Do weed seeds need oxygen to germinate
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Date and time: Sat, 05 Mar 2022 01:44:19 GMT
What are the factors that determine seed germination?
When you buy your packet of squash seeds each spring, you plant them in the ground, and expect plants to sprout! It seems like an easy process, but there’s a lot of plant physiology going on. Besides the current weather conditions, the seeds’ history, and their parents, play a role in how well they germinate. You might even be aware that seed packets are labeled with an expiration date. Let’s talk about what are those top factors that affect seed germination.
Seeds are designed to spread throughout the environment and grow into new plants through the process called seed germination. This process causes a seed to sprout. As seeds absorb water, stored food materials become hydrated. Enzymes in the seed become active, producing energy for the growing seed. The root (or radicle) is the first part of the seedling to emerge. It is the first indication of that a seed is viable, meaning it is possible for it to grow into a healthy plant. Roots provide all the necessary nutrients, minerals, and water for the growing shoot. Cotyledons are the parts that form into the first leaves of the seedling. So now, the plant is capable of obtaining energy from sunlight to do photosynthesis in order to make its own food. A few conditions must be present in order to properly germinate a seed.
Environmental conditions must trigger the seeds to grow. Among them, temperature plays the major role. Some plants require moderate to high temperatures, but others may need cold temperatures. For an example, spinach needs cold – if temperatures are 77°F, you’ll only get about half the amount of seeds to germinate than their ideal temperature of 59°F. But, coriander seeds will double their germination rate if the temperature is 77°F (vs the same 59°F). Germination temperature for vegetable crops such as beans, cucumber, okra, tomato, and pepper fall in the range of 60-85°F. Radish, cauliflower, lettuce, cabbage, and carrot need low temperatures, typically in the range of 45-70°F. So the ideal temperature depends on the plant species.
Moisture essentially brings the seed back to life. When the seed fills with water in a process called imbibition, it activates enzymes to initiate the germination process. On the other hand, too much water can cause seeds to rot instead of developing into a seedling. So, suitable moisture is needed to get the best results.
Like humans, seeds also need to breathe. Most seeds will not germinate under waterlogged conditions, because water is taking up all the air space in the soil. So, proper drainage is important to supply enough oxygen to the growing seeds. Aeration by plowing or mixing the soil can increase the available oxygen to grow. This is why getting your soil texture right before planting can really help with your yields. Adding in some compost and making sure your garden is appropriately drained can help in this regard, too.
On your package of seeds, there are instructions for how deep to plant your seeds. This is another area where optimal depth will depend on the plant. Small seeds typically need to lay on top of the soil for successful germination. Because of their small size, they only have stored food for a limited period of growth. If we put small seeds in too deep, lack of oxygen will limit seed germination, or the seedling will finish its food reserve prior to reaching the soil surface. On the other hand, large seeds need a deep planting location so that roots can grow deeply for proper anchorage.
In your home garden, when you start planting your seeds (vegetables, flowers, or any kind of herbs), the two most important things that you want to achieve are maximum germination and fast germination rate. However, due to some natural factors and environmental limitations you may have to put an extra effort to achieve the best possible results. These are some tips you can use to achieve the highest germination rate:
- Always plant seeds that are for that particular year. If you feel tempted to use seeds purchased in a previous year, do a germination test first (see bottom of linked page).
- Choose the ideal planting time, as noted on your seed packets. The ideal planting temperature range will be listed.
- Some seeds stay dormant and take a long time to germinate until they have enough moisture to grow. Pre-soaking the seeds before planting (usually overnight, on a wet paper towel), can help.
- If you haven’t had any luck germinating seeds soon after sowing on your outdoor seed bed in the past, you can begin planting seeds indoors. By doing so, you can protect your plant from any damages like wind, frost, or drought and later you can move the seedlings outdoors to continue to develop.
Answered by: Chathurika Wijewardana, Mississippi State University
To see a video of Dr. Wijewardana in action and read about her work, visit https://www.crops.org/about-crop-science/at-work/chathurika-wijewardana
Read the other blogs in our seed series!
About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.
The Science of Seed Starting
For success with seeds, it helps to understand a few key factors
S eeds are little miracles that transform the world. In the wild, these tiny amalgamations of DNA can be stored away for years, waiting for the right conditions to turn a barren desert, a fallow forest floor, or a trampled prairie into an oasis of bloom. In our gardens, starting seeds is one of the easiest ways to make more plants. Volunteer seedlings can be blessings, appearing year after year to fill holes in the landscape, or they can be curses, in the form of weeds that seem to spawn out of nowhere. I teach students with little to no background in growing plants, and it is always inspiring to witness their wonder when they see the first little leaves emerge from seeds they have planted. In that rich moment, they have become gardeners. What do seeds require to germinate? Here’s what you need to know.
What seeds need: The big three
Seeds need an environmental trinity to be able to germinate: the right temperature, abundant moisture, and adequate oxygen. If there is an absence of any of the three, you will not see emergence. Let’s look at each factor in more detail.
Photo: Carol Collins
1. The temperature must be right
Seeds, like Goldilocks, want their environment to be just right when they germinate—not too cold or too hot. At the extremes, germination may take weeks or months. But if temperatures are in the optimal range, seedlings will be up in a few days.
We see this in the garden when winter-annual weed seeds begin to germinate in late summer or fall after the temperature drops. Seeds have had access to moisture during the summer, but here they are popping up at the first signs of cooler weather. I have also seen the effect of temperature on germination in spring, when gardeners complain about warm-season crops like peppers taking forever to germinate. Usually they are placed on chilly windowsills and don’t emerge for a few weeks.
