When to use Preen

redsox_gwFebruary 23, 2009

If this is not the right forum for this question, please forgive me. We put Preen down in a particularly woodsy, weedy area in mid-late Fall. This area has some liriope and some bulbs that we inherited with the house, but it is very treed.

It get extremely weedy, as in waist high, a couple of times a season. In an effort to keep the weeds from germinating, we are trying Preen.

When is the optimal time to put Preen down in the Spring? Will the Preen have any effect on the bulbs?

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ken_adrian Adrian MI cold Z5


no other information is good enough ...

it is a pre-emergent.. which means it stops new seeds from germinating... ergo .. one would expect that it will have no impact on pre-existing bulbs or plants ...

with the few facts you provide.. i am guessing you are going about this improperly ...

first.. you need to ID the weeds in question ... preen will have no impact on perennial weeds ... as they do not come back from seed ... so you will have to kill those with something like roundup ...

it will knock back annuals ...

but it will have no impact on vines or other woody problems ...

i had a half acre i had to clear of poison ivy and a multitude of other problem plants .... it took me 5 years.. and spraying with generic roundup [credit] 3 times a year ...

you need a BIG plan ..... if you want to succeed ... the remedy is not going to be found in a container of preen ...



PS: in my z5 .. fall application of preen would not be useful .... since not much emerges in fall .. perhaps it is different in your zone ....

    Bookmark   February 23, 2009 at 4:09PM
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Maybe you don't realize how you come across in your posting, or do you always share information with others in such a pompous, annoying way? This is a friendly forum. Share information with kindness and in a friendly generous spirit, or don't share it at all.

Thank you,


    Bookmark   February 23, 2009 at 6:52PM
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mxk3(Zone 6 SE MI)

Hey, in defense of Ken - he is full of great info! I really don't think he means to come across as pompous and I never thought that of him, perhaps it's just how you're interpreting the way he types out messages.

And he IS right - Preen is a pre-emergent. :o)

    Bookmark   February 24, 2009 at 7:01AM
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The correct answer is: it depends on the type of weed you are trying to control. For perennial weeds (or bulbs), it will have no effect as Ken pointed out (except Preen contains a bit of a fertilizer I think in addition to the pre-emergent, so it could theoretically make those weeds grow more lushly). If you are using Preen to control chickweed, these seeds typically germinate in August, so putting Preen down a little bit early can prevent those seeds from germinating. I think one application lasts 3 months.

BTW, corn meal gluten is an organic alternative to chemical preemergents.

I agree that if you have waist high weeds several times a season that Preen will not fix your problem.

    Bookmark   February 24, 2009 at 10:29AM
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karen__w(z7 Durham, NC)

Like everybody says, whether or not Preen will help depends on the specific weed, and for most weeds a multi-pronged attack plan works best. Identifying and characterizing 'the enemy' is the first step. In using Preen with annual weeds, you need to apply it prior to germination and water it in. If you see seedlings already, it's probably too late (for this year), as it works by interfering with root growth in the seedling. I typically try to spread it right before a rain for the watering in part. The effect lasts 3-6 months unless you disturb the soil.

I've been trying to eradicate the annual weed stilt grass (Microstegium viminuem), which does grow waist high, from my property. Preen has been the central part of my stilt grass control plan, although early March is when I need to spread it and it's not widely available in the stores yet. I've learned to buy it in the summer/fall and keep it over the winter to be ready. One application in late winter does the trick for the year in my garden, at least for this particular weed. (I buy the formulation that doesn't contain fertilizer, as I see no need to fertilize along the driveway, road sides, and woodland floor.) After 4-5 years of Preen, pulling, and very occasional Grass-B-Gone, I've eradicated most of it. It's been important to understand the behavior of the weed to know how to make the most of each of my control meaasures, both chemical and mechanical. And for anyone who's waging the same battle, corn gluten doesn't do anything to microstegium.

    Bookmark   February 24, 2009 at 2:13PM
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Thanks for the information. I don't know if they are perennial weeds or not, this area is not one of my gardening zones at all and with 2 small children, I don't pay much attention to it.

I will try the Preen in late Winter before a rain and see where that takes us first.

    Bookmark   February 24, 2009 at 3:00PM
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Karen, good job on that Japanese stilt grass. That's one tenacious annual reseeder and I'm seeing it pop up in places it never has been before. Funny, we didn't have it in our woods until Hurricane Fran blew through....we wondered if the hurricane winds carried the seed from other properties with it. DH has tried weed whacking it down before it seeds, but it's impossible to do that everywhere.

    Bookmark   February 24, 2009 at 3:38PM
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gottagarden(z5 western NY)

What Ken said.

