Just use roundup/glyphosate to kill the weeds. There is no need for Preen and no reason to go tracking it all over the place on the bottom of your feet by walking through it all the time. If the joints need filling use whatever kind sand/grit you want. You will still get weeds. Spray them when you do. No preen. Crack Weeds will grow in 'moss' too, just not as abundantly.

Maybe you could mortar in some tile strips in the gaps

The reason to use Preen, too, is because there are already weed seeds in the joints and because more will blow in over time. Gyphosate kills nothing that comes in the future. Obviously, Preen is only needed in the joints, not on top of the blocks. After applying sweep the blocks and all of it will land in the joints. There won't be any to track around. Also, it wouldn't take the same application coverage that a regular landscape bed would require since the joints are only a small fraction of the space.

Mark my word Yardvaark, Preen has Landscape Fabric 2.0 written all over it.

Thanks everyone. I thought about fabric but it would be very difficult to get it in the joint. So I’ll spray roundup and then put Preen down. How often should I use Preen?

Read the label Paolo. Do you have kids or pets? Is this what you want in the cracks on your back patio that I assume gets used by humans? Read the label.

https://www.preen.com/labels/2463793.pdf

In the joints, it is out of reach of kids, pets, anyone, unless, of course, they are going to lick the joints. But that seems highly unlikely.

Read the label Yardvaark. Note all the cautions about potential damage to desired plants. Extrapolate that out to typical home owners behavior with chemicals. What kind of real world results are to be expected?

"Note all the cautions about potential damage to desired plants." Christopher, I think your concerns are unfounded. As a user of the product, in a yard that is total bed (no turf lawn) I have had no issues, ever. The likelihood that that plants in joints will be damaged does not apply here, since that's what the OP is trying to kill and keep from resprouting. It is not fair to a product to judge it based on its misuse by some. If that were the case, people should not be allowed to put gasoline in their cars, as I'm sure there will be mistakes made. Most homeowners are able to read and follow directions.

While it would not something I would use for this application, Preen is a pre-emergent herbicide that prohibits seeds from germinating fully. It has NO effect on any desired plants (or even existing weeds for that matter) beyond the seedling stage.........unless you are also growing them from seed, which would be unlikely in concrete expansion joints.

Anyone have experience with a torch. My gardener has suggested using a torch, says it will burn the existing seeds.

Weed torches are wonderful for this sort of application as well as with any rocked or graveled surface. And they are fun to use!

I am in Ontario and Roundup has been banned. Thinking about a torch but would a regular welding torch work? The area is not all that large so bending over is not a problem.

Is a welding torch very different from a regular propane torch you can buy at any hardware store? Those work perfectly well although not as easy to use (more bending required) than an actual weed torch

I take them out when I weed eat. Hold the weed eater so the string goes into the crack and excavates them out. Check out steppables.com if you want to plant something attractive in them.

Not a Pro.

Not sure if this was mentioned or not, I would try a power washer in those crevices to clear them out. Then apply Preen, etc.

+1 for weed torch they work awesome and they are a great charcoal grill starter too. I bought one to burn down cover crop stubble in the vegetable garden and now use it on the cracks in driveway and sidewalk with fantastic results. And they are cheap, I think mine was $45 at the big box store.

Thanks everyone. I’m thinking I’ll do a combo of roundup and preen. I’ll also do the torch because I built my own.

Preens mode of action to inhibit root growth. A seed has one shot at it. Preen inhibits the root growth of all plants. Some handle it better than others. I consider that an undesired effect on my plants. All of them.

Mark my word, Preen is Landscape Fabric 2.0

Christopher, I think you may need to do some additional research. The active ingredient in Preen - trifluralin - inhibits root growth only at the cellular level during mitosis. It is only effective immediately after seed germination. That's why it is a pre-emergent herbicide and has virtually NO affect on an already established root system. If it doesn't kill already established weeds - and it does not - then there is no reason to expect or evidence to support that it will have a negative impact on any other already established plant.

And absolutely no similarity to landscape fabric, which we all recognize to have next to no impact on weed suppression. In fact, the comparison is a bit of stretch - weed fabric that has little effect on controlling weeds and is just more of an inconvenience and a pre-emergent herbicide you claim is more effective in disrupting weed/plant growth than evidence supports.

Gardengal you will need a citation for this - trifluralin - inhibits root growth only at the cellular level during mitosis. Then please explain how mitosis isn't involved in cell division of other living plants. - Been a long time since that class. - Then explain how this chemical can be so smart and so targeted as to be "only effective immediately after seed germination" So you are saying at least we now have a chemical in the environment that is active at "the cellular level during mitosis." Hmmm?

Read the label gardengal. Collateral plant injury is an important topic.

Read the label gardengal. It pains me to imagine how many tons of this Magic Weeding Dust have already been liberally sprinkled on America's soils. You know in direct human habitat where they breath and absorb things through the skin like chemicals that are effective on the cellular level during mitosis.

Read the label gardengal. Water is Life!!!

Everybody should get them some Magic Weeding Dust and sprinkle it all around. Just Great.

Let me add: The similarity to landscape fabric is that Preen is equally doomed to failure and ridicule as a good idea for gardening.

How about some kind of caulk? It would probably take quite a bit. There is a caulk-like thing that I have seen that you can use to repair concrete that might work. I think the torch idea is the easiest, though.

