Green Manures

Eric Collier
Oct 21
3 min read

Updated: Nov 10

Green manures might feel like a modern sustainable ag trend, but the idea isn’t new at all. Farmers in ancient Greece, Rome, and China were already turning grasses and wild plants back into the soil to keep fields productive. So this practice has been around for a couple thousand years — we’re simply bringing it back into the spotlight.

So what exactly are green manures?

Think of green manures as cover crops grown with the intention of feeding the soil instead of feeding people. Many of the same plants used for cover cropping can also serve as green manure — especially legumes — because they team up with beneficial bacteria to fix atmospheric nitrogen. The difference is timing. When we incorporate the crop at the right growth stage, we capture that stored nitrogen and return it to the soil so the next crop can use it.

Why use green manure in the first place?

These crops aren’t harvested. Instead, they’re grown, then incorporated while still green and full of nutrients. As the plant tissues break down, they help form humus, build soil structure, and support the soil food web. Producers also use green manures to:

reduce runoff
manage nutrients more efficiently
suppress weeds and some pests
conserve soil moisture
increase infiltration
support beneficial microbes and fungi

In short — green manure feeds the soil, which then feeds your crops.

How do I choose which plants to use?

Start by looking at your soil’s needs. Are you trying to boost nitrogen, loosen compaction, or cycle nutrients more efficiently? The University of Hawai‘i Sustainable and Organic Agriculture Program maintains a Green Manure Cover Crops database specifically for Hawai‘i’s climates. It’s a fast way to see which species have been tested locally and what they’re best for.

When do I plant and incorporate green manure?Most producers plant green manure during breaks in rotation when the field isn’t being used for a cash crop. Seeds can be broadcast or drilled at recommended rates. No additional fertilizer is usually required.

The key is incorporating the crop right as it begins to flower or just before full bloom. That’s when the biomass and nutrients are at their peak. If you wait too long and let it go to seed, the plant uses the nitrogen it fixed, leaving less left in the soil for your next crop. After incorporation, allow about three to four weeks of breakdown time before planting the next crop.

Are there downsides?

The biggest challenge is time. Some growers struggle with the gap between incorporation and planting. Green manures also require a bit of rotation planning. And in Hawai‘i, bulk seed costs and shipping can be a hurdle — although local suppliers can help with that.

Bottom line — make your choice based on your goals.

Hawai‘i’s year-round growing climate makes rotation planning even more important because we don’t get a natural “winter pause.” Many producers choose mixes — combining legumes and non-legumes — to meet nutrient, structure, and timing goals in one planting.

Before you start, consider things like soil texture, temperature, pH, compaction, moisture, and existing residue.

Common green manures for Hawai‘i farms

Legumes (nitrogen-builders)

Cowpea

White sweet clover

Sunn hemp (tropic)

Pigeonpea

Lablab

Woolypod vetch

Non-legumes

Annual ryegrass

Barley