Manure compost & weeds/insects

Asked May 6, 2019, 12:35 AM EDT

Hi there! I applied 3” manure compost from Menards to my new flower bed (dug it up from grass last fall). Do I need to treat the composted dirt with weed killer now? Someone said manure compost will cause weeds if I don’t. Second, how long after adding compost can I take a soil sample for the lab? Do I need to wait a certain time for the compost to “sink in”? Thanks! Terra

Hennepin County Minnesota

1 Response

Thanks for the question. It is an interesting one.

Depending upon the type of manure and how it was treated, it may or may not contain weed seeds. I know some gardeners who will add the manure to their compost pile to compost it a second time before use. The downside of this, of course, is that would delay its use in your garden.

See the following:

If weeds are present in the manure they would most likely be seeds. If you treat the manure before using, it should be with a pre-emergent herbicide such as Preen. Remember if you do use Preen, it should be spread on the surface of the manure and then not be disturbed. Just adding weed killer to the manure probably would not work as such herbicides are only effective against actively growing weeds. Theoretically if the manure you purchased had already been composted, weed seeds should have been killed but this is entirely dependent upon how that composting was done.

I suggest a two-step approach:

If you saved the manure bag, there should be a number on it you could call for customer service. If you have already tossed the bag, then you would need to Goggle the manufacturer and obtain such a number. In any event, you would be asking the question as to the weed seed content of their product.

Secondly if you want to be on the safe side, apply Preen to the manure after you have spread and incorporated it into your garden. Then sprinkle the Preen on the surface and don't disturb things for a week or so. If you do this, you would not be able to plant any flower seeds in the garden for at least two or three weeks. Instructions that come with Preen should say when it would be safe to put in seeds after its application. Preen will not affect any flowers that are already up.

If you contemplate adding manure again in the future, it would be best to do this in the fall.

With respect to a soil test, waiting a week or so after incorporating your manure before taking a sample would give you the most accurate analysis. If case you don't have it, here is the University of Minnesota site at which soil testing is done:

Good Luck!!