You should mulch first then preen, or better yet, corn gluten.

Steve, do you know how self-righteous and annoying YOU sound. If you visited the forums regularly you would know that Ken is full of great info and takes time to spell out in detail what he has learned from experience. I usually agree with what Ken has written, but don't have time to type it all out. He makes the time.

    Bookmark   February 25, 2009 at 8:31PM
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The optimal time is when the weed seeds are germinating. This doesn't happen all at once but over a period of time - generally rather early in the spring and again in mid to late fall are the primary germination periods for most annual weeds, but they will germinate throughout the growing season if conditions are right. And the conditions always seem to be right in my garden! Since Preen is effective for around 3 months, I'd apply as soon as you feel temperatures - both air and soil - are warm enough to encourage growth. And you do want to remove any existing weeds first, as Preen will have no effect on them and you don't want to allow them to grow to flower or set seed. The label says it can be reapplied every 9-12 weeks to retard weed germination during the course of the season, however if you mulch after application, you will suppress a lot of weed development without additional (expensive) applications.

I weed, then apply Preen, then apply mulch over that. Between the Preen and a good thick layer of mulch, you should be able to keep the majority of the weeds under control.

Bare soil areas will always attract weeds - it's pretty much a fact of life. Nature abhors a vacume and she will fill it with whatever is easily available....typically some sort of undesireable plant/weed. One of the best weed controls is to plant the area with something YOU choose that will cover the ground and not let weeds take hold. Maybe consider some sort of woodsy groundcover?

    Bookmark   February 26, 2009 at 9:17AM
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I like the woodsy groundcover idea. Any suggestions as it is shaded over there after the trees fill out. Not deep, dark shade and not as light as dappled. Somewhere in the middle.


    Bookmark   February 26, 2009 at 1:58PM
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You could try epimediums, wild ginger, wintergreen. Vinca minor (periwinkle) is another choice but can be pretty aggressive in some areas. I personally have never had problems with it.

    Bookmark   February 26, 2009 at 2:40PM
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Best advice here is to get a head start early in the season pulling weeds, then lay down several inches of mulch once the bulb foliage has gotten to a decent size. Then use corn gluten for help with any weed seedlings that appear. The idea of planting shade-loving groundcovers to smother any persistent weeds is a good idea.

Preen has a number of disadvantages, including cost, potential toxicity (it's touted as a low-tox herbicide, but there are still questions about long-term impact, not just on you but on soil life and other creatures like amphibians). By using it you'll also eliminate the possibility of self-sowing of any desirable plants you add to the planting.

In my opinion we should leave pre-emergent synthetic herbicides for the farmers. There's little call for them in a well-managed landscape.

By the way - in defense of Ken I think he generally contributes good information and does not come across as pompous to me. I just wish he'd end a sentence now and again... ;)

    Bookmark   March 2, 2009 at 12:11PM
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I think if you do the research you'll determine that the active ingredient in Preen - trifluralin is essentially non-toxic to anything but aquatic life. And since it tends to have very high persistence in the soil and requires soil microbial life for degradation the issues of it leaching into groundwater and therefore affecting aquatic life OR causing issues for soil life is pretty much moot. All in all, it is about one the safest synthetic herbicides you can use.

And I'd also disagree on it's lack of appropriateness in a garden setting. With the time constraints many of us have in our busy lives, a "well-managed" landscape - or one that is maintained to the degree we'd all prefer - is difficult, if not impossible, to accomplish. Having tools in our arsenal that help us to achieve as close to this ideal as possible makes good, practical sense. I think some combination of a safe pre-emergent (like Preen or CGM), mulching, using groundcovers and the occasional necessary hand weeding works for just about any gardening situation to keep weeds at a minimum.

    Bookmark   March 3, 2009 at 12:54PM
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If one does the research, it turns out that Preen (trifluralin) can and does wind up in aquatic environments. It also has been found mutagenic in various species (aquatic and non-aquatic) and there is some concern about human carcinogenicity (one U.S. study suggeted a link between trifluralin use and non-Hodgkin's lymphoma). Check out the entry on trifluralin here. The bit about its "very high persistence in the soil" is not what I'd call reassuring from an environmental impact standpoint.

As herbicides go, Preen may be less toxic than most, but the question remains - do we really need to use a pre-emergent herbicide in our gardens on a regular basis? Does convenience justify the potential risk (long-term studies involving Preen use are lacking) and cost?

Use of mulching, groundcovers, and close enough planting of desirable species to prevent weed germination/growth along with spot weeding when needed will take care of the problem in the great majority of cases. Another question that needs to be asked whenever someone reports being overwhelmed by weeds - is the garden area under cultivation too big for the person to handle? Often we expend less work, get better results and greater enjoyment from more compact gardens.

