It’s September while I’m writing this. It’s raining pine needles. After a stressful summer for most plants, you can see pine trees with lots of brown needles ready to drop. Ironically, you can also see grasses that have suddenly become green again after some heavy rain. Mother nature is preparing for winter. A good question might be “What is she up to and why?” A reasonable (even if highly irritating) answer would be “It depends.”
Along the lines of “The more you know, the more you realize that you don’t know,” a word of caution here. The differences in soils between sand, clay, rock and loam; the differences in crops between perennial, annual, orchard, field and row crops; the differences in weather between wet, dry, cold and hot; and the differences in terroir between hills, meadows, forest, minerals and microclimates are all important in deciding how to prepare your ground for winter. Things I write or you read elsewhere have to be interpreted by your own developing sense of soil.
Having said that, a good general principle is that nature avoids bare soil. So if you are thinking “Let’s get all of these stems and weeds out and leave a clean bed for spring,” please stop. Multiple bad things happen when soil is bare. Rain and snow make heavier pieces of rock and sand sink and lighter dust and clay stay in the liquid. When that dries, the clay layer makes water run off without sinking in. The water goes on to create erosion. The microbes in the soil die of thirst. Living plants, dead plants, mulch… anything is better than bare soil, especially if that soil has a lot of clay.
Your underground livestock: bacteria, fungi, protists, nematodes and worms need air, moisture, food and shelter to survive the winter. Living plants are the best because fungi exchange nutrients with plant roots in exchange for sugars produced by plant leaves. Fungi can also live on just woody material like straw, sawdust, cardboard and wood chips. That slows way down under a cover of snow and ice. But even plants that die during the winter are good food for the other organisms. Take forage radishes, AKA tillage radishes, for instance. In hard soil and clay they tunnel down several inches. When they die, they leave passageways for water to seep in stopping runoff and food for microbes and fungi.
My soil is sandy. When we got here we never found rocks in post holes, but also no worms. Red wiggler worms burrow down several inches to survive the cold. But they don’t like sand. They like warm rotting stuff and give your whole biome a big boost. I am building up my soil year by year, but for worm insurance I am also building up a large compost pile. The worms will migrate to the bottom when it gets cold and survive. They also don’t like it too hot. Right now it is over 90° at the bottom of the pile, too hot for worms. Up a little higher it is 70° to 80° and has a lot of rotting produce, worm heaven.
Out in the vineyard there is a nice crop of green grass. Soon the grape leaves will fall on top of the grass. This is the natural mulch that makes deciduous forest soil so rich and is built up just before winter. Unlike a garden or flower bed, the vineyard is a perennial forest of vines and pasture. Some people would like to have all the dirt around the vines tilled up. That would disturb overwintering pests like leafhoppers and eliminate weeds that would “compete” with the vines for nutrients. I don’t look at it that way.
Green plants, especially grass, feed fungi during the winter with sugars in exchange for water and minerals. The more microbes you have living in the soil, the more nutrients are available in the spring. The nitrogen in green grass is especially valuable. I intend to mow the grass before adding mulch, such as woody material or sawdust, over it. The grass will help break down the mulch layer. The best thing would be to have animals eat the grass and return it as manure to the soil. I don’t have the right critters for that and am pretty sure most of them would prefer the grapes to the grass.
Part of the deal is that grass has huge roots. I have potted grape plants outside and grass sometimes gets in the pots. Most weeds pull out of the pot easily leaving just the grape plant in the soil. Not grass. Grass wants to take all the soil with it when you pull it. It thrives after being eaten down once and bounces back. The big roots prevent grass from being pulled by grazing animals and give it a food reserve to draw on for the rebound.
The grasses on the Great Plains built fertility for thousands of years before being plowed under. At first the rotting grass roots were very fertile and crops were great. But after having all the organic matter removed each year and laying fallow over the winters, they have lost 70% of the carbon and 70% of their water holding potential (For the Love of Soil, Nicole Masters). So I want to keep the grasses in my vineyard.