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Hort Update for July 5, 2022

Hort Update for July 5, 2022, Nebraska Extension,
Poison hemlock flowers. Image from Oregon Department of Agriculture, CC BY 2.0

Serious ConcernsMajor Symptom
1. Poison hemlock Naturalized weed; use caution to avoid sap contact with skin
2. July 5 growing degree days (GDD) Several Nebraska sites below, Understanding Growing Degree Days
3. Pest update Pests to watch for based on growing degree days (GGD)
4. Bagworm management Insects have hatched and are feeding; look for very small new bags on susceptible evergreens or those with a history of infestation
Timely Topics
5. Avoid turf fertilization Avoid fertilization in July and August
6. Japanese beetles Adults present and feeding; watch for skeletonized leaves on preferred plants
7. Tomato leaf spots Time to watch for leaf spot diseases on tomatoes
8. Squash vine borer Sudden wilting/death of cucurbit vines or plants
9. Biting insects Chiggers, ticks, mosquitoes
Continuing Issues
10. White grub management Still time for preventative applications
11. Yellow nutsedge control Apple-green, grass-like plant with waxy blades and a triangular-shaped stem

1. Poison hemlockNaturalized weed; use caution to avoid sap contact with skin

There has been great concern in recent weeks from home gardeners about poison hemlock, also known as poison parsley. Botanically known as Conium maculatum, poison hemlock is blooming right now with large flat clusters of white flowers so can be easily spotted.

Poison hemlock is native to Europe, northern Africa and western Asia. It was introduced to North America in the 1800s as a garden ornamental and marketed as a "winter fern". It is now naturalized throughout most of North America, including the entire state of Nebraska, often found along roadsides or in unmanaged areas such fence lines, windbreaks or field edges. It prefers shaded moist areas and does not tolerate mowing well, which can be used to aid in control.

The plant is infamously reputed to have been the cause of Greek philosopher Socrates death, his chosen poison when sentenced to execution under Athens law. All parts of the plant are toxic, containing several closely related pyridine alkaloids – naturally occurring organic compounds found in some plants.

Poison hemlock is a biennial, germinating from seed in spring and throughout summer. First year plants are short, growing into a small bushy rosette of fern-like foliage. The following year, plants begin sending up a flower stalk in late April/early May which can reach up to 6-10 feet in height. Plants bloom in June and July, set seed then dry out and die as the summer progresses. The plant's taproot dies along with the foliage. Reproduction is only via seed.

Leaves have alternate arrangement on stems, although they may be oppositely arranged on the upper stems. The foliage is bright glossy green and finely divided (bipinnately compound) similar to carrot foliage. Leaves are smooth and hairless. On the lower leaves, petioles are long and their bases wrap around the main stem. Upper petioles are shorter and may not sheath the stem.

Three main identification characteristics – stems are hairless, hollow between nodes, highly branched, with reddish-purple streaks or splotches. Stem coloration is often very faint in young first year plants, so inspect non-blooming plants carefully.

Blooms are composed of many clusters of tiny 5-petalled white flowers, creating showy flat-topped flower heads.

Look A-likes

  • Queen Anne's lace, aka wild carrot (Daucus carota) – no purple mottling on stems; usually 3 feet or less in height; white hairs on stems; blooms from June through September
  • Wild parsnip (Pastinaca sativa) – no purple mottling on stems; stout grooved stems; yellow flowers; blooms May to July
  • Water hemlock (Cicuta maculata) – does have purple mottling and hollow, hairless stems, but foliage is much coarser and not fern-like; also, unlike poison hemlock, has a cluster of fleshy taproots at the base of the plant; blooms July to September

All plant parts are toxic; dead canes can remain toxic for up to three years. Mature seeds are the most poisonous. Eating plant parts is the main danger to both humans and livestock, but sap can cause skin rashes and toxins can be inhaled affecting the respiratory system.

Use the same precautions when working with poison hemlock, that you would use with poison ivy. A simple touch to a stem or flower won't kill you, but sap on your skin could give you a rash. Warn clientele that people with a history of severe allergic reactions should be especially careful. Always protect yourself by wearing chemical resistant gloves, long sleeved shirt, long pants, shoes and socks. Small infestations can be pulled by hand or removed with a shovel. Large infestations can be cut down with a string trimmer. Be careful when using and cleaning equipment to keep sap off your skin.

Plant debris should be bagged, the bag tied closed and disposed of in the trash. Or pit compost plant debris by digging a deep hole and burying plants where they will not be disturbed or seeds able to germinate.

  • Don't burn dead plant material as toxins can become airborne.
  • Don't add plant debris to a compost pile.
  • Don't leave plant debris where children or livestock have access to it.

Herbicide applications are most effective on 1) young seedlings, 2) in fall when 1st year plants are in the rosette stage or 3) in early spring on 2nd year plants before flower stalk development begins. Do not use herbicides on large mature plants; instead cut off the flowers to prevent seed development, mow large plants down and use herbicides to kill the first-year plants. Tell clientele to keep children and livestock away from areas where plant debris is left on the ground.

