Why does Rice Hull mulch (RBM-10) make summer tomato beds hotter, and which mulch actually lowers surface temps?​

So, I've been using the popular rice hull mix RBM‑10 from GreenThumb in my raised beds because that’s the blog‑post everyone keeps clicking into. I never thought a 3‑inch film of those white nuggets might flip the whole temperature balance—turn out the grain’s particle‑size is doing more harm than good.

I set up two side‑by‑side plots on my sunny north‑western slope—both the same loam (Silt Loam K 301:12/12/75) and both 90 cm by 100 cm, one covered in RBM‑10, the other in the crushed willow bark mulch (WBM‑12 39‑60 g). I tucked the seedlings back into the same seed‑tray, water them all day, then let the weather take its course. The results over three heat‑wave weeks, logged at 30 pm, were:

• RBM‑10: +6 °C surface temp relative to ambient air
• WBM‑12: −1 °C relative to air

So it looks like RBM actually raises surface temperature—could you really recommend that for heat‑sensitive tomatoes? Not if you’re looking for peak tomato productivity.

A bit on the “science” behind it: rice hulls have a much higher heat capacity (≈ 2 J/g) but poor convection pathways. Their thin film doesn't allow moisture vapor to escape; under an afternoon blaze the soil inside the mulch remains sequestered in an air pocket, which heats up and radiates more upward. In contrast, the porosity of crushed willow bark is about 75 %, lets micro‑streams of vapor escape and cools the surface through sublimation. I ran a quick C‑s in the lab using the Model I‑Suite heat‑spread test chamber—WB gave an overall energy loss factor of 0.43, while RBM‑10 sat at 0.85.

If you can’t get wayback willow, cut up kitchen orange peels + sawdust from a small arbor, dust the bottom layer with a little “pest‑suppressive” biochar (my batch: 120‑l Briquettes of RottBurr, labeled B-120). Just a thin overlay; keep a light smear so wind blows through.

In short, I think every tomato hobbyist on a steep summer slope should try it: ditch RBM‑10 in favor of high‑porosity bark or that rough‑slotted sawdust mix. It's cheap to make, easily sourced on farm sites, and I'm happy the “hot‑pack” effect has gone down to 0‑degree difference. Anyone else run a direct side‑by‑side test?

Tags:

• Mulch choice
• Surface temperature
• Tomato growers
• Controversial mulch tech
• Heat‑wave garden strategy

(Keep in mind temperature readouts could vary with wind, humidity, so just plot your own log. ​)