Contrasting Alliums

Two of our alliums (the onion family) give us contrasting stories about mulch: Onions and Garlic

Onions

Some old-timey gardeners will tell you to never mulch onions, claiming it will spoil the crop. Onions, however, are one of the garden’s thirstiest crops and do quite poorly in dry soil. In fact, they’re one of the few crops that really can use a little more than an inch of water per week, making them seem like a perfect candidate for a thick mulch. However, if they get too much moisture while they’re bulbing (the last few weeks before the tops fall), it can lead to several diseases that seriously affect storage quality, including a much higher percentage of the crop having “sour skins.”

Sour skin is the name for a bacterial disease that causes a few middle bulb layers to partially dry and then rot prematurely. It’s spread, in part, by heavy rains that splash the bacteria into broken skins and leaf axils. It’s an attempt to avoid soil wetness during bulbing that I think originated the advice not to mulch onions. Knowing all of this, we still mulch them (they need something), but we’re careful to mulch them very lightly, only giving them a sprinkling of straw that we crumble and crunch in our hands, breaking it into four-to-six inch and smaller stalks, as we apply it carefully between the rows before the plants are too big. This light mulch is often almost completely eaten up by resident earthworms by harvest time.

Garlic

Conversely, garlic, onions’ fellow allium family member, is often the most heavily mulched crop in the garden, getting six inches (plus or minus two inches) of loose straw spread over the planting bed after placing cloves in the ground. This thick layer slowly compacts down to a three-or-four-inch layer by spring and remains thick enough to block out almost all weeds until garlic is harvest in late July or early August.

Being planted in fall, garlic often needs this thick layer to temper winter cold snaps. Garlic starts growing soon after being put in the ground, having decent root systems and possibly even some small leaves by the time winter fully arrives. But all of this growth is hidden under thick straw mulch for a good reason. While garlic can withstand quite cold temperatures, its nascent leaves (and possibly even the entire plant) can be killed if exposed to extreme cold for too long. In northern areas with thick winter snow coverings, the snow often provides enough insulation, but sometimes it’s not present or not deep enough to adequately shield the garlic from extreme cold. In these cases, a thick mulch is a great backup. In areas that get temperatures dipping into the teens or 20s (-10 to -5º C) for a few days straight with little snow cover, a thick mulch is a must to keep the new leaves and bulbs warm enough to withstand the sustained cold. Plus, the mulch keeps the ground evenly moist and weed-free throughout the following growing season, both of which garlic loves.