Module 6.1: Crop Life Cycles and Environments
Module 6.1: Crop Life Cycles and Environments sxr133Climate, soil resources, and the organisms in the environment influence which food and fiber crop plants humans can produce. To overcome environmental resource limitations, humans also alter the environment to produce food and fiber crops.
Plant Life Histories
Plant Life Histories azs2Plants need light, water, nutrients, an optimal temperature range, and carbon dioxide for growth. In a natural environment, the availability of plant resources is determined by the:
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soil fertility, soil depth, and soil drainage
- climate: the seasonal temperature and precipitation distribution
- competition with other plants, herbivory by other organisms, and pathogens
- the frequency of environmental disturbances (for example from fire, floods, and herbivory).
In some environments, nutrients, light, and water, are readily available and temperatures and the length of the growing season are sufficient for most annual crops to complete their lifecycle; we will refer to these as high resource environments for crop production. High resource environments tend to have soils that are fertile, well-drained, deep, and generally level, as well as growing seasons with temperatures and precipitation that are optimal for most plant growth. In general, in environments where competition for resources among plants is low, annual plants with more rapid growth rates tend to dominate (Lambers et al, 1998). Consequently, humans tend to cultivate annual plants with high growth rates in high-resource environments.
By contrast, in low-resource environments plant growth may be limited due to soil features and/or climatic conditions. Soils may be sloped, with limited fertility, depth, and drainage; and/or the growing season may be short due to extended dry seasons and/or long winters (with temperatures at or below freezing). In natural ecosystems, resources can be limited due to competition among plants, such as in a forest or grassland where established plants limit the light, water, and nutrients for new seedlings. And in these environments where resources are limited, plants with slower growth rates and perennial life cycles tend to succeed (Lambers et al, 1998), and perennials are often the primary crops that humans cultivate in resource-limited environments.
Annuals
Annuals azs2Annual plants grow, produce seeds, and die within one year. In general, annual plants evolved in environments where light, water, and nutrients were available, and they could consistently reproduce in one year or less. Where resource availability is high, plants that can germinate and grow rapidly have a competitive advantage capturing light, nutrients, and water over slower growing plants and are more likely to reproduce. To ensure the survival of their offspring, annuals allocate the majority of their growth to seeds (often contained in fruit); and they tend to produce many seeds.
Human selection of annual crop plants typically further selected for large seeds and/or fruit. Some examples of annual crop plants are corn, wheat, oats, peppers, and beans (see photos). What are some other examples of annual crop plants?
Annual crop plants are generally categorized into one of the three seasons that falls in the middle of their plant growth life cycle: spring, summer, or winter. For instance, summer annuals are generally planted in late spring, grow and develop through summer, and complete their lifecycle by late summer or autumn. Winter annuals are generally planted in early autumn and germinate and grow in autumn. Depending on how cold the winter is where they are cultivated, winter annuals may grow slowly in winter or become dormant until spring. In spring, they grow, flower, and produce seed by early to mid-summer (See Figure 6.1, Annual Crop Types). After an annual crop is harvested, in some regions farmers may be able to plant another crop, such as a winter annual crop after a spring annual crop, this is referred to as double-cropping (cultivating two crops in one year). If only one crop is cultivated in a season, the soil may be left exposed until the next growing season. Leaving crop residue on the soil can reduce erosion, but planting another crop with live plant roots and aboveground vegetation provides better soil protection against water and wind erosion. Alternatively, a cover crop may be planted after the harvested crop to protect the soil from erosion and provide other benefits until the next crop is planted. Cover crops are typically annual crops that can establish quickly; you will learn more about cover crops in Module 7.
Figure 6.1.6: Annual seasonal crop types with their approximate growing seasons in Northeastern US
The image is a chart titled “Annual Crop Types” in bold green text at the top. Below the title, there is a horizontal timeline spanning from January (Jan) to December (Dec), with each month abbreviated and arranged in order from left to right. The chart visually represents three categories of annual crops—Spring annual, Winter annual, and Summer annual—using colored arrows to indicate their growing periods throughout the year.
- Spring annual is shown in blue with an arrow starting around April and ending around July, indicating that these crops are typically grown during late spring to mid-summer.
- Winter annual is represented in purple with two arrows:
- The first arrow starts around February and ends around June, showing crops planted in late winter and harvested in early summer.
- The second arrow starts around September and continues through December, indicating crops planted in fall and continuing into winter.
- Summer annual is shown in orange, with an arrow starting around May and ending around September, representing crops grown during the warm summer months.
Below the timeline, there is a legend listing examples of crops for each category:
- Spring annual (blue text): oats, spring barley, spring canola, peas, leafy greens, broccoli.
- Winter annual (purple text): winter wheat & barley, cereal rye, hairy vetch, crimson clover, triticale, winter canola.
- Summer annual (orange text): corn, soybeans, sorghum, peppers, string beans, cucumber, summer squash, eggplants.
The overall design uses color coding (blue, purple, orange) to clearly distinguish the crop types and their respective growing seasons. The background is white, and the text is in black except for the colored crop names and arrows, which match their category colors.
Perennials
Perennials azs2Biennials are plants that live and reproduce in two years, and at the other end of the life-cycle spectrum are perennial plants that live for 3 or more years. Perennials evolved in environments where resources were limited often due to competition with other plants and their growth rates tend to be slower than annual plants (Lambers et al, 1998). In these resource-limited environments, often plants cannot germinate from seed and reproduce by seed within one year. Therefore, to increase their opportunities for successful reproduction, perennials evolved ways to grow and survive for multiple years to successfully produce offspring. Perennial crops are typically cultivated in environments that may also have a climatic limitation such as a short growing season or dry climate, or where a plant's ability to access resources may be limited due to frequent disturbance such as grazing.
