Why are seeds germinated

Seeds generally "wake up" and germinate when soil moisture and temperature conditions are correct for them to grow Miles and Brown Each seed type has individual needs--take a minute and read about their specific germination requirements. Temperature, moisture, air, and light conditions must be correct for seeds to germinate.

All seeds have optimal temperature ranges for germination Table 1. The minimum temperature is the lowest temperature at which seeds can germinate effectively.

The maximum is the highest temperature at which seeds can germinate. Anything above or below this temperature can damage seeds or make them go into dormancy. At optimal temperatures, germination is rapid and uniform. All seeds need correct moisture to initiate internal processes leading up to germination.

In field soil this is generally about percent of field capacity. A fine-textured seedbed and good seed-to-soil contact are necessary for optimal germination. Aeration in the soil media allows for good gas exchange between the germinating embryo and the soil. Seeds respire just like any other living organism. They need oxygen and produce carbon dioxide CO 2. This carbon dioxide needs to be able to move away from the seed. If the soil or media is not well aerated due to overwatering or compaction, the CO 2 will not dissipate and seeds can suffocate.

Not all seeds have the same light requirements. Most seeds germinate best under dark conditions and might even be inhibited by light e. However, some species e.

Don't confuse seed light requirements with what seedlings need. All seedlings require sunlight. Seedlings will become leggy and fragile and will not produce to their potential if they do not have sufficient light. Soil temperatures should be taken by inserting a soil thermometer inches deep into the soil surface and noting temperature.

Adapted from Kemble and Musgrove Some viable seeds might not germinate. Many seeds have developed a dormancy or sleep period. Seed dormancy is a condition that prevents germination even under optimal environmental conditions. Why would it benefit seeds to not all germinate when conditions are right?

In nature, staggering germination keeps some seedlings safe from possible bursts of bad weather or herbivores that might eat them. Seeds of plants that grow best in the spring have self-selected to germinate only after cold winter temperatures have passed. For seeds to come out of dormancy, we have to break their physical or chemical dormancy factors. Seeds might have a hard or thick seed coat physical dormancy.

This can be broken by soaking or scarifying scratching the surface the seed. Other seeds have internal chemical or metabolic conditions that prevent germination chemical dormancy. Factors affecting seed dormancy include the presence of certain plant hormones--notably, abscisic acid, which inhibits germination, and gibberellin, which ends seed dormancy.

For example, the membrane within the seed coat of some seeds forms a barrier that is permeable to water but not to oxygen. Cool temperatures also allow the seed to digest some of its food reserve, giving it energy. For these seeds, putting them in the refrigerator for a specific period of time allows them to gain sufficient oxygen and energy to germinate Colorado Seed Laboratory To find out whether or not your seed is viable, do a germination test.

Wrap seeds in a moist paper towel, wait days, and count how many seeds germinate. Illustration 1: Steps of seed germination. If you save your seed from the year before, think about this: the life of a seed can be cut in half by an increase of just 1 percent in seed moisture or by an increase in storage temperature of just a few degrees.

A simple rule of thumb is that the sum of the storage temperature in degrees Fahrenheit and percent relative humidity should not be greater than The primary root, called the radicle, is the first thing to emerge from the seed. The primary root anchors the plant to the ground and allows it to start absorbing water. After the root absorbs water, the shoot emerges from the seed. In dicots, the shoot has three main parts: the cotyledons seed leaves , the section of shoot below the cotyledons hypocotyl , and the section of shoot above the cotyledons epicotyl.

The way the shoot emerges from soil or growing media follows two main patterns. In some plants, the section of the shoot below the cotyledons elongates and forms a hook, pulling the cotyledons and the growing tip through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air.

For example, beans germinate this way. This is called epigeous germination. In other plants, only the section above the cotyledons expands, leaving the cotyledons underground where they soon decompose. This is called hypogeous germination. Peas, for example, germinate this way Raven, Ray, and Eichhorn In monocot seeds, the primary root is protected by a sheath coleorhiza , which pushes its way out of the seed first.

Then the seedling leaves emerge covered in a protective sheath called a coleoptile Raven, Ray, and Eichhorn After the shoot emerges, the seedling grows slowly while the storage tissue of the seed diminishes.

Soon, the plant develops a branched root system or taproot. Dicots have two seed leaves in the shoot that emerge from the germinating seed. Tomatoes, peppers, cabbage, beets, lettuce, beans, cucumbers, and squash are all dicots. A few vegetables are monocots, including sweet corn, onions and asparagus.

Monocots have one seed leaf in the emerging shoot. Once the shoot, with its one or two seed leaves, emerges from the soil or growing media, we call the plant a seedling.

Still quite fragile at this stage, the young seedling is vulnerable to diseases and to environmental stresses such as high temperatures and dry soil. Making sure that conditions are optimal for growth will ensure that young seedlings can grow rapidly and uniformly. Seeds purchased within a year of when they are to be planted rarely fail to germinate.

However, seeds are often stored from one year to the next, and if they are stored improperly or for too many years, they can loses vigor and germinate poorly when planted. A simple germination test can show whether or not stored seed is still viable. To test seed for germination, count out a sample of at least twenty-five seeds. Wrap seeds lightly in a moist paper towel, keep the paper towel moist but not soggy for five to ten days. Unwrap the paper towel and count how many seeds have germinated.

We know that seeds need proper conditions to germinate rapidly. Whether seeds are planted into trays in the greenhouse or directly in the field, the goal is to have all seeds germinate close to the same time and grow at the same rate.

A uniform tray of transplants or a uniform field of seedlings is easier to manage and will lead to a better crop. Uneven germination due to slow growth, differences in soil moisture or temperatures, or planting depth of the seed, can result in seedlings of different sizes.

This can especially cause problems when transplanting a tray of seedlings; half are ready to plant in the field, and the other half are too small, with root balls that don't slide easily out of the tray cells. In the greenhouse, one way to achieve rapid, uniform germination is to use germination mats under the trays. These mats allow you to set the temperature according to seed requirements. Make sure you maintain optimal temperatures for your crop see Table 1. Providing good air circulation during germination and early seedling growing will help to control diseases in this early stage.

We cannot control conditions in the field like we can in the greenhouse, but we can still take steps to make sure that seeds planted directly into the field germinate uniformly. A fine-textured seed bed provides good growing conditions, ample seed-to-soil contact, and the ability to plant to a uniform depth. Planting when the soil temperatures are near optimum will hasten germination and emergence of the seedlings. Sometimes in the rush of spring planting, seeds are sown in soils that are too cold.

This can result in slow germination, weakened and diseased seedlings, and even plant death. It is much better to delay planting until soils warm up. The optimal temperature for growing seedlings may be different from optimal germination temperatures. Table 2 shows the range of day and night temperatures that are best for growing seedlings in a greenhouse, where temperatures can be controlled.

Cooler temperatures generally slow down growth, and warmer ones speed up seedling growth. All seedlings need ample light to grow. If light levels are low or if seedlings are too crowded as they grow, the stems will stretch as the plants seek more light, resulting in weak, "leggy" transplants.

Consider supplemental lighting if greenhouse light levels are low. The length of time that seedlings need to grow in the greenhouse before they are big enough to transplant into the field varies by crop.

Planting seeds too deeply causes them to use all of their stored energy before reaching the soil surface. Dry conditions mean the plant doesn't have enough moisture to start the germination process and keep it going.

Some seed coats are so hard that water and oxygen cannot get through until the coat breaks down. Soaking or scratching the seeds will help break down the seed coat. Morning glories and locust seeds are examples. Other seeds need to be exposed to proper temperatures.