This section focuses on three areas of plant growth adn development. Growth and development require photosynthesis and respiration processes within the plant. External evnironmental factors affect photosynthesis and respiration depending on air exchange, water levels, light and temperature. Finally there are factors which limit growth and development such as plant hormones, seed dormancy juvenility, flower induction and photoperiod, fruit development and plant hardiness.

Photosynthesis and Respiration

Photrosynthesis is the creation of chemical compounds with the aid of radiant energy (light) or more specifically:  formation of carbohydrates from carbon dioxide and a source of hydrogen (such as water) in the chlorophyll-containing cells (as of green plants) exposed to light. Photosynthesis takes place in the leaves and stems where chlorophyll absorbs light as energy to combine carbon dioxide (CO2) from the air and and water (H2O) into carbohydrates (such as C₆H₁₂O₆), producing enrgy for the plant and excreting the extra oxygen into the atmosphere.

Factors affecting photosynthesis include anything that affects each contribution to the process such as too much or too little of light, water, nutrients, soil pH (acidity or alkalinity), compact soils, air pollution, moisture, temperature and pests. Assessed individually, they can be modified to optiize the plant’s growth and development.

Respiration is the opposite of photosynthesis. The plant needs oxygen to break down sugars to create energy and provide the nutrients to the plant tissues. The result is excess carbon dioxide and water. Unlike photosynthesis, respiration occurs both night and day. Plant growth occurs when photosynthsis outpaces respiration. Stree occurs when respiration exceeds photosynthesis such as when a sunny plant has too much shade. It simply can not grow properly.

External factors affecting growth and development

Air exchange is critical as the plant needs both carbon dioxide and oxygen at different points in its life cycle. Air exchange is controled by the opening and closing of stomata (small holes in the leaf) in response to light and water loss as shown in the diagram.

The opening and closing of the pores or stoma, is regulated by light and water. For example, in excessive heat, water escapes from the leaf so the stoma close down to prevent dehydration. The leaf may wilt due to lowered water pressure. Closing of stoma also prevents the uptake of liquid such as herbicides. Hence, herbicide labels list optimum temperatures when application is effective. If the herbicide is applied outside those recommendations, it is unlikely that the plant will absorb enough of the herbicide to effectively kill the plant.

Water levels in the air and in the soil affect the plant in specific ways. A minimum amount of water is needed to support the plant due to water pressure. Too much water for the wrong plant will rot the roots so they can not maintain adequate water pressure so the plant wilts. Too little water will produce the same effect of a wilted plant but for the opposite reason: too little water can not provide enough pressure to support the plant. Humidity (water in the air) in moderation for the specific plant will support the plant but too little (think tree in the desert) or too much (think cactus in the rainforest) will kill the plant. In our area, excessive humidity over long periods of time stress plants.

Light exposure is needed for photosynthesis in all green plants however not all green plants can tolerate the same amount of light exposure. In the plant cells, cholorophyl abosrbs lith to synthsize carbohydrates. plants become light saturated in full sunlight. Some plants (sunflowers) have sufficient rates of photosynthesis to manage full sunlight while other plants (ivy and ferns) have a much slower rate of photosynthesis and require much less light to thrive. In that case, too much light will exhaust the ability of the plant to photosynthsize and ultimately kill the plant.

Temperature ranges exist for every plant. Some require extreme heat and cold (think desert mountains) others require more limited temperature ranges such as in the rainforest. However, all plants require some change in temperature to move through there life cycle and for seed germination. For example, plants native to this area often require a 6-12 week period of below freezing for the seeds to go through a stratification process. In nature, seeds require certain conditions in order to germinate. Seed stratification is the process whereby seed dormancy is broken in order to promote this germination. As a practical matter, sowing seeds for coneflowers, asters and other natives in our area in the spring will be unlikely to produce plants for the summer. Those seeds need to be sown in the late fall and winter to allow the seeds time to stratify so they can germinate properly.

Factors limiting growth and development include plant hormones, seed dormancy, flower induction, fruit development and plant hardiness.