    Bookmark   March 3, 2009 at 7:03PM
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I try very hard not to use anything that isn't organic. I admit there have been times when certain things just get out of control and I will use something BUT... what has worked the best for me is to weed late in the fall then lay down layers of newspaper and add manure or compost and shredded leaves over that. It's really not that hard and my garden LOVES it. I have worms as big as my pinky finger.
I hope you're able to get some control over your weeds so you are able to enjoy your gardens.

    Bookmark   March 20, 2009 at 9:11PM
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mary4b(4b WI)

If you're a lazy gardener like me, preen is a godsend. It works for about 3 months, so in my climate, I use it before anything appears in my garden in very early spring. Then I use it again in June or July, when I get around to it and this pretty much helps me through the season, along with weeding and mulch. I never buy the one with fertilizer.

    Bookmark   March 30, 2009 at 11:52AM
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gardenfanatic(MO zone5b)

Ken's right - it won't work on perennial weeds that are coming back from last year. But to prevent the new weed seeds from germinating, put it down when the forsythia in your area starts blooming. That's the indicator that the soil is warm enough for seeds to start germinating.


    Bookmark   March 30, 2009 at 10:45PM
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I think the all caps thing makes people think there is some attitude behind the statement. LOL. It's so easy to take things the wrong way in this internet age.

I have used the method someone recommended on here to great success. I put a layer of cardboard on the ground and turned the soil upside down on top of it. I didn't use any chemicals at all. This method really works.

You should also research the Lasagna bed method.

Since it sounds like you already have plants growing there, you may need to dig them up and move them to the side before creating your flower bed. I went around some rose bushes with the cardboard and have been amazed how well this method works. Turning the grass upside down and putting it on top of the cardboard rots the grass. It really works. I found it on this forum some place. You'll still have to remove weeds from time to time.

    Bookmark   March 31, 2009 at 12:15AM
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veilchen(5b southern Maine)

I use Preen in my paths to keep the annual weeds down. Without it I'd be spending half my summer pulling weeds. It will not work on perennial weeds or things that are already growing, as stated above. It does last a good 3 months or more, so timing has to be right to 1) stop the annual weeds before they germinate in the spring; and 2) have the longest lasting effect as possible. It's possible on zone 6 you will have to reapply it in midsummer. I can usually get by with one application per growing season, but by the end of the summer I can tell the effect is wearing off, esp. if we have had a lot of rain.

    Bookmark   April 7, 2009 at 3:35PM
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What I use for path weeding is a cheap propane torch (available at building supply stores). A short application of flame and the weeds are scorched out. No environmental concerns (apart from minimal fuel usage).

    Bookmark   April 7, 2009 at 10:07PM
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linnea56(z5 IL)

Anyone know if this works on Yellow woodsorrel? The last few years IÂve had more and more of this pest coming up. ItÂs a fully planted large perennial bed so removing the plants or Round-up is not an option.

    Bookmark   April 8, 2009 at 8:19PM
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A pre-emergence product must be applied before germination and can be applied even in the snow- but with limitations on product life, i.e. 3 months for Preen, timing is of importance as far as being economically feasable. As for a groundcover for a wooded setting you might try Sweet Woodruff- I found it to be perfect in my case and has pretty white blossoms nearly all year.

    Bookmark   May 7, 2010 at 7:52PM
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We messed up. We used Preen on our new garden just before planting seeds because we got in a hurry since it was going to rain. Is there anything we can do to counteract the Preen, or will we have to wait until next year to plant a garden in this spot?

Here is a link that might be useful: Garden Web

    Bookmark   May 17, 2010 at 10:24AM
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buyorsell888(Zone 8 Portland OR)

You can buy potted starts as it won't affect them, it only effects seeds.

    Bookmark   May 17, 2010 at 6:20PM
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If you use organic preen (primarily corn gluten), you can plant some seeds after applying: mostly peas and beans. For other seeds, you need to use transplants if you've already applied, or wait till seedlings are 3 inches tall before applying. Check the package for details.

    Bookmark   June 12, 2010 at 11:58AM
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no defending ken here. info or not, he is a pompous idiot. dump the caps, and the arrogance...then give some info ken.

    Bookmark   August 8, 2011 at 3:41AM
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Hi anonymous, this type of statements belongs to Youtube rather, I think. Personally, if the information is right and useful, I do not care whether it comes with the caps or not. It would be MUCH WORSE if it was with CAPS AND WRONG!!!

    Bookmark   August 8, 2011 at 6:10AM
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ken should feel honored - he's the subject of the one and only GW post by our anonymous friend.

    Bookmark   August 8, 2011 at 10:20AM
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