Trays of seeds that need warmer temperatures can be put on a seedling heat mat or on top of a refrigerator, freezer, or water heater. Just be sure that once leaves emerge you get them into the right light conditions as quickly as possible. If you are trying to keep seeds cool, try placing cardboard over them and moistening the soil underneath them once a day.
Photo: Carol Collins
2. Water gets things going
Germination is a bit like starting a car. Think of temperature as the key that controls whether germination starts or not, and water as the fuel that really gets things going. Once water is imbibed by the seed, many metabolic processes prime the seed for the emergence of root and shoot.
Seeds must have close contact with the growing medium to make sure that they can get enough water to germinate. Some species, such as basil (Ocimum basilicum), even produce a mucilage that plumps up around the seed to help ensure that it is surrounded by the wet stuff.
A common issue with seeds germinating is that they do not get enough water. There are multiple approaches to make sure seeds stay wet. Seeds by themselves can be placed in damp paper towels in baggies before sowing into a tray. If the seeds are large enough, they can be soaked overnight in a saucer. Seeds sown directly in trays can be covered with moist newspapers or put into a mist chamber to keep the surface of the planting medium moist. If seeds are planted outside, morning and evening irrigation is usually enough to keep them hydrated. If the weather is extremely dry, you can plant seeds in a slight trench made with a hoe to catch water (photo) or cover the seeds with cardboard and moisten underneath once a day.
Photo: Boyd Hagan
3. Oxygen is the often forgotten element
Seeds use oxygen too! This may surprise some people, since the focus on gases with plants is usually on the carbon dioxide plants use to make sweet sugars. Just as you need plenty of oxygen when you exercise, a seed must take in oxygen to break down its stored food supply during germination. If seeds are planted too deep, if they sit in too much water, or if a thick crust forms on the soil, they may be starved of oxygen. Avoid creating these conditions when sowing.
Troubleshooting: What if my seeds don’t germinate?
If a viable seed is exposed to the appropriate temperature, moisture, and oxygen levels and nothing happens, a condition called dormancy may be to blame.
Dormancy makes sense from a plant’s perspective. Interacting with their environment over time, many wild plants developed mechanisms to prevent germination until conditions were favorable for seedling survival. Some vegetables and flowering annuals grown by humans over the millennia have had seed dormancy bred out of them, but many species still show these basal traits.
Three common dormancy scenarios (and how to overcome them)
1. Water can’t penetrate the seed coat
Some seeds have a very thick seed coat that may also contain water-repellent molecules. A thick coat makes sense for seeds that must endure freezing and scorching weather, traveling through animals’ digestive systems, or being trampled by herds. Under conditions like these, natural selection would have favored seeds with a robust physical barrier over thinner-walled counterparts.
To encourage germination, you can scarify (damage) the water-resistant seed coat. Use fingernail clippers, a file, or sandpaper to remove the seed coat until you see the underlying flesh. Some horticulturists soak the seeds in hot water (140°F) or acid, but these techniques require more safety precautions.
Seeds with thick coats
- Eastern bluestar (Amsonia tabernaemontana, Zones 3–9)
- Kentucky coffee tree (Gymnocladus dioica, Zones 3–8)
- Texas bluebonnet (Lupinus texensis, Zones 3–8)
- False indigo (Baptisia australis, Zones 3–9)
- Goat’s rue (Tephrosia virginiana, Zones 3–9)
2. Hormones are preventing germination
Many temperate perennial and woody species that shed seed late in the season have evolved a hormonal strategy to prevent seeds from germinating before winter. The two main hormones involved work against each other: abscisic acid induces dormancy, and gibberellin releases it.
For these seeds, a cold, moist treatment called stratification can shift the hormones from inducing dormancy to promoting germination. The name comes from horticulturists who stacked seeds outside in layers during the winter to expose them to dormancy-breaking weather conditions. The hormonal shift that occurs during stratification is like an alarm clock, letting seeds know it is time to wake up. If stratification does not occur, the seeds will snooze until they get their cold-and-moist beauty rest. An easy way to stratify seeds is to place them in a damp paper towel or soilless substrate, seal them in a labeled plastic bag, and place the bag in the refrigerator for one to three months. In some cases you may need to alternate warm and cold stratification to allow the embryos (baby plants) to fully develop.
Seeds that need cold and moisture to germinate
- Red buckeye (Aesculus pavia, Zones 4–8)
- Butterfly milkweed (Asclepias tuberosa, Zones 3–9)
- Pawpaw (Asimina triloba, Zones 5–9)
- Persimmon (Diospyros virginiana, Zones 4–9)
- New York ironweed (Vernonia noveboracensis, Zones 5–9)
3. The seeds have no access to light
It makes sense that some seeds would need light to germinate. Because a plant cannot move once it sprouts, waiting until there is ample light is a good adaptation to have. We observe this requirement in many species that grow on the forest floor or in swamps where water is usually abundant; thus, it is believed that light would be a better trigger for when to germinate. (Interestingly, spider lily [Hymenocallis occidentalis, Zones 5–8] actually has a green seed coat that must photosynthesize for germination!) When planting light-sensitive seeds, sprinkle them on top of the growing substrate, lightly press them into it, and then place them under lights or outside to germinate. Because they are on the surface, take extra care to ensure that they stay moist.
Seeds that need light to germinate
- Lettuce (Lactuca sativa, annual)
- Oakleaf hydrangea (Hydrangea quercifolia, Zones 5–9)
- Cardinal flower (Lobelia cardinalis, Zones 3–9)
- Foxglove beardtongue (Penstemon digitalis, Zones 3–8)
Jared Barnes, Ph.D., is an assistant professor of horticulture at Stephen F. Austin State University in Nacogdoches, Texas.
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