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Lesson 8: Introduction to Turfgrass Weeds

Another important issue to be concerned about to insure that weed management programs are successful is weed seed dormancy. Dormancy is the most important characteristic of weeds which enable them to persist. Weed seed dormancy exists in three catagories (innate, induced, and enforced).

Innate Dormancy

This dormancy is considered to be an inherent property of the seed. When the seed is fully developed and shatters from the parent plant and lands on the ground, it will possess inherent dormancy if it has been genetically transfered from the parent to the progeny. Seeds that are innately dormant do not germinate immediately as they must first undergo a process called “after-ripening”. After ripening occurs as a response to changes in environmental conditions. For example, freezing and thawing cycles often will be sufficient to “break” an innate dormancy condition. Wetting and drying cycles are another example. Most conditions that are necessary to terminate innate dormancy are most active at the soil surface. The five most common causes of innate dormancy are:

  1. rudimentary embryos
  2. physiologically immature embryos resulting from inactive enzyme systems
  3. mechanically restrictive seedcoats
  4. impermeable seedcoats
  5. the presence of germination inhibitors.

Induced Dormancy

Weed seeds that have an induced dormancy have been exposed to some unique exposure to the environment. For example, high levels of carbon dioxide in the rootzone or abnormally high soil temperatures. In any case, this exposure to environmental extremes causes internal changes in the seed. As a result, when environmental conditions return to “normal” the seed usually does not immediately germinate. A certain period of time must pass in order for the seed to re-acclimate to “normal” before it can germinate. This passage of time can be a few days or even a few weeks.

Enforced Dormancy

Weed seeds that have an enforced dormancy exist that way because of an interaction with the environment. Seeds need moisture, oxygen, proper temperature, and sometimes light in order to germinate.

When an enforced dormancy exists, one or more of these requirements are missing. Probably the best example of enforced dormancy is when a weed seed has been buried too deeply into the soil. When such a conditions exists and the weed seed has a light requirement for germination, it will not germinate until the condition preventing germination is eliminated. Everyone has observed what happens when a plow turns the soil over. in a matter of days, hundreds, if not thousands of seeds begin to germinate.

The principal difference between induced and enforced dormancy is that when the causal environmental condition is removed, a seed that is dormant due to an enforcing agent will immediately germinate, while a seed that has an induced dormancy will not.

Often dormancy mechanisms work together. For example, a weed seed may have an innate dormancy mechanism that is eventually eliminated by some form of after-ripening. However, it may have become buried in the soil and is then subjected to an enforced dormancy.

Being aware of how weed seeds can persist due to various dormancy mechanisms is important. Planning the proper timing of herbicide applications can be influenced, in the sense that, applications of preemergence herbicides should not be made if the target weed seed is in a dormant state anyway. Crabgrass, which has an innate dormancy mechanism, will not germinate immediately after the seed shatters from the parent plant. Since crabgrass is a summer annual, and produces seed in the late summer, if it the shattered seed would immediately germinate, it would be killed by the first frost and thus not be a problem the following late spring. That is also why timing the application of preemergence herbicides for crabgrass control in the spring works best.

The crabgrass goes through an after-ripening process during the winter and early spring months which eliminates it’s innate dormancy. The innate dormancy mechanism had prevented the crabgrass from germinating in the fall, thus allowing it to avoid the frost which would have killed it.

Chemical Control Strategy

The next step in a successful weed management program is to select the proper chemical control strategy such as:

  • fumigation
  • preemergence
  • pre/post emergence
  • total vegetation control
  • renovation

Once a strategy has been determined, proper herbicide choice is imperative. Proper application–strictly following the herbicide label using precisely calibrated equipment is the next step.

Finally, the success of the herbicide application must be monitored to assess efficacy and facilitate the scouting and mapping needed for successful IPM programs (which will be discussed in a later lesson). Prior to the establishment of the turf, there are certain preventative measures including: appropriate species, clean seed, good site preparation, correct time of year, method and clean mulch � that can be used.

To review the necessary steps for successful weed management in turfgrass:

  • Step One � Proper Weed ID
  • Step Two � Ask how the weed got there
  • Step Three � Fix the cultural program that has an issue
  • Step Four � Select a proper herbicide
  • Step Five � Apply a herbicide using properly calibrated equipment and assess final outcome of program devised


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How to Control Weeds in Your Flower Garden

Everyone has different standards of neatness. That’s true in the garden as well as in the house. Gardens that look weedy to one person, may look pretty tidy to another. But p ersonal standards aside, there are actually some good reasons to keep weeds under control. Weeds complete for valuable moisture and nutrients, can shade or crowd the plants you’re actually cultivating, and if they’re allowed to go to seed, they be a bigger problem in the future.

Weeding is a chore you can’t avoid. But you can dramatically decrease the amount of time you spend doing it and that’s a benefit everyone can appreciate!

Weed Early and Often

The time to start managing weeds is very early spring. Difficult perennial weeds like dandelions are able to grow in surprisingly cold soil. If you get them out while the soil is still moist and before they are in active growth mode, it’s easier to remove them, roots and all.

As the soil warm up, annual weed seeds start to germinate. In most areas of the country, weed pressure is greatest in the spring and this is a critical time to assert your dominance. Weed weekly to remove these plants while they are still small. You’ll be surprised how quickly this can be done when the plants are young and tender.

Starting a new garden area? Don’t despair! Controlling weeds can be very difficult for the first couple years. As you turn the soil and shape new garden beds, you are bringing weed seeds up to the surface where there’s enough light for them to germinate. Plus, all that freshly exposed soil is like a red carpet for airborne weed seeds (dandelions!) as well as invasive creepers (creeping Charlie!).

The photo at left was taken in a public garden in England. Weeds sometimes overpower even the best of us!

Banish Seed Heads

The key to long term weed control is vigilance. Make it your goal to let no weed go to seed. A single plant can produce hundreds or even thousands of seeds that will be around to plague you in the months and years to come.

Some weeds inevitably go unnoticed until they start flowering. Whenever and wherever you find these rascals, be swift about removing them – especially the flower heads. Weeds have an uncanny ability to go from flower to seed in a blink of the eye.

Minimize Soil Disturbance

Dig only when you need to dig. Most soils are filled with weed seeds that can remain dormant for decades. Digging brings these seeds to the surface where there is enough light for them to germinate. When you’re weeding, it’s important to remove the roots, but when you do, try to disturb the soil as little as possible.

Mulch the Soil

Mulch has many benefits, and one of the big ones is weed suppression. Covering the soil with a 2” layer of fine-textured mulch (shredded leaves, compost or ground bark mulch) deprives weed seeds of light and will drastically reduce the number of them that can germinate.

Organic mulches improve the soil as they break down and also help to conserve soil moisture. You and your plants will receive many benefits from this one activity.

Pinpoint Watering

Weeds love overhead sprinklers because they don’t discriminate. Weeds get just as much water as flowers. To keep weeds in check, focus water on desirable plants and avoid wetting nearby surfaces where weed seeds lie in wait.

Though it’s enjoyable to water with the “sprayer” setting on your hose nozzle, consider turning the dial to jet or soaker instead. Adding a wand will let you direct water to specific plants without bending over.

Plant Densely

Give weeds an inch and they’ll take a foot. To prevent them from getting started, keep your flower beds nice and full — just short of crowded. The fuller they are, the more completely the foliage will shade the soil surface. And without light, most weed seeds won’t germinate. Aim to have the foliage of one plant just touching the foliage of its neighbor.

For more tips about tending a flower garden, you may be interested in reading: How to Water Your Plants and How and Why to Fertilize Your Plants.