My thinking has changed. I used to mulch everything and always.
But in Coastal SoCal mulch is not helpful in winter because it keeps the soil cool, slows plant growth. Use it only in various places to keep the soil moist where you are waiting to plant, or for erosion prevention. Take your mulch away a week or more before planting so the soil can warm up.
Understanding contradictory practices! The University of Georgia’s Robert R. Westerfield, Extension Horticulturalist says: “Mulch keeps the soil cooler in summer and warmer in winter, improving both root growth and nutrient availability. At the end of the growing season, organic mulches can be tilled into the soil to further increase the soil’s organic matter content and water-holding capacity. Finally, mulches impart a neat, trim look to gardens and reduce the incidences of mud-splashed flowers and vegetables after heavy rains, which can lead to disease problems.” I’m thinking the warmer in winter part may be in colder climates than ours, like where the soil can freeze, where there are very cold air temps, mulch acts as a buffer. Here in coastal SoCal, if sunlight can’t get to the soil, the soil can’t get warmer from IT.
Unless you are not planting in an area, and won’t be doing a green manure cover crop, NO, DON’T dig your mulch into the soil! Fall soils are cooler, decomposition is slower. The decomposition process requires Nitrogen, but so do your plants. Plants put in that soil will be slower, stunted, may never produce. However, if you can’t help yourself, make the ratio of straw to soil very little, and add well composted manures or instantly-ready-to-use bunny poop. And if you are vermicomposting, have worms available, put them to work in the area where the straw is near the surface. Red wriggler worms feed at the surface. INSTEAD of digging old mulch in, much better to add yummy already made compost to increase organic matter and water-holding capacity.
It is true you could have some mud splashed veggies, but how many of your SoCal winter veggies are near the ground or need it? Broccoli, cauliflower, grow up. Kale gets up as you harvest lower leaves. Peas use less ground space if they are up a trellis keeping clean. Cabbages are low, yes, but they are in a tight round head that outer leaves protect and water runs right off. Turnips, beets, carrots, radish are in the ground. That leaves lettuces and late strawberries. Yes, mulch about them to keep the leaves and berries clean. Use your common sense.
Remove and throw mulch away if there have been any diseased plants above or near it, and/or if you have any pest infestations. For example, it is a nesting materials place for ants, that tend aphids – aphids that love broccoli heads, curly leaf kale and young cabbage leaves. If you do use mulch there, keep it away from the base of your plants. Ant nests may also be associated with plants that support large populations of other honeydew-producing insects – soft scales, mealybugs, or whiteflies. Bagrada Bugs like to scurry under mulch to hide and lay eggs. Since winter’s Brassica plants – broccoli, cauliflower, kale – are the Bagrada Bug’s favorites, especially no mulch under or near them!
Save it! Bag it if it is still usable. Keep it dry. No mice allowed.
Use it in composting layers! Already partially decomposed, your composting will go a tad faster! Put it handy beside your compost area if possible. If you have veggie kitchen waste available, layer it in. 1 inch wet, 2 inches dry (straw), 1 inch wet, 2 inches straw, repeat, repeat, repeat! Your wet could be a 1/2 inch layer of mowed grass. Not too thick on grass because it mats and stops oxygen flow. Turn the pile once a week or when you can. If you have animals you could give old straw mulch to them for bedding. If not, and you just don’t have the time to do compost layering, and have the space, get out the pitchfork and toss it in a pile, water it down a bit, maybe cover it some and throw a couple boards with rocks or concrete blocks on top to hold the cover in place, and forget about it until next spring and you feel feisty again!
If you are leaving an area unplanted, instead of mulch, grow a cover crop there that is rich in Nitrogen, feeds the soil, is a green manure! Legumes take N from the air and deposit it into nodules on the roots! Fava, a legume, is a great winter cover crop, producing one of the highest rates of compostable organic material per square foot! They get tall, so if you need less shade, one great mix you can put in that gets 4′ to 5′ in height, is vetch, Austrian Peas, bell beans, and oats (in Santa Barbara area, buy it at Island Seed & Feed bulk). The oats have deep roots that break up the soil. Each year I dedicate a different area or two for soil rest and restoration.
The one way winter mulching acts like a charm, is erosion prevention. If you have some berms or have established some water capturing wells, cover the berms or sides of your wells, with mulch just thick enough to prevent them from dissolving to flatness!