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Hort Update for August 2, 2021

Hort Update for August 2, 2021, Nebraska Extension,
Serious ConcernsMajor Symptom
1. Overseeding lawns Best time for seeding - August 15 to September 15
2. August growing degree days (GGD) Lincoln Airport 7/1/21 GGD -  2300,  Understanding Growing Degree Days
3. Fall weed control Fall best time to control perennial, biennial and winter annual weeds
Timely Topics
4. Fall lawn fertilization Get ready for fall lawn fertilization
5. Get ready for fall tree planting Time new plantings for September & October; root starter products are not needed

1. Overseeding lawnsBest time for seeding - August 15 to September 15

Overseeding is used to increase density of thinned lawns, introduce disease resistant cultivars, and to convert Kentucky bluegrass lawns to turf-type tall fescue. The optimum time to seed cool-season grasses is between August 15 and September 15. Keys to success include using quality seed, obtaining seed to soil contact, and uniform irrigation following seeding.

Quality Seed
Purchasing quality seed is essential for long-term performance of a turf area. Seed cost is insignificant compared to long-term maintenance costs (you get what you pay for with seed). Poor quality seed can lead to problems like introducing rough bluegrass, Poa trivialis to the site. If good quality seed is used, a lawn may last for twenty years or more with no reseeding if maintained properly.

Purchase seed from local, high quality seed wholesalers or retailers and purchase their more expensive seed; which virtually insures quality seed and dependable cultivars. Pay attention to label details on the seed bag such as purity, germination, and other crop seed. Know what the preferred ranges are for these details. This information is available in the publication Improving Turf in Fall.

Site Preparation
When seeding into an existing turf stand, it is important to ensure good seed to soil contact. Seeds that get hung up in thatch or other foliage can dry out and not establish properly. To achieve seed to soil contact while overseeding, mow the area to 1.5 inches or as low as feasible to reduce competition from established grass. Aerify the area, punching 20 to 40 holes/sq. ft. with the largest tines available, making at least 2 to 3 passes over the lawn. Power raking prior to aerating will also help increase seed-soil contact. Apply a starter fertilizer over the entire lawn at 1.0 - 1.25 lbs. P2O5/1000 sq. ft.

Seed with a drop seeder or a power overseeder (also called a slit seeder or slicer-seeder) which is a machine that drops seed into small grooves cut into the soil. Make 2 to 4 passes over the lawn in different directions with either type of seeder to insure uniform seeding. Use recommended seeding rates for the type of turfgrass being planted. For Kentucky bluegrass, seed at a rate of 1.5 to 2.0 pounds per 1000 sq. ft. and for turf-type tall fescue, seed at 6.0 to 9.0 pounds per 1000 sq. ft.

After Seeding Care
After seeding, lightly water the newly-seeded area as much as 3 to 4 times daily to keep the soil surface moist. Light, frequent irrigation is the rule until seedlings are rooted. Once rooted, irrigation frequency can be reduced and irrigation depth increased.

Mow frequently to limit the competition from the established turf. Mow at 1.5 to two inches until new seedlings have been cut at least two times, probably 4 to 6 weeks after seeding. After that, gradually raise the mowing height back to 3.0 to 3.5 inches where it should remain. Four weeks after germination, apply a starter fertilizer again at 1.0-1.25 lbs P2O5/1000 sq. ft. unless a soil test indicates this is not needed.

Most broadleaf herbicides should not be applied until after the second mowing of the seedlings, which may be 4 to 6 weeks after seeding. However, some broadleaf herbicides like carfentrazone (QuickSilver from FMC) can be used sooner after seeding. Purchase herbicide products accordingly and be sure to follow label instructions for your specific herbicide.

When converting KBG to tall fescue, multiple years of overseeding may be required to increase the percent of tall fescue in the lawn.


3. Fall weed controlFall best time to control perennial, biennial and winter annual weeds

Identification of problem weeds is critical before attempting control. Annual weeds, such as crabgrass, foxtail, knotweed, oxalis and spurge, will naturally die in just a few weeks. Applying herbicide to kill them now is a waste of time and chemicals. Preemergent herbicide should be applied next year the first week of May to kill germinating summer annual weeds. 

However, perennial, biennial and winter annual weeds are best controlled in fall because:

  1. winter annual weeds germinate in late summer/early fall and can be controlled with preemergent herbicide applied at that time,
  2. winter annual weeds that have already germinated are small and more easy to control than when they mature in spring,
  3. newly germinated or first-year biennials are now in the rosette stage which is ideal for control,
  4. perennial broadleaf weeds are translocating stored energy (and properly applied herbicide) below ground, and
  5. cooler temperatures reduce the likelihood of injuring turf or ornamental plants.

Winter Annuals - Preemergence herbicide should be applied by early September for control of winter annuals like annual bluegrass, downy brome, goosegrass, henbit, little barley and speedwell.  Winter annuals are or will soon be germinating; preemergence herbicide should be in place beforehand so it kills the seeds as they germinate.  Common landscape preemergent herbicides include bensulide (Bensumec), dithiopyr (Dimension), oxadiazon (Ronstar G), pendimethalin (Pendulum) and prodiamine (Barricade). Other management options for winter annual control include maintaining a healthy and vigorously growing lawn or landscape bed plant cover to crowd out weed invaders. A good layer of organic mulch 2-3 inches thick will also minimize weed germination.  

