Q. I have seen white asparagus in grocery stores. Is this a different kind of asparagus, and can it be grown here?
A. In much of Europe, white asparagus is preferred to green. Recently, white asparagus has become more widely available in the United States, as well. White asparagus has a somewhat milder flavor than green asparagus but can be cooked and used in much the same way.
White asparagus is not a different variety than green asparagus, it's just grown in a different way. The emerging spears are shielded from sunlight, usually by hilling up loose soil or compost around them. Without light, green chlorophyll cannot develop, so the asparagus spears remain white.
If you already have an asparagus patch, you can harvest your own white asparagus. Select several adjacent asparagus plants and hill up loose soil over the crowns just when the rounded spear tips break the soil surface. Every couple of days, gently pull back the soil and harvest the blanched spears, then push the soil back in place.
Another way to keep asparagus from turning green is to place a lightproof wooden box (or even an opaque plastic 5-gallon bucket) over the plants, making sure the edges are firmly nestled into the soil to prevent light leaks. Then you can simply lift the box to harvest spears.
Q I have a crabapple that had nice pink flowers in spring, but by mid-summer the leaves were falling off and the bark was cracking. What's wrong with it?
A While all flowering crab apples are beautiful when they bloom in spring, the best cultivars are resistant to disease, which keeps them looking good all year. Cultivars that are susceptible to foliar diseases tend to look ratty by mid-summer.
Apple scab, a fungal disease that causes leaf drop on crab apples, is the likely culprit in your case. Many older crab apple cultivars are susceptible to apple scab, which causes brown spots on leaves and fruits. Severely infected leaves can turn yellow and drop off the tree in mid-to late summer. Susceptible cultivars include 'Hopa,' 'Radiant,' 'Almey' and 'Sparkler.' Cultivars with good disease resistance include 'Donald Wyman,' 'Prairifire,' 'Sugartyme' and 'Red Jewel.'
Cracks in the trunk that are long and vertical might indicate frost cracking caused by sudden temperature changes during the winter. These usually heal on their own, though the cracks may re-open in subsequent winters. If the cracking is more like flaking or ridging, it may just be normal changes in the bark that occur naturally as trees mature.
Q. I'm worried that the lack of snow cover this winter will damage my garden. Are we likely to have a lot of dead perennials in the spring? Is there anything I can do to protect my plants?
A. Snow cover can be a gardener's best friend, but for the second year in a row Twin Cities area gardens have lacked that nice white blanket. Snow is a good insulator that helps to hold the soil's heat and prevents frigid air from infiltrating the soil. Bare soil loses heat, which allows the ground to freeze to greater depths. The lack of rainfall we experienced in fall compounds the problem, since dry soil also allows for a deeper freeze.
There are many hardy perennials that should make it through the winter just fine even with minimal snow cover. Tough, long-lived perennials such as peonies, hostas and false indigo (Baptisia australis) are unlikely to be damaged. Others, however, might be more prone to winter damage because of lack of snow cover. Two groups of perennials at higher risk are those that are marginally hardy and those that have shallow root systems, which are prone to frost heaving.
Despite their name, many hardy hibiscus (Hibiscus sp.) are marginally hardy, which means they are susceptible to winter damage in Zone 4. Newer cultivars such as 'Fantasia,' 'Sweet Caroline,' 'Fireball,' and 'Kopper King' have improved hardiness, but these and all hardy hibiscus should be mulched each winter. While so-called hardy mums are indeed hardier than typical cushion or football mums, they also benefit from winter mulch.
Shallow-rooted perennials are also more susceptible to damage in open winters. These plants, including coral bells (Heuchera sanguinea), painted daisy (Tanacetum coccineum) and European wild ginger (Asarum europaeum) can be damaged in late winter or early spring when fluctuating temperatures cause the soil to freeze and thaw. Freezing and thawing can push the crown and upper roots of a plant above the soil, where they can dry out or freeze. Applying winter mulch to these plants will help regulate the soil temperature and avoid the freeze-thaw cycle.
Winter mulch (dry straw, hay or leaves) should be applied in late fall when the ground starts freezing. You could still go out now and spread an 8-to 12-inch layer of straw over your perennial bed. That might help, though it's difficult to say if any winter damage already has occurred. Next year, be sure to water perennials thoroughly until the ground freezes, then apply winter mulch. That way, you won't be dependent on a blanket of snow that may not appear.
Nancy Rose is a horticulturist, writer and photographer. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.