Asked July 1, 2019, 12:18 PM EDT

At what soil temperature is the soil too hot for plant life? If applying an organic mulch such as compost, at what temperature (of the mulch) does that mulch become dangerous to plant life (such as grass that is growing next to the application of mulch)? For example, if I put down compost in my landscape beds as a mulch and that mulch is hot, is there a certain temperature range where it becomes dangerous to those plants?

Marion County Oregon

3 Responses

I'm not sure there is one answer to your question about temperatures, as plants vary widely in their temperature tolerances (think USDA Hardiness Zone Map). In your example of applying mulch that is still in hot stages of the composting process, I suspect that the thinness of the layer would allow heat to dissipate before much damage were done. The roots of grass are very resilient, and so would likely outgrow any damage done to the blades. Mulches are usually spread in thin layers, but I would think that even a four inch layer of mulch could not retain its heat for very long.

I understand that plants have different needs. Here in Marion County, if we are referring to native species, what temperature would you say is too hot for plant health? A soil temperature of 95 or hotter? Is there any data that shows what temperature is too hot? I haven’t been able to find any.

We have had multiple cases this year where applying mulch has fried customers grass along edges of beds. We took the temperature of it and our bark dust was at 120 degrees. Would that be considered too hot to apply near plants? From your response, it sounds like not.

I'm going to recommend that you contact Neil Bell, Home Horticulture Agent in Marion County. I suspect direct interaction or a couple of phone calls will be required.;
1320 Capitol St NE #110, Salem, OR 97301