Often only thought of the first sign of spring when shopping, we see Crab Grass Preventer bags stacked up high. More often than not, the herbicide is bundled along with a fertilizer. The problem is the actual timing - the timing of the pre-emergent application and the timing of the fertilizer.
Many universities studies have shown applications of fertilizer too early in the season, before the grass is allowed to green up naturally, forces excessive top growth. Top growth (shoot growth / blade growth) comes at the expense of root growth. During spring, before the growth flush that necessitates mowing twice a week, the roots of turf grass are actively growing deeper and stronger as the soil warms, even though the grass seems to be still in winter dormancy mode.
Turf grass managers, hobbyist and knowledgeable home owners will refrain from applying a nitrogen based fertilizer before the grass actually wakes up in the spring, merely only applying a straight pre-emergent packaged alone. The most commonly used products, available to both professionals and homeowners are known both by chemical names and trade or brand names.
- Siduron / Tupersan
- Halts, PreM / Pendimethalin
- Dimension, Preen / Dithiopyr
- Barricade, Stonewall, Guardrail / Prodiamine
Due to marketing by major brands, the term Crab Grass Preventer is commonplace. While the above herbicides do prevent crab grass from taking hold in ones lawn, here's something to remember. Crab Grass germinates when soil temperatures are consistently above 55 degrees and those soil temps are maintained. Crab grass is mainly seen in the warmer months - late spring into early summer. This is a couple / few months past when the stores are stocked with their first arrivals of spring in March. So not only would applying a fertilizer be too early, but the crab grass herbicide can be applied too early - if crab grass is your main objective to prevent.
Often it is said, watch the forsythia. When they bloom, it's time to drop if you want to prevent crab grass. Both forsythia bloom and crab grass germination coincide at the same soil temperature so the visual is a tell tale sign that it's time to go to work.
However, the above herbicides will also prevent a very common nemesis - Poa Annua. So applying a pre-emergent early in spring, before poa has a chance to germinate, does have its place in a lawns annual regiment. When poa annua is your enemy, applying to early is always better than applying too late. So while the forsythia is a great visual for crab grass - other things can be used to prevent poa annua too. When you start seeing tulips and daffodils sprouting and beginning to leaf, now is when you need to apply a pre-emergent for poa annua. Again, better early than late.
But - timing, timing and timing the pre-emergent for poa annua is the nut. Knowing the life cycle is most critical to prevention. The very sad truth with poa annua is, not only is it considered an annual - meaning it has a life of one season, new studies are showing different strains are also a perennial, meaning it lives again the following year after going dormant, i.e. never dying, never living only for one growing season. The annual strain is the most common and the one we as lawn owners can prevent with pre-emergents.
Poa annua, the annual strain is basically a winter annual. Winter annuals, like poa annua, germinate in fall once soil temperatures fall below 70 degrees and the cooler temperatures are maintained. Once soil temperatures are favorable, poa annua germinates and grows until frost, when it too goes dormant at the same time turf grass does. Then come spring, poa annua breaks its dormancy and flourishes like most turf grasses do, then produces a massive seed crop before dying off once the heat of summer arrives in force. Comes fall, all those viable seeds lie awaiting for the cooler soil temperatures to germinate, and the vicious cycle continues on and on, year in and year out.
Having said that and knowing the cycle, how can one use pre-emergents for the prevention of poa annua? The answer is called split applications. This means knowing the maximum amount of herbicide product allowable per year, in order to maintain healthy turf grass without injury. A spring application is great for crab grass and any viable poa annua seeds that care to germinate, but the main focus should done around Labor Day, before the soil temperatures begin to fall into favorable ranges for germination, which is when the majority of the most common form of poa annua germinates. So if 6 Lbs of herbicide is allowable annually, 3 Lbs in spring and another 3 Lbs in early fall would be a split application, giving you coverage throughout the entire germination season.
Split application. Once in spring and another in fall. That's easy huh? Well not if Mother Nature throws you a curve ball it isn't! Case in point, summer 2009 on the east coast was extremely cool. Every gardener and production farm lost their tomato crop to early blight due to much cooler than normal temperatures. These cooler temperatures made soil temperature fall below 70 degrees. Now guess what liked that and germinated? Yes, poa annua flourished. The problem was twofold. Not only was it the soil temperatures but also the lack of residual from spring pre-emergent applications. Some commonly used products only having a residual affect of 60 days, lost their protection by then. This is an example of when three applications would have been needed against poa annua but caution must be used, referring to the annual allowable amount applied.
