rabbits - a rodent that can be a pest of landscape plants.
raceme - flowers are borne on a structure called the inflorescence.  The peduncle is the main stem of the inflorescence. When the flowers are attached to the main stem (peduncle) individually on short pedicels (stems), this is called a raceme.
rachis - in compound leaves, the extension of the petiole along which the leaflets are disposed.
radicle - this is the first embryonic root, that emerges from a seed.
ray flower - a petal like unit on a flower where the corolla appears to be a single long, narrow petal. These structures are commonly found on the flowers of members of the Asteraceae Family (aka Compositae Family).
See disk flower.
receptacle - the enlarged end of a flower stalk that bears the organs of a flower and eventually form the fruit. 
rejuvenation pruning - old plant tissue is normally less productive of flowers or fruit and is more prone to disease and insect problems. Rejuvenation pruning refers to a drastic removal of old tissue in order to allow or encourage new tissue to grow.

For example, old canes on lilac shrubs tend to flower less each year and they shade out the younger canes beneath them. These plants may be cut to the ground after blooming to allow all the new canes to emerge and grow. This will improve the blooming and overall health of the plant.

A less drastic way to rejuvenate such a plant is to cut out 10-20% of the oldest canes every year. This pruning rotation will allow new growth to compete with the older, less productive canes.

relative humidity - the amount of moisture in the air compared with the maximum amount of moisture that the air could hold at saturation. Warm air can hold much more moisture than cold air. The relative humidity of air, therefore, decreases when it is heated.
remedial pruning - pruning to remove broken, dead, diseased, dying or heavily shaded branches.
renovation -  periodically over years or decades, a flower bed or border will need renovation. This will be warranted when plants become overgrown and crowd each other or seedlings spread plants to unwanted areas.

In clay soils, time may cause compaction of the soils and new tilling is needed to renew the air spaces. Infestation by certain invasive weeds may signal the need for renovation. Finally, a renovation may be just be desired to make a change in the plant combinations.

repellants - one of the common techniques available to home gardeners for mitigation of damage to beds and borders by animals. Repellants consist of materials that are put on or near the plants which make them either taste or smell bad to the potential eaters such as  deer, rabbits, slugs, etc.

There are commercial products, home remedies and other items such as soaps that will work as repellants. However, the key to all repellants is keeping them on the plants at the proper strengths throughout the entire growing season. Most replents will wash off or lose their effectiveness with exposure to rain or ultraviolet light.

Generally, the repellants do not fail but the persistence of gardeners may.

repotting - transferring a plant into the next larger size pot when it outgrows the one that it is in.
resistance - (adj. resistant) - 1) plants of the same species are usually susceptible to the same disease and insect problems. Through selection or plant breeding, cultivars may be developed that are resistant to the common problems. These plants are less likely to get the disease and, if they do get it, the impact will be much less. Note that they are not immune which would mean there is no chance of them getting the disease. For example, maple trees are immune to apple scab and will never get it.

2) may apply to insects which are no longer killed by a certain insecticide. Whenever a stomach or even a contact poison are used on an insect pest population, there are always a few individuals who, for some reason, are not killed. When these individuals breed, a certain percentage of their offspring will also not be killed. As this passes through many generations, eventually, very few of the individuals are killed and the insecticide is no longer effective.

One way to avoid this situation is to alternate insecticides from different chemical families. Switching insecticides periodically will prevent the buildup of resistant populations.

respiration - during the daylight hours, plants carry on photosynthesis which build carbohydrates and sugars. In the dark hours, the plant takes those carbohydrates and turns them into plant tissue such as flowers, leaves, stems and roots through a process called respiration.
Restricted Use Pesticide

- the Environmental Protection Agency regulates the production and use of pesticides in the United States. They have created two general categories that apply to all pesticides.

General Use Pesticides - These are materials that have relatively small potential for misuse and/or threat to the applicator. Anyone can go to the store and purchase these products. No training or knowledge is necessary on the part of the purchaser.

Restricted Use Pesticides (RUP) - These are chemicals that pose a special risk to the applicator or the environment if improperly used. To legally purchase or use these products, the person must have passsed one or more examinations to receive certification as a commercial applicator.

revert or reversion - many variegated plants are the result of a single colored plant developing a sport with a different colored leaf. This variant part of the plant may be separated and vegetatively propagated to produce more variegated plants.

