Cooperative Extension: Garden and Yard

There are a number of reasons why seeds fail to germinate and/or why germinated seeds fail to thrive. The UMaine Analytical Soil Lab does not test for soil toxins because there are hundreds of them and they require specific tests–i.e., there isn’t a test just for toxins present/not present in soil. You could submit a soil test if you are concerned about the fertility or pH of the soil, but I’d be quite surprised if the mulch were the culprit, unless they added something extremely salty (e.g., unwashed seaweed compost). If it were contaminated by pesticides, I would expect to see nearby plants dying as well. It’s more likely that something else was causing the failure to germinate, as well as the germinants’ lack of vigor.

The most common reasons for failure to germinate are:

1) Lack of viability in the seeds (even new seed occasionally has bad batches, and older seeds can lose viability quickly under non-ideal storage conditions). Poor seed quality could also result in something like radishes sprouting, but not doing well.

2) Fungal pathogens in the soil. Germinating seeds and newly emerged plants are highly susceptible to lethal diseases like damping off. Overly wet conditions drive these infections, so it is possible that the mulch trapped moisture and kept the seeds too wet, especially in the wet summer we’ve been having.

3) Poor soil conditions–compacted or crusted soil can inhibit the free flow of air and water that seeds need to sprout successfully. Nutritional content/deficiencies do not usually impact seed germination, but they can absolutely work against newly germinated seedlings like the radishes after they’ve emerged.

4) Improper planting technique–you sound like an experienced gardener, so this probably isn’t the problem, but seeds planted too deeply (e.g., if the seeds were buried under a thick layer of mulch instead of being troughed or planted in hollows), too shallowly (light inhibits germination in most seeds, they mostly prefer it dark), or at the wrong time of year (e.g., planting cool season crops after it has grown hot) can inhibit successful germination.

5) Predation by wildlife–small mammals love nothing better than a buffet of tasty seeds or tender young germinants, so you may have trouble with the “neighbors in your garden.” I’ve personally had an entire bed of newly planted seeds decimated by a chipmunk infestation.

The Univ of MD Extension has a nice page that goes into further detail on this topic here.

So, I can’t rule out that there is something toxic in the mulch (it’s worth asking your property manager where they got it), but it would be surprising to me and I’m not sure how you would go about verifying it. Again if the mulch is the problem from a toxin standpoint, I would expect to see a lot of dead plants in the immediate area. It’s more likely that there was an issue with the seeds themselves, or that the mulch contributed to the problem mechanically (by being piled too thickly, preventing the seeds from emerging). It is theoretically possible that the mulch wasn’t fully composted, and that decomposing material in it was using up nitrogen at the barrier between mulch and soil, but this isn’t normally a huge problem and would really only be an issue for the poorly performing radishes. Soil nutrition doesn’t have much impact on seed germination because the seeds rely on their internal stores until they sprout, put roots out into the soil, and begin to photosynthesize.

My advice would be to get the soil tested for nutrition and pH, add a thin layer of compost annually to improve soil texture, follow good seed planting technique, and obtain a new batch of seeds for next season. And despite this experience, I always recommend the proper use of mulch. If those don’t work, you could also start the seeds indoors and transplant out larger plants.

With apologies for not being able to give you a more definite answer, I wish you happy gardening, nonetheless.