Sound like a tough enemy yet? Well consider this Latin name, poa pratensis. That is Kentucky Bluegrass. So both Kentucky Bluegrass and poa annua are in the the same family. In fact, poa annua is often referred to as 'Annual Bluegrass' for this very reason. This is why we have such problems.
So below, note the commonly used and available pre-emergents and their residual values. These time frames are at times noted as time that is needed before safely reseeding or over seeding. If turf grass can not germinate, neither will poa annua.
Chemical name - Siduron
Trade name - Tupersan
Delays - Can be applied prior to seeding cool-season grasses and anytime after germination.
Limited grassy weed protection.
Prevents crab grass germination when sowing turf seeds.
Offers no protection against poa annua.
Effective for 30 days.
Chemical name - Pendimethalin
Trade name - PreM, Scott's Halts
Delays - Applications made after turf has filled in and at least 4 mowings.
Pendimethalin's herbicidal effects are related to inhibition of cell division and cell elongation. It is absorbed by plant roots and shoots. Pendimethalin is not absorbed by the leaves of grasses. Only very small amounts are taken up by plants from the soil. Once absorbed into plant tissues, translocation is limited and pendimethalin breaks down via oxidation. Pendimethalin adsorbs strongly to soil organic matter and clay and does not leach through the soil to contaminate groundwater. See http://pmep.cce.cornell.edu/profiles/extoxnet/metiram-propoxur/pendimethalin-ext.html
Pendimethalin has an estimated half-life of 60 days. Soil temperature, moisture and type have been found to influence Pendimethalin persistence. Pendimethalin half-life in a sandy loam was 98 days at 30C but was 409 days at 10C. A later study indicated an increase in half-life from 43 to 101 days as temperature was decreased from 30C to 10C in a clay loam. See http://www.jstor.org/pss/4045113
NORTHERN GRASSES - Kentucky bluegrass, Perennial ryegrass, Fine fescue and l Fescue
This product controls weeds as they germinate but will not control established weeds.
The efficacy of this product will improve if the application is followed by one-half inch of rainfall or its equivalent in sprinkler irrigation.
If this product is not activated by rainfall or irrigation within 14 - 30 days, erratic weed control may result.
Over-application can result in turf stand loss, turf injury, or soil residues.
*Do not use on Bentgrass or Poa Annua (greens and tees) or injury may occur.
For crabgrass and Poa annua protection
2.3 to 5.4 lb/1,000 sq ft. (100-233 lb/acre) as the initial application prior to weed germination in the spring
Apply a repeat application of 1.5 to 2.3 lb/1,000 sq ft (67-100lb/acre) after a minimum of 4 for extended control or where heavy weed infestations are expected.
Chemical name – Dithiopyr
Trade name - Dimension, Preen Crabgrass Control
Delays - Applications to well-established turf with good root system and uniform stand and has received at least two mowings.
Use of this product on certain varieties of Fine Fescue may result in undesirable turf injury. The following Fine Fescue varieties have been found to be sensitive to this product: 'Atlanta', 'Banner', 'Beauty', 'Biljart', 'CF-2', 'Enjoy', 'HF-93', 'Highlight', 'Ivalo', 'Jamestown', 'Koket', 'Majenta', 'Mary', 'Pennlawn', 'Tamara', 'Tatjana', 'Waldorf', and 'Waldina'. Not all varieties of Fine Fescue have been tested.
*Reseeding, overseeding or residual of treated areas with this product should be delayed until 12 weeks from the time of application.
DO NOT apply to flowers, vegetables, shrubs, or trees.
Dithiopyr is both pre-emergent and post emergence herbicide. The early post-emergence crabgrass control is only effective on crabgrass if applied before crabgrass has displayed its fifth leaf or first tiller.
NORTHERN GRASSES - Kentucky bluegrass, Perennial ryegrass, Fine fescue and Tall Fescue
This product may be applied as a single application, as a split application, or as a sequential application for crabgrass control in the spring, summer, or fall.
DO NOT apply more than 11.47 lb of this product per 1,000 sq ft per application, and no more than 34.41 lb of this product per 1,000 sq ft per year.
For single applications made in the spring or early summer, this product should be applied at the appropriate rate corresponding to one of the three control programs listed in Table 1 below, depending on the user's location, the turfgrass mowing height, and whether the use is considered to be pre-emergence or early post-emergence at the time of the application. The duration of residual weed control provided by this product is directly related to the total rate applied, but will vary somewhat depending on weather, weed pressure, turfgrass competitiveness, and the user's location within a region.
