Mulch can be a positive addition to any garden. It helps preserve moisture, protects tree roots, and can help keep emerging weeds at bay by depriving them of sunlight. Some mulches are also pre-treated with herbicides or insecticides to deter weeds or insects (check labels to be sure). Also, mulch visually defines the space and gives a nice neat look to your flower beds.
Adding mulch isn't as easy as randomly throwing some handfuls. Knowing when to mulch (and when not to) can help you get the most out of this garden material.
When to Mulch
Broadly speaking, the best time to mulch is in spring after a light rainfall. Mulching in spring helps flower beds look vibrant and deters weed growth.
Don't mulch too early in spring, or the mulch will slow the ground warming up. Wait to mulch until your spring bulbs have all come up. If you have trees that drop a lot of litter in spring (such as maples that drop buds or helicopter seeds), then you might want to wait and mulch after this happens, so that the litter doesn't fall on your fresh mulch.
Since one function of mulch is preservation of moisture, just after a light rainfall is considered a great time to apply mulch. Spraying the surface of the mulch lightly with water after you spread it helps it set and stay in place.
Mulching in Summer
Some gardeners like to freshen up their mulch in summer, for various reasons. There might be more weed growth than expected due to heavy rain. Some mulches might fade and adding a new layer can refresh the color. Also, in times of drought, a bit of extra mulch can help retain more moisture after watering.
Mulching in Fall
Mulching in fall is often part of the garden routine as the season winds down in many growing zones. Mulch can provide protection from winter cold and dryness. For some shrubs, such as roses or hydrangeas, a lightweight natural mulch (such as pine straw) can be a better choice than heavier mulch such as wood chips, as it is less likely to freeze in case of excessive moisture.
You can also use fallen leaves for an effective winter mulch. Decaying leaves will also add nutrients to your soil and provide a place for early spring insects (including beneficial pollinators) to lay their eggs.
Mulching in Winter
Winter is a bit late for mulching in most areas. But if it looks like your shrubs could use some extra protection from harsh weather or the temperatures are about to get much colder than usual, a bit of extra mulch can help protect the root systems of shrubs. Mulching in winter can also help protect your spring bulbs from squirrels who might dig them up.
Mulch is a preferred breeding ground for jumping worms, an invasive worm species and growing problem in the United States. Jumping worms can deplete the nutrients in garden soil and cause other negative impacts. They or their larvae may arrive in bags of commercial mulch, so be on the lookout for them. Check with your local Cooperative Extension for advice on how to deal with jumping worms in your garden.
When Not to Mulch
Don't mulch just before major rainfall is expected. Too much rain might cause your mulch to run right off your flower beds or away from the base of a tree. You also shouldn't mulch right before a very windy day.
Once your mulch has been established for roughly three week, it will be less vulnerable to being shifted by weather conditions.
What month should you mulch?
The best months to mulch are mid to late April, after spring bulbs have started to push up through the ground but before spring weeds get too active.
When should you not mulch?
It's best not to mulch just before a heavy rainstorm. Also, holding off on mulching until after spring tree litter (such as maple buds) falls will keep your mulch beds looking neater.
When should I mulch before winter?
It's good to mulch before the first hard frost to help protect the roots of your shrubs and perennials. In most colder zones this means before mid November.
Jumping worms. University of Minnesota Extension Office