How to repot a plant

Q When should I repot my houseplants? They've been growing in the same containers for several years.

A Repotting in spring takes advantage of the energy boost the plant receives from longer days and more intense sunlight. But you can -- and should -- repot a houseplant whenever it needs it, regardless of the time of year.

Signs a plant needs repotting:

• It's grown so much that it overwhelms the container.

• You have to water every day or two because the pot is filled with roots and water rushes right through.

• The plant isn't putting on new growth, even though it receives ample light, water and fertilizer.

Give it a test:

Examining the root ball will let you know for sure if your plant needs to be repotted. Here's how to do it:

Tip the pot over and give it a good rap on the side of a kitchen counter. The soil and root ball should pop out pretty much intact.

If you see lots of roots twining around in the shape of the container, or making a circular mat at the bottom, it's time to repot. If you see mostly potting soil, with roots scattered throughout, just return the plant to its container; no harm done.

How to repot:

• Choose a container that's only one size larger than the current pot. Make sure it has drain holes so the soil doesn't stay too moist, which invites root rot.

• Add enough fresh potting soil to the container bottom so the plant will sit at the same depth as it did in the previous pot.

• If there's a dense, circular mass of roots, wiggle some of them loose with your fingers.

• Settle the plant into the container and firm potting soil around the sides.

• Water the soil thoroughly, then put the plant back in its former location.

• Because you've added new potting soil, it shouldn't be necessary to fertilize for several weeks.

Pine needles as mulch Q We have several mature pine trees on our property. Each fall they lose a lot of needles, which we've allowed to build up over the years. Can we use these needles as mulch?

A Fallen pine needles are commonly used as mulch in Southern states, where they're called "pine straw." They're an excellent mulch here, too, because they're organic and when they eventually break down, they enrich the soil.

Pine needles are acidic, but that actually works in their favor. Acidity makes them less likely to get moldy as they age. And their ability to acidify the soil is a plus. Most Minnesota soils -- certainly in the Twin Cities area -- are alkaline, and most plants grow best in slightly acidic soil.

Use pine needle mulch around flowering perennials and spring bulbs as well as young trees and shrubs. If you can, add more needles every year or two to maintain a depth of 3 to 4 inches. Because pine needles are somewhat slow to break down, they're not as good for mulching flowering annuals or vegetable gardens, where the soil is turned every year.

Pruning wisteria Q How do you prune wisteria?

A You probably have Wisteria macrostachys, also known as Kentucky wisteria. This lovely plant blooms quite reliably in the Twin Cities area, producing hanging clusters of lavender-blue flowers. Though fast-growing, it's not as vigorous, nor are its flowers as large and fragrant, as the popular Japanese wisteria that thrives in milder climates.

Here's how to prune wisteria:

• Once new growth is obvious in spring, prune back any stems that show signs of winter die-back.

• Wait until Kentucky wisteria finishes blooming to do any additional pruning.

• To encourage more blooming, shorten side branches so they extend no more than 36 inches from the main stem.

• Thin out some of the stems if growth is so dense that light won't penetrate well.

• If the vine is as tall as you'd like it to be, you can prune back the top of the main stem. This results in more branching and potential future blooms.

If you're growing Japanese wisteria and hoping that proper pruning will make it bloom, you'll be disappointed. Although the vines survive our winters quite readily, producing plenty of leaves, they rarely bloom.

Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.