Paolo, that torch is freakin' beautiful!!!

"Preen inhibits the root growth of all plants." Christopher, I'm sure you believe this with your heart and soul, but I'm also sure you're a non-user ... and consequently couldn't have as much experience with Preen as someone who uses it regularly ... regularly meaning on a schedule according to directions. Since my mom's yard (what I take care of now) is one gigantic landscape bed with no lawn, the whole yard, with the exception of a couple of reseeding beds, gets Preen. There is literally nothing showing signs of struggle on account of it. But it is a massive weeding time saver for me. At the beginning, I resisted using it because I wasn't in the habit of it. And I was filling a five-gallon bucket with weeds about once per week. Now, it is more like 1-gallon every 3 or 4 weeks. Since I have to travel some to get there, it would extremely difficult to keep up without it.

Yardvaark, I read the label. It is quite explicit about the potential for unintended plant damage. It also said do not put preen on the foliage or in the root zone of desired plants. What percentage of the people you interact with on this forum do you think could effectively use Preen as a weed management system year to year? I would put that percentage in the low single digits. Of the few who might be successful at effectively suppressing weeds without damaging other plants in the landscape by dousing their soil with a chemical toxic to aquatic life every three months into perpetuity, it is not a landscape I would want to work in, live in, or have children or pets in on a regular basis. You know that breathing and skin absorption thing that happens with living organisms when you go rolling around in the dirt and shrubberies.

I would not even wish success with Preen as a weed management program on anyone. How could you want or suggest that a Garden Where Nothing Ever Germinates is a good thing?

"How could you want or suggest that a Garden Where Nothing Ever Germinates is a good thing?" That's not what I'm suggesting, but where those things are weeds, it's a good thing! :-) It's not actually "nothing ever germinates" ... as a percentage of weeds slip through the net. And in reseeding beds where I want germination, I don't put Preen. Problem solved.

Snarkiness of previous comments aside, I HAVE read the label. And quite thoroughly. I am a licensed pesticide applicator as one of the requirements for being a horticultural consultant and studying and understanding how various pesticides work is part of my brief. I am not sure what Preen product you are familiar with - there are several - but mine, Preen Garden Weed Preventer, makes NO comments about any "collateral plant damage". The only cautionary statements included on the label (other than proper usage instructions) is that the product is for home use only and should not be used on "plants grown for sale or other commercial use or for commercial seed production or for research purposes". It does state that it should not be used on ajuga, pachysandra or myrtle unless well established. Googling the Preen label will confirm that as well.

Aside from that, it makes NO restrictions on what plants it can be used with and in fact, provides a rather detailed and extensive listing of hundreds of common garden plants it can be used with safely.

Sorry but I do not feel the need to do the research for you or provide citations re: trifluralin....it's all readily available if you just look. I'm sure you will find several references discussing mode of action for this chemical compound including very detailed explanations of how it works on developing - not already established - roots. But put your scientist hat on......it is far too technical for the average homeowner.

I'm not sure what Preen label you read, but the one I linked to above clearly states,

"Do not over apply; injury may occur to subsequent crops yada yada ....."

"To avoid plant injury, do not apply when foliage is wet."

It is for use "around ESTABLISHED plants and transplants. It can be used after flowering plants have germinated and are two to three inches tall." In other words, don't use it on newly planted beds that need to establish roots in the ground.

"This product is not for use on a lawn." Now why do you suppose the manufacturer would not recommend using a perfectly fine pre-emergent that inhibits root growth on the lawn?

There is so much room for plant damage on that label you could grow a banyan tree in it. 'Injury' sure sounds like 'collateral plant damage' to me.

I am a lapsed pesticide applicator and working gardener who knows full well how chemicals get used on houses and in gardens in the REAL WORLD. In real gardens.

There is so much room for plant damage on that label in the hands of your average non-gardening home owners, it boggles the mind that you could some how overlook it.

Not to mention, "This pesticide is extremely toxic to freshwater marine and estuarine fish and aquatic invertebrates ….." That most assuredly is Collateral Damage.

"Aside from that, it makes NO restrictions on what plants it can be used with -." Uh yes it does. "Do not use with crops other than those listed on the label." It is a violation of federal law to use this product in a manner inconsistent with the labeling. But you know that.

Not in the mood to discuss mitosis eh? Fine.

Mark my word. Preen is Landscape Fabric 2.0. There is no such thing as Magic Weeding Dust and never will be.

OK Christopher, you are welcome to have the last - if not the most impressive or convincing - word on the topic. You obviously have serious issues with this product..........I and many thousands of other gardeners do not. Like any other commercial herbicide on the market, it will have cautions and restrictions but they are far fewer with Preen than with a host of other regularly used products.

And unless you have a lot of really stupid garden customers, I think you are underestimating their ability to read, understand and follow label directions. Applying Preen directly to water sources seems really farfetched to me but in a garden, your so-called marine "collateral damage" is pretty much a moot point. Trifluralin binds tightly to soils and is nearly insoluble in water so according to Cornell University, "leaching and groundwater contamination by trifluralin is not expected to occur."

I can't tell you how many times I've seen the Pigeon River flowing high, muddy and full of dirt, a nice compact water shed full of humans using Preen and all kinds of other things.

Average homeowner here. I use Preen without problems. It saves me lots of aching muscles 💪!

how about horticultural vinegar?