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Monday, August 29, 2016

Useful Tools to Determine Soil Moisture I: Tensiometers

Karl Foord, Extension Educator - Horticulture

Generally the most limiting element in maintaining uniform plant growth and high quality produce is water. A plant can be experiencing water deficit prior to our observation of wilting. Such deficits can lead to slower growth rates, pollen mortality and loss of flowers, lighter fruit weight, and blossom end rot, among others. It is best to avoid having the plant experience any water deficit.

However, how can you know that the plant is in water deficit? The truth is that you can only know this indirectly. One can estimate soil water content by taking a soil surface sample and feeling the water content with your hand; this will of course vary with soil type and again extrapolates the soil water content within the root zone. This method will require consistent attention to growing conditions including temperature humidity and rainfall.

What  other options might allow a more direct sampling of soil water content within the rooting zone of the plant?

Soil Water Potential

Soil water potential is a measure of the amount of energy required by the plant to absorb or extract water from the soil matrix. A soil water potential value of 0 (Zero) means that the soil is saturated, water moves freely, and the plant need not expend energy to absorb water at this potential. However as water is removed from the soil by the plant, more energy will be required by the plant to extract water overcoming the forces holding water molecules to soil particles. The soil water potential is measured in centibars and as the values become more negative the difficulty in obtaining water becomes greater. One tool to measure water potential is a tensiometer.


A tensiometer is a hollow tube with a porous ceramic tip at one end (Figure 1).
Figure 1

The tube is filled with water and sealed with a cap, and has an attached vacuum gauge. The porous tip allows the water in the tube to be connected to the soil water matrix. As plants use water and the soil dries out, water is also drawn out of the tensiometer. Because the tube is sealed, a vacuum is created and measured by the gauge. Plants can easily extract soil water when the soil water potential value is 10 centibars (CB). At higher values such as 35 CB it is more difficult for the plant to extract water. Figure 2 shows the face of the pressure gauge.
Figure 2

Figure 3 shows the cycling of tensiometer readings over a period of time. The plants use water and the pressure increases into a range between 20 and 38. The sloped lines indicate the water draw down and the straight lines indicate a watering event. The gauge reading returns to near zero following the watering event .
Figure 3

By placing tensiometers at two depths, we have a way of knowing when to turn the water on and when to turn it off. A short tensiometer (6") is place in the middle of the active root zone, and a longer tensiometer (12") is placed below the active root zone. When the short tensiometer reads between 30 and 35 CB begin irrigation. As irrigation begins the 6" gauge will begin to move to zero. But when should you stop irrigating because the 6" gauge is in the middle of the root zone? When the 12" gauge begins to move to zero it means that the water is beginning to exit the active root zone. At this point the irrigation should be turned off (Figure 4).
Figure 4
 We will take a look at some other tools used to measure soil moisture in the second part of this blog.

Friday, August 19, 2016

Bugs on milkweed

Jeffrey Hahn, Extension Entomologist

Home gardeners have been finding orange and black insects on their milkweed and related plants. Although some people think they are boxelder bugs, they are actually insects called large milkweed bugs, Oncopeltus fasciatus.
Large milkweed bugs are colorful and conspicuous and
fortunately harmless.  Photo: Jeff Hahn, U of M Extension

Adults grow as large as ¾ inch long. They are mostly orange and black including a black horizontal band across the center of the body and black on the end of the wings (the membranous section). A portion of the head is reddish orange. The immature nymphs are mostly orange with black wing pads and smaller than the adults.

Large milkweed bugs prefer to feed on common milkweed but will also feed on other related species. They often feed in large groups making them conspicuous on the plants. Despite their appearance, they do not harm milkweed nor any insects, like monarchs, they may also be on the plants. No action is necessary if large milkweed bugs are found in your garden.

Thursday, August 18, 2016

Sap beetles in gardens

Jeffrey Hahn, Extension Entomologist

Some home gardeners have been finding sap beetles in some of their fruits and vegetables. These beetles are generally small, between 1/8 – ¼ inch long, oval, and dark colored. Some sap beetles have orange spots on their wing covers.
A picnic beetle, a common type of sap beetle.
Photo: Tom Murray
Sap beetles are attracted to fermenting smells and will attack fruits and vegetables, such as tomatoes, sweet corn, raspberries, strawberries, and muskmelons, that are damaged, overriped, or rotting. They often are just a nuisance, although it is possible for them to move to and damage ripening fruit.

