To Till or Not to Till?

December 15th, 2015

Veggie-garden “old-timers” will tell you that the vegetable garden should be rototilled at the end of every season to “put it to bed for winter.”

Tilling, especially in fall, is a good way to waste nitrogen in the soil.

That’s the time to work in manure, get rid of weeds and left-over plant parts, and generally have the soil smooth and ready to go for spring. Right?

Maybe not because research indicates that this traditional season-ending practice is a bad idea.

The latest study to cast doubt on fall tilling comes from Penn State’s College of Agricultural Sciences where Dr. Denise Finney led a 5-year farming study into the relationship between tilling and that most important of plant nutrients, nitrogen.

Finney found that when you till in the fall and let the ground bare, you not only waste some nitrogen to the air, but you also increase the amount of nitrogen leaching into and polluting groundwater.

Her findings translate into the home garden.

She says if you’re going to till, it makes much more nitrogen-sense to do it in spring when you immediately follow it with a planting that will use the nitrogen that tilling stirs into action.

If you must till in fall, she says, nitrogen loss can be counteracted by either planting a winter cover crop or at least covering the ground with an organic material, such as chopped leaves or left-behind dead plants from this year’s crop.

“To me, if you know you have diseased plants or pest materials, it makes sense to remove that,” Finney says. “But in my home garden, I leave the healthy material in the garden. That’s organic matter. There’s no need to remove that unless it poses a health threat.”

Come spring, last year’s dead plants will make a semi-decayed mat that will act as weed-deterring mulch… one that also returns nutrients to the soil. Legume crops such as peas and beans are especially good sources of nitrogen.

In a small garden, Finney says a good alternative to fall tilling is letting healthy but frost-killed plants behind and adding a light layer of chopped leaves. Then in spring, either plant directly into that or till it in along with a light layer of compost. Then immediately plant.

In a larger garden, cover crops can take the place of lugging leaves and compost.

Finney says one of the best cover crops is planting oats in September and October. Those tie up nitrogen in fall, then return it to the soil as they die over winter and decay in spring. Tilling usually isn’t needed.

If you keep the vegetable space going until late October and don’t mind tilling in spring, cereal rye is Finney’s recommended cover crop.

Clover is another good fall cover crop that mines nitrogen from the air and delivers it into the soil when tilled in.

Any of those are better options than the conventional wisdom of tilling in fall and letting the soil bare over winter – a move that can also lead to erosion if your garden is on a slope.

“In any tilled system, you have to add compost or other plant material or you’re going to eventually deplete the (nitrogen) ‘bank account,’” Finney says.

Adding a nitrogen fertilizer in spring and summer solves the nitrogen need for plant growth, but the down side is that that’s easy to overdo, which in turn causes nitrogen to leach into groundwater and increase pollution problems downstream.

It’s also wasteful… as is tilling at the wrong time.