Use Program #1 for pre-emergence control at sites where the turfgrass is cut high (e.g., homeowner lawns). This program provides 3-5 months of pre-emergence crabgrass control. This program should not be used for early post-emergence crabgrass control.
Use Program #2 for pre-emergence control at sites where (a) turfgrass is cut low (e.g., golf fairways), and (b) turfgrass maintenance or weed control has not been conducted during the previous year. This program provides 4-6 months of pre-emergence crabgrass control. This program may also be used for early post-emergence control at sites where turfgrass is cut high (e.g., homeowner lawns).
Use Program #3 for pre-emergence control at sites where (a) turfgrass is cut low (e.g., golf fairways) and (b) turf maintenance or weed control has not been conducted during the previous year. This program provides 4-6 months of pre-emergence crabgrass control. This program may also be used for early post-emergence control at sites where turfgrass is cut low (e.g., golf fairways).
Subsequent, sequential pre- and/or post-emergence applications should be made where longer periods of control are desired.
Where split fertilizer applications are recommended, the rates in Table 1 may be split across applications made 6-10 weeks apart and prior to crabgrass emergence.
Used as directed for crabgrass control in the spring, this product will also control (at the"Program 3" rates) the following weeds when applied prior to their emergence:
- ryegrass (annual & perennial)
- bluegrass (annual)
- lespedeza (common)
- foxtail (yellow & green) *Also controlled at the "Program 2" rates.
- oxalis (buttercup, creeping & yellow woodsorrel)
- purslane (common)
- speedwell (corn)
- spurge (prostrate & spotted)
2.86 lb/1,000 sq ft‡
4.13 lb/1,000 sq ft‡
5.73 lb/1,000 sq ft
Chemical name – Prodiamine
Trade name - Barricade, Stonewall
Delays - Wait 60 days after seeding or until after the second mowing, whichever is longer.
This product is a selective pre-emergence herbicide that provides residual control of many grass and broadleaf weeds in established turf grasses (excluding golf course putting greens) and lawns; landscape ornamentals; and established perennials & wildflower plantings. This product controls susceptible weeds by inhibiting weed seed germination and root development. Most effective weed control will be obtained when this product is activated by at least 0.5 inch of rainfall or irrigation prior to weed seed germination and within 14 days following application.
*Do not apply this product to putting greens or areas where dichondra, colonial bentgrass, velvet bentgrass or annual bluegrass (Poa annua) are desirable species.
This product will not harm most established ornamental trees, shrubs, and flowers.
This product may be applied as a single application or in sequential applications to control weeds germinating throughout the year. All applications must be made prior to germination of the target weeds. This product will not control established weeds. Maximum use rate selection should be based on turf species. The length of time of residual weed control provided by this product is related to the rate applied.
Maximum Application Rate per Calendar Year
.20% Prodiamine granular
NORTHERN GRASSES - Kentucky bluegras, Perennial ryegrass, Fine fescue and Tall fescue
Tall Fescue (including turf-type) 750 lb/acre (17.22 lb/1,000 sq ft) 1.5 Lbs AI per Acre
Kentucky Bluegrass and Perennial Ryegrass
500 lb/acre (11.48 lb/1,000 sq ft) 1.0 Lb AI per Acre
Creeping Red Fescue
375 lb/acre (8.60 lb/1,000 sq ft) 0.75 Lb AI per Acre
When used in accordance with this label, this product will provide control of the following weeds:
1. In many areas a single application of 330 to 750 lb/acre of this product will control the above listed weeds and goosegrass. However, under heavy goosegrass pressure and/or an extended growing season, most effective weed control can be obtained by making an initial application of 330 to 500 lb/acre followed after 60 to 90 days by a second application at doses that would not exceed those given in the Maximum Annual Rate Table. Do not exceed the maximum rate for turf grass species listed in the Maximum Annual Rate Table above.
2. Suppression only. Sequential applications may be made so long as the total amount of product applied does not exceed the maximum annual application rates recommended for each turf species. All applications must be made prior to germination of weed seeds.
3. Applications for this weed should be made in late summer, fall or winter prior to germination.
WHEN TO OVERSEED AFTER APPLICATION (residual)
This product will inhibit the germination of turf species if overseeded too soon after application. Follow rates and intervals in the table below for best overseeding/reseeding results.