Plant hormones (phytohormones) are chemicals produced by plants that regulate their growth, development, reproductive processes, longevity, and even death. They are formed naturally in the plant. Gardeners use these substances or synthetic substances that are similar to natural plant hormones to influence the plant development. People who raise orchids for example or those who propogate plants from cuttings, frequently use plant hormones to increase the likelihood of rooting.

Seed dormancy is the state in which seed is unable to germinate, even under ideal growing conditions. Species that have dormant seed have evolved dormancy because it is useful in survival. Plants utilize dormancy so that seed can endure unfavorable conditions and not all germinate at the same time and are killed by unfavorable weather. We discussed examples of this above.

Flower induction is required for seed production to continue the urvival of the speacies. One of the most important environmental factors affecting flowering induction is Photoperiod. Photoperiod is defined as the time plants are exposed to light in a daily cycle. Plants can be classified based on photoperiodic response in day-neutral, short-day and long-day plants. Day neutral plants form flowers regardless of day length. Examples are common summer flowering annuals such as geraniums, begonias and impatiens. Short day flowering plants require a long period of darkness. Short-day plants form flowers only when day length is less than about 12 hours. Many spring- and fall-flowering plants are short-day plants, including chrysanthemums, poinsettias and Christmas cactus. Long day flowering plants require more than 12 hours of light. Many of our summer-blooming flowers and garden vegetables are long-day plants, such as asters, coneflowers, California poppies, lettuce, spinach, tomatoes and potatoes. To apply this in real life, planting a long day plant in an area of part shade (only 6 hours of sun or less) will limit the opportunity for that plant to thrive while planting short day plants in full sun in the summer will not induce early blooming.

Fruit development has specific requirements readily affected by outside factors. The first step in making fruits is pollination of the flower. Then, bees, bats, birds, and even the wind spread pollen from one flower to another. In an area with few pollinators such as many inner city areas, pollination is limited creating a cycle which continuesw to decrease the vegetation of the area. Pollination sets off the second step, the process of fertilization, which results in a fertilized seed contained within the flower’s ovary. Once this happens, the petals of the flower will fall away, leaving an immature fruit that begins to grow. Inside the ovary, the seed produces hormones that cause the cells of the ovary wall to multiply, expand, and thicken. Over the growing season, the “mother” plant receives sunlight, water, and nutrients from the soil to keep growing, helping the immature fruit to continue growing larger. Eventually, the fruit will release a hormone called ethylene that signals the ripening process. Ethylene causes enzymes to be released that make the fruit change colors and become softer, sweeter, and delicious to eat! Ehtylene realeased from fruit can affect nearby fruit to ripen. Consider picking green tomatoes at the end of the season. Putting the green tomatoes in paper bags with or without a banana can cause the ethylene in the limited space to continue ripening of the green tomatoes for freach tomatoes even into winter.

Hardiness of plants describes their ability to survive adverse growing conditions. It is usually limited to discussions of climatic adversity. Thus a plant’s ability to tolerate cold, heat, drought, flooding, or wind are typically considered measurements of hardiness. Certain plants can only tolerate temperatures no lower than 50 degrees while others can drop below freezing. Identifying plants for a given hardiness zone is critical for their success. St. Louis is in the 6a hardiness zone which is a change from zone 5 several decades ago. It is the identifiable effect of climate change which is causing plants to die in some places where they thrived and thrive in some places where they would not have survived before. Below is the national hardiness zone map. Below that is the hardiness map zoomed into the St. Louis area. Notice, the St. Louis area is actually composed of two different hardiness zones with plants tolerating colder temperatures doing better in the city than in the outlying areas.

As an example, when I bought my house in the county (colder winter temperatures), the previous owner planted azaleas. One group did really well while the other group was windburned and stunted every year for the first two years. Applying the above information, I needed to supplement the stressed azalea plant in the winter with an anti-desicant sprayed on its leaves to prevent the cold winds from dehydrating them. Now both groups of azaleas are thriving. When designing for gardens or greenspaces, the variety of the plant, not just its genus and species must be considered for long term sustainability.

Bottom line: All of these factors must be considered to create a sustainable garden or greenspace. Hence the adage: plant the right plant in the right place.