The best management for sap beetles is to pick fruits and vegetables regularly as they ripen and remove any damaged or overripe produce in your garden and dispose of by burying or bagging them. This helps eliminate smells that could attract them to your garden. However, once sap beetle find your garden, they can be challenging to eliminate.

Insecticides, such as carbaryl or permethrin, can kill sap
Pick up fallen and damaged fruit to prevent
attracting sap beetles to your garden.
Photo: Jeff Hahn, U of M Extension
beetles and reduce their numbers. However, the challenge is that there is an interval of time between when a product is applied and when the fruits or vegetables can be safely harvested. You can find this information by looking for the Days to Wait to Harvest number on the pesticide label.

Depending on the product, this can take days or even weeks. By then the sap beetles have likely returned. If that is the case, try to use a product with as short of a time interval as possible.

See also Sap beetles in home gardens

Friday, August 12, 2016

Extension publications: New & Revised in 2016

The following list is updated with links to new and revised online Extension publications as they become available. The publications are new unless otherwise indicated.


Monday, August 1, 2016

Lawn Irrigation Survey and Water Saving Strategies

The Metropolitan Council and University of Minnesota Extension are conducting a survey to assess irrigation practices throughout the 7-county Twin Cities Metropolitan Area.  This survey is part of a larger project with the ultimate goal of reducing water use in the home landscape.  You can help us by taking 10-15 minutes to answer a 30 question survey regarding your irrigation practices.  All survey participants will be entered into a drawing for 1 of 10 Visa Gift Cards ($50 value).  Additionally, we are conducting irrigation audits for many properties throughout the Twin Cities.  To have your home irrigation system audited, please complete the survey and indicate that you would like to receive a free audit.  To access the survey, please follow click the hyperlink below: 


Basic water saving strategies for home lawns      
Pay attention to the weather
During a Minnesota summer we may see heavy periods of rainfall followed by extended periods of drought. Homeowners with lawns should adjust irrigation practices accordingly. Operating irrigation controllers in manual mode is one way to monitor and cut down on water use, rather than using an automated schedule.
Select turfgrass species that use less water and can tolerate drought
Choice of grass species will impact irrigation requirements. Traditional turfgrass species for Minnesota include Kentucky bluegrass, perennial ryegrass, fine fescue, and tall fescue. The fescue species offer the best drought tolerance potential. 
Adjust irrigation programs to conserve water
To encourage rooting and drought tolerance, lawns should be irrigated infrequently (one time or less per week) with a sufficient volume of water (up to 0.5 inches). Set irrigation programs or sprinklers to water during the morning hours, because daytime irrigation is often lost through evaporation or wind deflection.    
Implement water saving technologies
Rain sensors connected to irrigation controllers are vital to conserving water. There’s no need for an automatic sprinkler system to be used when it’s raining.
Conduct an audit on your irrigation system
Irrigation auditing is one great way to conserve water.  Irrigation contractors will often perform this service for you if you have a contract with them.  Auditing an irrigation system includes three basic steps: 1) checking system components including sprinklers, valves and controllers, 2) conducting a performance test, and 3) programming the controller.  For more information on conducting an irrigation audit, visit this website:

Friday, July 29, 2016

Ground-nesting solitary wasps

Jeff Hahn, Extension Entomologist

There have been a lot of questions about solitary wasps lately. The most common questions have been about cicada killers (Sphecius speciosus) but residents have also seen great golden digger wasps (Sphex ichneumoneus), steel-blue cricket hunters (Chlorion aerarium), and sand wasps (Bembicini).
Cicada killer returning to its nest with a paralyzed cicada. 
Photo: Jeff Hahn, UMN Extension

These wasps are generally large insects. Cicada killers range in size from 1 – 1 ½ inches with a stout body, black and reddish brown thorax, amber colored wings, reddish brown legs, and a black abdomen with yellow bands. Great golden digger wasps are about one inch in length, a more slender body with a black head and thorax covered with short golden hair with a reddish-orange and black abdomen and reddish-orange legs. They have smoky, dark colored wings. Steel-blue cricket hunters are also about one inch in size and relatively slender with iridescent dark blue bodies and wings. Sand wasps are smaller, most are close to ½ inch in length and are typically black and yellow.