Occasionally, a variegated plant will "revert" to the solid color of the original plant. This often happens to a single bud or division of the crown. Remove this part of the crown (reversion) to help maintain the rest of the plant in the desired variegated color scheme.

reversed - pertaining to two plants with exactly the opposite pattern of variegation. For example, one hosta has a green center and white marginal variegation, its reverse would be one of the same type but with a white center and green color on the margins.
revolute - this term relates to flower petals or leaves whose margins are rolled, bent or curled outward. See involute.
rhizome or rhizamotous - a rhizome is a stem that grows beneath the soil allowing new plants to develop at the nodes along its length.

A few hostas, especially some of the smaller types, multiply in this manner forming a loosely spreading clump. Some people mistakenly describe these plants as stoloniferous but a stolon is an above ground stem like those on strawberry runners.

The species, Hosta clausa, is a rhizomatous type. 

ring spot - disease symptom characterized by yellowish or necrotic rings surrounding green tissue
rock garden - most commonly used to describe a small, usually hillside, garden meant to reflect an alpine site with alpine plantings. However, with the exception that they all include rocks of some sort, the term can also describe many other types of ornamental beds.
rodenticide - the suffix, icide, means "to kill". Rodenticides are, therefore, meant to kill rodents such as mice, voles and other similar pests of landscape plants.
root - generally the underground portion of a plant that serves to anchor the plant and absorb and conduct water and minerals up to the leaves.
root cap - a thimble-like mass of cells that covers and protects the growing tip (meristem) of a root.
root hair - the extremely small, hair-like roots often only a single cell in width. They are the most delicate roots and the most efficient in absorbing water and nutrients from the soil. Root hairs are most easily lost during droughts, flooding and whenever the plant is moved.
root bound - describes a plant which has been growing in a container so long that the root system has run out of room to expand. In this case, the roots will tend to circle around on the inside of the container and proliferate the growing media. At this point, the plant will begin to lose vigor and will show signs of lack of water quickly.
rooting hormone - certain plant hormones are able to encourage the development of roots when applied to freshly cut stems or, in some species, roots. These substances may be purchased in the form of a powder or liquid for application during propagation by cutting.

Some of the commonly used rooting hormones or compounds include IAA, IBA and NAA.

root pruning - the cutting or removal of some of a plantís roots. This may happen inadvertently during the moving and transplanting process. In container plants, roots that grow out the drainage holes may be allowed to dry out and die which is called air pruning.

Another type of root pruning is when the roots of trees are cut by a shovel or hydraulic tree spade out to the perimeter of the root ball needed for transplanting. This is ideally done one or two season's before the actually digging to encourage the proliferation of roots inside the anticipated root ball.

One technique often suggested for stimulating flowering in wisteria vines is to prune the roots with a shovel a foot or two away from the main stem. It is not always effective, however.

rootstock - grafting involves the combination of two plants into one. The upper part of the graft is called the scion. In the case of hybrid tea roses, this would come from the plant that has the beautiful flowers but which has roots that are not winter hardy.

The lower part of the graft is called the rootstock or stock. In hybrid tea roses, this would be from some "wild type" of rose that is extremely winter hardy but does not produce beautiful flowers.

Rootstock of apple trees are often meant to cause the tree to be smaller than normal i.e. dwarf. This makes them easier to spray and for harvesting of the apples.

rose - 1) beautiful flowering shrubs from the Rosaceae Family

2) the "nozzle" where water emerges from a watering can. Different roses will result in different patterns of flow from a fine shower to a coarse drizzle.

rosette - a cluster of leaves that radiate from the base of the stem of a plant near the soil surface. In biennial plants such as foxglove (Digital purpurea), the first year a rosette of leaves is produced which lives through the winter. The second year, the plant flowers and dies.
Rossing - Hosta propagation technique which can help to speed up the propagation of slow multiplying cultivars.

More info...

rot - the breakdown and disintegration of living or dead plant tissue as a result of infection by a fungus or bacteria. Generally speaking, problems such as root rots are due to an overly wet environment. Poor drainage often provides the right conditions for fungi to infest roots and cause them to rot.
runcinate - coarsely toothed to incised, with the teeth pointed toward the base of the leaf
runner - refers to an above the ground stem (stolon) that spreads along the surface of the ground and sends out roots and leaves at the nodes.
rust disease - a fungus disease that forms spots or blotches that are orange and resemble rust on metal. Many rust diseases jump back and forth between two alternate hosts.

Cedar-apple rust spends the spring and summer on the leaves of apples or crabapples. In the fall, the spores jump to a red cedar (Juniperus virginiana). In the spring, the red cedar forms an orangish ball with tentacles on its foliage which produce spores to jump back to the new leaves of the apple.



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