant which usually occurs in early spring by a pathogen that has overwintered on the site. Apple scab is a fungal disease of apple and crabapple trees. If the primary infection is prevented when the leaves first unfold from the bud, further infection later in the year will be diminished greatly.
progeny - the offspring from sexual reproduction. It is usually the result of the combination of genetic material from two individuals which leads to genetic variation in the offspring or progeny.
protectant - a material that is applied to a leaf surface before an infection.  Most fungicides act as protectants and must be on the leaf when the fungal spores land to be effective. Once the fungal spores have germinated, the fungicide will not "cure" them but will prevent spreading to uninfected leaves.
pruinose - See glaucous
puckering - See seersuckering
pyramidal - the width of the plant is greatest near the ground and tapers to the apex of the plant


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