Biennials - Weeds with this lifecycle grow into a flat rosette of foliage by the end of the first year. These rosettes are the ideal stage for post emergent herbicide spot spray control in fall. Common biennial weeds in home landscapes include common burdock, common mullien, Damesrocket,  poison hemlock, speedwell, sweet clover, some thistles (bull, Flodman, musk, plumeless, scotch, tall and yellow-spine), western salsify, wild carrot, wild parsnip. Any of the herbicides listed below for controlling perennials weeds can also be used against biennial weeds, as long as the site is listed on the label. Make applications anytime after September 1, while plants still have green foliage. 

Perennials and germinated winter annuals - Apply herbicide beginning September 1st and again in 4 to 5 weeks if any new growth emerges or green tissue remains.  If weeds are not killed completely, weakening them with fall herbicide applications makes them more susceptible to winterkill. Weeds are killed more slowly as daytime temperatures drops. Ideally daytime temperatures should be between 65-85º when applications are made. Selective broadleaf weed control products include triclopyr, quinclorac, carfentrazone, sulfentrazone, dicamba and 2, 4-D. 


4. Fall lawn fertilizationGet ready for fall lawn fertilization

Early September is a one of the best times to fertilize cool season turfgrass like Kentucky bluegrass and tall fescue. On older lawns (10 to 15 or more years), one fall application may be all that is needed as older lawns typically only require two fertilizations per season. Recommend a fertilizer with at least a 50% slow release nitrogen source.

On younger lawns, two fertilizer applications during fall are recommended; one in late August/ early September and one about mid-October. For the first application, use a slow release nitrogen source. For the last one, use a fast release nitrogen source. Avoid fertilization after late October. Applications much past this time are less efficient because plant uptake is low. This causes nutrients to leach away during winter or linger in soil until spring; resulting in too early succulent growth.  A specific winterizer-type fertilizer is not needed, but have homeowners buy a fertilizer with their particular fertilizer spreader settings listed on the label.

Cool Season Lawn Calendar - Eastern Nebraska, Nebraska Extension 


5. Get ready for fall tree plantingTime new plantings for September & October; root starter products are not needed

Planting Depth - September and October are good times to plant trees. A common mistake made is planting too deep. Once planted too deep, a tree grows slower and has increased stress issues over its lifetime. To avoid planting too deep, always find the root flare (where the first major lateral root grows off of the trunk and the trunk flares out slightly) before ever digging the hole. Once found (you’ll likely have to remove soil on top of the root ball to find the root flare), only then dig the hole just deep enough so the first lateral root will be just below the soil line and/or the trunk flare is visible above ground. If the soil in the hole is loosened beneath the root ball, it will sink after planting and end up being too deep. As little as two inches too deep can cause issues. Always set the root ball on undisturbed soil. It is more important to dig a wider hole to loosen soil around the sides of the root ball because tree roots grow outward. Backfill the hole with the same soil removed. Do not use amended soil in the planting hole only.

Stem girdling roots - Fixing stem girdling roots is an important practice when planting woody plants. Heavily matted root systems of annuals and perennials also need to be addressed at planting for plants to perform their best. Clients won’t be happy with plants that perform poorly or fail to thrive in the landscape due to underlying root issues. Even quick-growing annuals often fail to develop a good root system if circling roots are not fixed. Their roots often stay in a tight mass and fail to spread out through the soil. Plants can be popped out of the ground even several weeks after planting, showing little root development. 

Matted roots - If a plant’s root ball is matted with roots, either pull the root ball apart with your hands or make several cuts down the side of the root ball to loosen it.  Pull the root mass apart and spread the roots out in the planting hole.  Annual plants with a mat of roots at the base of the root ball also need attention. Tear the base mat of roots off or gently pull the root system apart so it can be spread out in the planting hole. Plants will quickly develop new roots and establish a good root system.

Root starter products - Too much phosphorous from root starter fertilizers can be harmful. If clients ask if a starter fertilizer is needed at planting, the answer is usually no. Only a soil test can determine if phosphorous, the common nutrient found in starter fertilizers, is needed. However, as a general rule, landscape soils in Nebraska has plenty of phosphorous (P). This nutrient binds to soil and does not readily leach out of soil like nitrogen does. And P is readily available in the warm soils found with fall planting. The buildup of phosphorus in soil can cause plants to grow poorly and even die. Excessive soil phosphorus reduces the plant’s ability to take up required micronutrients, particularly iron and zinc, even when soil tests show there are adequate amounts of these nutrients in soil. Phosphorus buildup is caused by excessive use of inorganic fertilizer or the use of composts and manures high in phosphorus. High soil phosphorus levels also can threaten streams, rivers, lakes and oceans.

According to Professor Ed Gilman, University of Florida School of Forest Resources and Environmental Horticulture, tree establishment is determined by many factors. Fertilization and the addition of root stimulant products have little or no effect on how quickly a plant re-establishes on the new site.

Practices that encourage growth

  • loose soil, proper irrigation
  • mulch ring 8 feet in diameter or more around planting hole
  • root flare slightly above soil surface
  • leaving top of tree intact (no pruning at planting)

Limits growth

  • compacted soil
  • little or no irrigation
  • grass and weeds close to trunk
  • planting too deeply
  • pruning at planting

Little or no effect

  • peat or organic matter addition as backfill
  • root stimulant products
  • fertilizing at planting
  • adding mycorrhizae
  • adding water absorbing gel crystals


Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Nebraska Extension is implied. Use of commercial and trade names does not imply approval or constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.