Great golden digger wasp with a captured katydid.
Photo: Jeff Hahn, UMN Extension
Their biology is similar to each other. They nest individually in burrows but are gregarious, i.e. there are many nests in a small area. They hunt specific insect prey, paralyze them, and bring them back to their nest to feed to their young. Cicada killers seek out cicadas; great golden digger wasps hunt katydids, steel-blue cricket hunters capture field crickets and sand wasps, depending on the species, look for a variety of different insects, especially flies.

These wasps can be common in lawns, gardens, areas adjacent to sidewalks and driveways, and areas with patio stones. Typically they are found nesting in well drained, light textured or sandy soils. The females are not aggressive and while they are capable of stinging will only do so to defend themselves. Males can aggressively guard a territory and will challenge other males (even people) but they lack a stinger cannot harm us.
Aggregation of dozens of great golden digger wasp nests (with
dozens more to the right).  Photo: Jeff Hahn, UMN Extension

They do not injure turf, nesting in areas where the lawn has thinned or existing bare spots. There can be a large quantity of dirt that is dug up which can be unsightly. It is possible that their tunneling can undermine patio bricks but nothing worse than that. Some people are frightened by them because of their size and perceived threat.

There are a couple of options for dealing with ground nesting solitary wasps. The first is to ignore them. They are not causing any real harm and are not dangerous to people. They will go away eventually on their own sometime during August. However, if they were present this year, they will probably nest in the same area next year.

Sand wasp with paralyzed leaf-footed bug prey.
Photo: Jeff Hahn, UMN Extension
If control is desired, it will be necessary to treat the nests with an insecticide. Generally spraying the nests is not very effective. Instead, apply an insecticide directly into each individual nest entrance. Dusts are most effective; granules and sprays can also help reduce numbers. Effective active ingredients include permethrin and carbaryl. If you rather, you can also contact a lawn service to treat the cicada killers for you.

Resistance does not equal immunity

M. Grabowski, UMN Extension
Crabapple 'Adams' has good resistance to apple scab but has
some leaf spots this year. M. Grabowski, UMN Extension

Planting disease resistant plants is a great way to reduce disease problems in the garden without pesticides or the added time and labor needed for many cultural control practices. A disease resistant plant is able to defend itself against a plant pathogen. In some cases, a resistant plant is very successful in its defense and the gardener will not see any visible symptoms of disease. In other cases the plant may develop low levels of disease but is able to slow the pathogen and prevent severe damage from disease. These plants may be marketed as disease tolerant, moderately resistant, or resistant.

This year, weather conditions have been highly favorable for apple scab, a fungal disease of apple and crabapple trees. The apple scab fungus infects both leaves and fruit. Leaves have olive gray to black spots with a feathery undefined margin. Severely infected leaves turn yellow and fall off mid-summer. This year, due to weather conditions favorable for apple scab, many susceptible varieties of apple and crabapple have thin see through canopies with small piles infected leaf debris collecting on the ground below.

Severe apple scab leaf infection on a
crabapple variety susceptible to apple scab.
M. Grabowski, UMN Extension 
Gardeners may be surprised to see that even apple and crabapple varieties that are resistant to apple scab have some leaf spots and even some leaves turning yellow. It is important to understand that these disease resistant trees have not lost their resistance to apple scab; they are simply fighting against a strong and thriving pathogen population this year. Despite the presence of a few leaf spots, apple scab resistant varieties have thick full green canopies and very little leaf loss. This means that the stress placed on the apple scab resistant varieties is significantly less than the stress on the defoliated susceptible varieties.

Full canopy on crabapple variety ' Adams' despite some apple scab.      
M. Grabowski, UMN Extension 

Apple scab susceptible crab apple variety with
a thin see through canopy due to leaf loss.
M. Grabowski, UMN Extension

For a list of apple scab resistant crabapple varieties and more information about management of apple scab read the UMN Extension publication on apple scab

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