To survive for multiple years, perennials allocate a high proportion of their growth to vegetative plant parts that enable them to access limited resources and live longer. For instance, they often invest in extensive and deep root systems to access water and nutrients, or in tall and wide-reaching aboveground stems and shoots to compete for light, such as bush and tree trunks and branches. Perennials also store reserves to regrow after growth-limiting conditions such as drought, freezing winters, or disturbance such as grazing. Carbohydrates, fat, and protein are stored in stems and roots, or modified stems such as tubers, bulbs, rhizomes, and stolons. In many plant species, these storage organs can produce root and shoot buds that can grow into independent offspring or clonal plants; this is called vegetative reproduction. Although most perennials reproduce both through seed and vegetative reproduction, in resource-limited environments where plant competition is high, the large storage organs and their reserves offer vegetative offspring plants a competitive advantage over starting from seed.
Perennial Crops
Humans have cultivated and selected perennial crop plants for their vegetative plant parts, storage organs, fruit, and seeds. For instance, the leaves and stems are the primary plant parts harvested from perennial forage crops (crops in which most of the aboveground plant material is grazed or fed to animals). Horticultural perennial crops that are harvested for stems and leaves include asparagus, rhubarb, and herbs. And in some cases, a perennial crop's storage organs are harvested each year, limiting the plant's ability to complete its perennial lifecycle and effectively reducing its cultivated lifecycle to an annual. Examples of such crops perennial crops that are cultivated as annuals include potato, sweet potato, and taro, Tree, shrub, and vine food crops managed as perennial crops are typically cultivated for their fruit and seeds, such as apples, stone fruit (ex. peach, plum), plantains, nuts, berries, and grapes (see photos below).
Crop Life Cycles and Environments
Crop Life Cycles and Environments azs2Annual plants are typically cultivated in high-resource environments and regions with:
- climates that have sufficient precipitation and temperatures for plants to complete their life cycle each year
- soils that soils tend to be relatively flat and well-drained, and are not prone to erosion when they are tilled or planted to an annual crop each year
- high fertility soil
Annual crops produce grain and fruit crops within one growing season. Grain crops are typically a concentrated source of carbohydrates, protein, and sometimes fat, that can be cost-effectively stored and transported long distances, enhancing their market options and utility. Grain and oilseed annual crops are often processed for multiple uses and markets. For instance, oil is extracted from soybean for industrial and human uses, and the remaining meal is high in protein that is used for both human food products and livestock feed.
If conditions are not ideal for annual crops, farmers sometimes use management practices or technologies to improve conditions for crop growth such as irrigation to compensate for the lack of precipitation or black plastic to warm the soil in environments where temperatures may limit plant growth.
Regions, where perennial crops dominate the landscape, tend to have soil or climatic limitations such as steep or hilly slopes that are prone to erosion, shallow or poorly drained soils, soil nutrient limitations; limited precipitation and soil moisture availability, short growing seasons, or temperatures outside of optimal plant growth temperatures. In these environments, farmers may produce annual crops that are adapted to the environment, such as spring or winter wheat that grow during the cooler season or drought-tolerant annuals such as sorghum and pearl millet. Or farmers may use technologies and management practices, particularly for high-value crops, to improve conditions for crop growth such as tile drains, irrigation or season extension technologies.
See illustration and comparison of plant life cycles, the time and forms of reproduction. Can you name a specific crop plant example for each type of plant life cycle?
Text description of the Figure 6.1.14 image.
The image is a black-and-white diagram titled “Figure 1. Weed life cycles” and illustrates the growth and reproduction patterns of four types of weeds—Perennial, Biennial, Winter Annual, and Summer Annual—across the months of the year. Each life cycle is represented by a horizontal timeline labeled from January to December, with sketches of plant stages and seed dispersal points.
- Perennial weeds show continuous growth throughout the year, with mature plants appearing in spring and summer, and seeds dispersed in late summer and early fall. These plants persist year after year.
- Biennial weeds germinate in the first year, grow vegetatively, and then flower and produce seeds in the second year, typically during summer.
- Winter annual weeds germinate in fall, overwinter as small plants, and then grow rapidly in spring, producing seeds by early summer before dying.
- Summer annual weeds germinate in spring, grow through summer, and produce seeds in late summer or early fall, completing their life cycle within one year.
The diagram uses simple line drawings to depict stages such as seedlings, mature plants, and seed dispersal, providing a clear visual comparison of how different weed types align with seasonal changes.
Perennials and Soil Conservation
Perennials and Soil Conservation azs2Because perennials allocate a high proportion of their growth to vegetative structures and regrow for many years, they can: i. protect soil from erosion; ii. return organic matter (carbon-based materials that originated from living organisms) to the soil, providing multiple soil health benefits; and iii. remove carbon dioxide from the atmosphere, potentially sequestering (storing) carbon in the soil or aboveground plant biomass. Forests, for example, sequester carbon above-ground in trees and in below-ground root systems.
Perennial grasses, in particular, have dense, fibrous roots that protect soil from erosion well and are valuable plants for soil conservation. In addition, over the years, some perennial roots and aboveground plant tissues die when environmental conditions limit growth (ex. drought, winter, grazing), and accumulate organic matter and nutrients in the soil. The majority of the most fertile and deep agricultural soils of the world were formed under natural perennial grasslands, whose deep root systems accumulated organic matter in the soil which contributed many beneficial soil properties, as well as carbon sequestration. Some annual crops can also contribute to conserving soil and add organic matter to the soil if a large portion of the crop residue is left on the soil surface, such as corn stalks left on a field after the grain is harvested.
