QThe large white pine in our yard tends to turn yellowish every winter. It has not turned as yellow so far this year, perhaps because of the milder weather. Is it normal for white pines to turn yellow? Does the tree need fertilizer?
AThe needles of some white pines (Pinus strobus) do develop a rather unattractive yellowish or olive green color in the winter. Some other evergreens that also tend to show winter discoloration include red or Norway pine (Pinus resinosa), Jack pine (Pinus banksiana) and arborvitae (Thuja occidentalis). Most trees that discolor in winter recover a more normal green color in the spring.
There seems to be quite a bit of variability in needle color among individual specimens of any of these evergreens. Summer needle color among white pines can vary from olive green to silvery blue-green. The degree of yellowing in the winter also varies greatly. However, if an individual tree tends to yellow, it seems to do so consistently from year to year. The needle color is determined by genetics, sort of like eye color among humans.
Among these evergreens, a number of cultivars have been selected for good foliage color and resistance to winter discoloration. There are many arborvitae cultivars so selected. Unfortunately, very few white pines have been selected for good foliar color and resistance to winter discoloration. Korean pine (Pinus koraiensis), while not widely available, is gaining popularity in part because most specimens retain a good blue-green color through the winter. This pine has long, thin needles in bunches of five and looks similar to white pine.
If a yellowed evergreen fails to regain a green color in the spring, or if an evergreen starts turning yellow during the growing season, problems may be brewing. Yellowing foliage often indicates root problems, usually overly wet soil. Most evergreens require well-drained soil, although arborvitae is an exception. Check the soil moisture around the tree. Also check to see if the tree is planted too deeply; you should be able to see the roots starting to flare out just at the soil line. Planting too deeply reduces the amount of air reaching the roots, which in turn can kill roots and eventually the whole tree. Also check for trunk injuries and insect or disease symptoms.
Most trees in home landscapes benefit from a yearly application of fertilizer. A tree that is lacking nitrogen may have yellowish foliage. Have your soil tested to find out if fertilizer is needed; often there is plenty of phosphorus and potassium, and all that's needed is nitrogen. Have the pH of your soil tested also. White pines can develop iron chlorosis (which shows as yellowed foliage) in high pH (above 7.0) soils. If your soil is on the high pH side, use an acid fertilizer and mulch heavily with pine needles and shredded oak leaves, both of which help acidify the soil.
QSome of my crocus and squill started sending up leaves in the warm weather early this month. I was worried about the foliage freezing so I covered it with straw. Is this a good idea? Can bulbs be damaged by having leaves frozen?
AWhile many bulbs are hardy enough to make it through our winters without mulch, I think its a good idea to cover any of those that started sprouting in this oddly warm early winter. Frozen foliage won't kill the bulb, but it could reduce the bulb's food reserves, since it will have to produce more foliage in the spring. The bulbs need foliage to produce food reserves during the growing season, which is why you never should cut back bulb foliage when it's still green.
You can add mulch to garden beds any time during the winter. If you have lavender plants, for example, and you opt to mulch them instead of letting them tough it out, you can still run out and pile some straw around them. A nice way to recycle the Christmas tree is to cut off branches and use those to cover a bulb or perennial bed. Of course, if we've already had weeks of subzero temperatures and no snow cover, the damage may already have been done.
Remember that mulch also keeps the ground cool in the early spring; this is a good thing, especially for bulbs and early-emerging perennials. If these plants come up too early, a late cold snap can freeze them. Many gardeners remember the early April temperature plunge several years ago, when many emerging tulips and other bulbs were frozen to the ground.
QWhich ornamental grasses have the showiest seed heads during the winter?
AMost varieties of miscanthus (Miscanthus sp.) grass have fluffy plumes of seeds that stay on the plant most of the winter. Within this genus there are several species and many named cultivars. Cultivars of Miscanthus sinensis are some of the best grasses for winter interest. The cultivars 'Purpurescens,' 'Silberfeder' (Silverfeather), 'Autumn Light,' 'Malepartus' and 'Graziella' are just some of the miscanthus available. The first two cultivars listed are the hardiest. The species Miscanthus sacchariflorus, Chinese silvergrass, has pretty seed heads, but it spreads aggressively by rhizomes and is too invasive for landscape use.
Though less showy, other ornamental grasses can provide winter interest also. Switchgrass (Panicum virgatum) has airy seed heads that catch the light nicely. While the seedheads of Karl Foerster feather reedgrass (Calamagrostis x acutiflora 'Karl Foerster') fall apart by mid-autumn, its striking bleach-blonde upright foliage provides a highlight in the winter perennial bed. Little bluestem (Schizachyrium scoparium), silver spikegrass (Spodiopogon sibericus) and moorgrass (Molinia caerulea) are some other possibilities.
Ornamental grasses are gaining in popularity. For more information, check for books on ornamental grasses at the library. The University of Minnesota extension publication Ornamental Grasses for Cold Climates is very informative.
--Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. She spends her spare time gardening, inside or outside, depending on the weather. Please address gardening questions to her at the Minnesota Landscape Arboretum, PO Box 39, Chanhassen MN 55317. She will answer questions in this column only.