Why germinate

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.

Tomato and pepper seedlings may take five to seven weeks to produce, while cucumbers and squash are ready to transplant after three to four weeks in the greenhouse. But all transplants need to be hardened off before going from the greenhouse out to the field, or they will be damaged by the harsher conditions. To harden off seedlings, gradually expose them to conditions they will have in the field. Plants may show some signs of wilting, but do not let plants wilt excessively.

After a day or two, weather permitting, set the trays outside of the greenhouse for five to seven days prior to planting.

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.

Then, true leaves that look like the leaves of the mature plant appear. These leaves, unlike cotyledons, photosynthesize light into energy, allowing the plant to grow and develop. We know that seeds need optimal amounts of water, oxygen, temperature, and light to germinate.

If we don't create the most optimal environment possible, then plants tend to germinate slowly and unevenly. Generally, greenhouse space is limited, so we want plants to germinate as quickly as possible. Uneven germination can also cause problems. If you have ever had to transplant out a flat of seedlings where half are ready to plant and the other half are too small with root balls that don't slide easily out of their cells, you will understand why.

One common option to achieve optimal germination temperature in growing media is to use germination mats. But have you ever stopped to really think about how, exactly, this seemingly magical transformation actually happens? Like how does a seed take that first step from seed to seedling?

Most people learn at school that plants grow by getting energy from the sun via their leaves , and moisture and nutrients from the soil via their roots. But where does a seed get energy and moisture to grow? Not exactly. Seeds come in all shapes and sizes.

Some tropical rainforest orchids have seeds that are smaller than a pinhead—so small,in fact, that they are like dust. At the other end of the scale is the enormous Coco de Mer seed which can be up to 40 centimetres long and weigh as much as 18 kilograms, about the same weight as a medium-sized dog!

In flowering plants, seeds develop in a fruit. The fruit protects seeds but also helps with their dispersal from one place to another. Sometimes the fruit is nice and soft and delicious, like a berry that attracts animals who then accidently carry the seed to a new home.

Other times the fruits are hard and woody, like those of a banksia or eucalypt. This is the Svalbard Global Seed Vault—a seed bank established by the Norwegian government which holds copies of more than 4, plant species from seed banks around the world. The vault holds over 4, plant species including essential food crops such as beans, wheat and rice.

The northernmost place in the world with its own airport, Svalbard is the perfect place for the seeds to be delivered from around the world for cold storage. Even if the power fails, the ambient temperature on this chilly island 1, kilometres beyond the Arctic Circle will keep the seeds frozen without extra cooling.

In fact, experiments show that some of the tissues inside of seeds remain active, and even carry out some basic metabolic processes, such as cellular respiration GLOSSARY respiration a chemical process whereby energy is released from glucose. The embryo gets energy by breaking down its food stores. During aerobic respiration:. Seeds need the right temperature to germinate, and this varies depending on the species of plant and its environment.

Some need fluctuations in temperature. Some need very cold conditions for a few weeks or even months before they will germinate at a higher temperature. This ensures that cold climate seeds, for example, delay germination until after winter. What about light, you might wonder? Such seeds can lie dormant for years, until, say, a tree falls, opening up a gap in the forest canopy and exposing the seed to light.

Interactive How seeds germinate 1 Next Reset. Nearly all seeds are in a sort of suspended animation, called dormancy, until conditions are just right for them to germinate. Dormancy means that, even when exposed to water, oxygen and the right temperature, a seed may delay germination until it gets certain other environmental and chemical cues.

It is only when the seed is in permanent darkness that the dark form of phytochrome is active for long enough to trigger germination.

This mechanism is even more interesting This enables such seeds to germinate as soon as they receive direct sunlight, but to remain dormant whilst they are under other plants which would give a developing seeding too much competition.