Dear Master Gardener: I like your column very much. I would like to see a column about plum trees in northern Minnesota. I live north of Brainerd and have greatly enjoyed my plums, but there are a number of challenges with them. I have Toka, Waneta, Sapalta, Compass and Pipestone plum trees. I am particularly concerned about how to prevent and treat plum curculio, and when to prune them.
Answer: Plum trees can definitely pose a bit of a challenge in northern Minnesota. Like all stone fruits, they bloom very early in the spring and if the delicate flowers get damaged by freezing temperatures, it means no fruit that year. Spring is known for temperature fluctuations, but it’s worth taking a chance on the weather if you enjoy eating plums. If you don’t get fruit one year due to freezing temperatures, it doesn’t mean you won’t get them in a subsequent, milder year.
Plum trees are relatively easy to grow and manage. They should be planted in full sun in well-drained, composted soil. To ensure fruit, plant two different, compatible varieties spaced 12-20 feet apart (depending on the mature size of the variety.) It will take about two to five years after planting a one or two year old tree to get fruit.
Prune trees annually to maintain healthy fruiting wood and to maximize light penetration into the tree. Remove any broken, dead, or diseased branches, and any downward-growing branches. If two limbs are crossed or entangled, remove one of them completely at its base. Remove suckers and watersprouts.
All the plums you are growing are hardy to zone 3. The Toka plum is sometimes called the bubblegum plum, due to its sweet, candy-like flavor. It is often used as a pollination partner because it produces tons of pollen. Toka was developed in South Dakota and has been around since 1911. Waneta, a 1913 South Dakota introduction, has red-skinned fruit with yellow flesh that is juicy and sweet. Sapalta, a Canadian introduction, and Compass are both unique crosses between a sand cherry and an unknown species of plum. Pipestone is a University of Minnesota plum tree that was introduced in 1942. It has sweet, juicy yellow fruit.
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The plum curculio weevil infests fruit trees shortly after the end of bloom and can cause significant damage. Plum, apple, apricot, pear, cherry, and crabapple are hosts. This pest can be more abundant on fruit trees adjacent to woods. The females try to lay eggs in the fruitlets, leaving the characteristic crescent-shaped scar on the skin. If an egg develops into larvae, the larvae feed on the inside of the fruit, causing internal damage and some of the fruit to drop. The larva appears in June and is active for about two weeks. If you only have a few infested trees, plum curculios may be shaken out of them. Place a white sheet under the tree, shake the branches to make the adult weevils fall onto the sheet, roll it up and dunk it in a bag or bucket of soapy water to kill the insects. This technique works best early in the morning when the weevils are sluggish and more likely to fall than to fly away. Clean up fallen fruit under trees in which eggs or larvae may be developing. Although it is very time consuming, you can put mesh bags around each fruit to prevent insects from reaching them. I’ve used Ziploc bags with the corners cut off on my apples and Jackie Burkey (a Master Gardener and frequent co-contributor to this column) ties footies (she orders them in large quantities) around hers. It may work on plums, especially the larger ones.
Dear Master Gardener: Is the burning bush really banned from being sold in Minnesota now? The shrub is so pretty in the fall.
Answer: In the mid-1800s, the winged burning bush (Euonymus alatus) was introduced to North America as an ornamental shrub. The bright red fall foliage has made it an attractive and popular landscape plant for years. According to the Minnesota Department of Agriculture, propagation and sale of this plant are prohibited in Minnesota. In 2020 it was declared a Restricted Noxious Weed. MDA states that although Restricted Noxious Weeds are not required to be controlled or eradicated by law, landowners are strongly encouraged to manage these invasive plants on their properties in order to reduce spread into new areas.
Dear Master Gardener: Does watercress grow in Minnesota?
Answer: The USDA Natural Resources Conservation map shows that watercress occurs throughout all of the continental United States, except for North Dakota. It is found in cool shallow water, muddy banks, streams, wet ditches and pond margins. Watercress (Nasturtium officinale) is a hardy, perennial herb that originated from Europe. It is the same watercress you can find at the local grocery store in the produce section. It is considered to be noxious and invasive in 46 states. In Minnesota it is considered a widespread and problematic invasive of clear streams, springs and brooks. It spreads out on the surface of the water, choking out native plants.
It is edible and can be harvested wild, but it is important to take it from clean (tested) water, as you run the risk of ingesting harmful parasites, pathogens, toxins, or getting giardia. Harvest it before flower buds appear; otherwise, the flavor decreases in potency and it becomes inedible. Cut the stems at the waterline with a scissors or pull clumps out by the roots and cut the roots off later.
According to Utah State University you can grow watercress in your garden. Place potted plants in a bucket with 2 to 3 inches of water so the media stays wet and the roots are submerged under water — simulating the saturated conditions of a stream. They recommend changing the water once or twice a week. You can also plant watercress by an existing water feature in the yard, locating the plant where the soil stays saturated with water.
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You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at umnmastergardener@gmail.com and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.
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