Give your plants a boost from below this autumn
As branches go bare, it may seem that plants have shut down to wait out the winter. In fact, there’s still a lot of growing going on in our gardens and woods — but it’s underground.
In the soil, beneath the mulch in our gardens and the layer of fallen leaves in the woods, tiny roots are lengthening, cell by cell. In fact, the roots of many kinds of trees have a growth spurt in fall.
The flower bulbs we’ve planted in the fall are also growing. Not their stems, leaves and flowers — those are already formed inside the dormant bulb and waiting until spring to emerge. But in March or April, that unfurling plant will need roots to absorb water and nutrients from the surrounding soil. Autumn, before the soil freezes, is when the bulb plant builds its root system.
The trees, shrubs and perennials we planted this year are growing roots too. They had limited space for roots in their pots or their burlap wrappings. Now that they’re in the ground, they’re starting to expand their root systems as they settle into their new homes.
The roots that are growing are not the large, gnarly anchoring roots that we sometimes see surfacing around trees, but the almost microscopically fine threads called feeder roots. They are so fine that they can snake between particles of soil. These threads branch and branch again, probing for water and chemicals that the plants need. Cooperating with beneficial fungi that form even finer threads, they create a wide underground network that extends far beyond the plant we see aboveground.
What does this seasonal root growth mean for a gardener? “It means your plants need watering in fall,” said Sharon Yiesla, plant knowledge specialist in the Arboretum’s Plant Clinic.
“Plants need roots to soak up water, but they need water to grow more roots,” she said. “They often need a boost.”
Autumn rains can help, but they may not be sufficient. “Climate change has made our rainfall much more variable and less reliable than it once was,” Yiesla said. “We have to really pay attention to how much moisture is in the soil now, and err on the side of watering the plants that are likely to need it.”
Any new plants will need water. Those will include fall-planted bulbs, perennials installed this year, and any trees or shrubs installed in the last two to three years. To store the water they need for root growth now and a good start in spring, plants should receive the equivalent of 1 inch of rainfall weekly. For trees and shrubs, water both the root ball, right around the trunk, and the surrounding area. Water deeply so the water soaks in to encourage new roots to grow deeper into the soil. “Wait a few days before you water again,” Yiesla said. “Then keep it up until the soil freezes.”
Autumn watering also will benefit evergreen trees and shrubs, whether they are new or established. Evergreens keep their leaves through the winter, and they are in danger of drying out in the dry cold of winter. Watering them in the fall so they can store water in their roots, stems and leaves will help them resist this winter kill. A layer of mulch over the root zones of all plants, including trees, shrubs and perennials, will snuggle growing roots like a comforter. It will insulate the soil so it takes longer to freeze, keep moisture from evaporating, prevent bitter cold snaps from penetrating the soil and nurture the many microorganisms that interact with the roots.
“We can see plants slowing down in fall, but we don’t see that they are still at work and they still need our help,” Yiesla said. “Watering and mulching in fall will help them get ready for their new year.”