Selective broadleaf herbicides

  • Residential landscapes – 2,4-D, dicamba, triclopyr
  • Livestock producers – 2,4-D ester + dicamba, Grazon P+D (picloram + 2,4-D), Streamline (aminocyclopyrachlor + metsulfuron)

In addition to the products above, the non-selective herbicide glyphosate (RoundUp) is also effective but should be used carefully to minimize damage to turfgrass or other desirable ornamental plants.

Legal Status in Nebraska
Poison hemlock does not have noxious weed status and is not a legally regulated plant in Nebraska. Plants or infestations do not need to be reported. Control is not required but is encouraged.


2. July 5th growing degree days (GDD)

Location Accumulated Growing Degree Days
Grand Island, NE - Airport 1469
Lincoln, NE - Airport 1472
Omaha, NE - Airport 1489
Norfolk, NE - Airport 1345
North Platte, NE - Airport 1271
Scottsbluff, NE - Airport 1097


3. Pest updatePests to watch for based on growing degree days (GDD)

Examples of insects that have or may emerge soon in Nebraska based on growing degree days for your area. Understanding and Calculating Growing Degree Days.

GGD (base 50) InsectLifestage present at this GGDType of Damage
950-2150 Japanese beetles Adult emergence leaf feeding
1000-2000 Emerald ash borer Peak adult emergence D-shaped holes on trunk
1200-1800 Fall webworm Caterpillar feeding Leaf feeding & webbing
1250-1350 Pine needle scale 2nd generation egg hatch Needles turn yellow/brown
1375-1500 American plum borer 2nd generation Cambium tunneling on trunk, especially around wounds
1500 Pine needle scale 2nd generation hyaline stage (control target) Needles turn yellow/brown
1500-1600 Cooley spruce gall adelgid 2nd generation adults active (control target) Enlargement & browning on new growth shoots
1600-1700 Walnut caterpillar egg hatch; caterpillars Leaf feeding
1700 Zimmerman pine moth Adult flight Tunneling at branch junctions; popcorn masses of sap and sawdust
1800-2200 Banded ash clearwing borer Adult emergence Tunneling in phloem & sapwood
1900-2050 Euonymus scale 2nd generation egg hatch Yellow spots on leaves; defoliation; branch death
1925-1950 Magnolia scale egg hatch Sticky honeydew on foliage; black sooty mold; branch dieback


4. Bagworm managementInsects have hatched and are feeding; look for very small new bags on susceptible evergreens or those with a history of infestation

While bagworm populations were not high in 2021, monitor conifers for this serious pest. Eggs typically hatch from mid-May through early June, depending on weather conditions. Not all eggs hatch at the same time so it's best to wait until the majority have hatched before applying insecticide. The recommended control time is mid to late June; although effective control can still take place into July since larvae are small enough to control and they continue to feed until September. Bagworms are easiest to kill in June when small and susceptible. At that time Bacillus thuringiensis, Bt, which poses little to no hazard to non-target insects or birds and mammals, works well. If the early window for treatment is missed, larger bagworms can be treated with synthetic pyrethroids. 


5. Avoid turf fertilization in summerAvoid fertilization in July and August

Fertilization of cool season turfgrass is best avoided during July and August. It promotes leafy growth at the expense of roots and increases irrigation needs as well as susceptibility to heat, drought and pests, like brown patch disease. If needed, avoid applying fertilizer to drought-stressed or dormant turf, or when temperatures are over 80F. Select a slow-release nitrogen source. While more expensive, these rarely burn leaf blades even when applied at temperatures above 85F. Calibrate spreaders to be sure the correct rate is applied. Maintain a mowing height of 3 to 3.5 inches and use sharp mower blades.


6. Japanese beetlesAdults present and feeding; watch for skeletonized leaves on preferred plants

Adult beetles feed on leaves, flowers and fruits of more than 300 plant species. Linden trees, roses, grapes and soybeans are a few favorite foods. The beetles feed over a 4 to 6 week period beginning in late June. They eat green tissue between leaf veins causing leaves to appear lacy. Severe defoliation can stress trees and reduce yields in orchards and crops. 

The larvae, a grub, feeds on turf roots starting in late July/early August. If grub populations are high enough, the turf area affected may turn brown and the sod may pull up like carpet. Note: Treating for the grubs will not prevent beetle damage as new beetles fly in.

Reduced risk beetle control consists of hand-picking or knocking beetles off into a bucket of soapy water (7 PM at night is a good time), or exclude them from plants by using plant covers i.e. floating row covers, where feasible. Fine mesh nets can be placed over roses. Two organic sprays, Neem and Pyola, can protect plants for about 3-7 days.

Chemically, some relief will be gained by treating leaves with carbaryl, bifenthrin, or other pyrethroid products. These all provide about 2 weeks of protection after thorough coverage. 

Because insecticides affect pollinators, try to spray in the evening when bees are not foraging and after plants have finished blooming. Follow label instructions to avoid harming pollinators and damaging plants.

Dealing with Japanese Beetles, Nebraska Extension


7. Tomato leaf spotsTime to watch for leaf spot diseases on tomatoes

Tomatoes are susceptible to a number of diseases whose symptoms begin as leaf spots. Most common are the fungal diseases early blight and Septoria leaf spot, and bacterial spot & speck.

Both fungal diseases begin near the bottom of the plant as leaf spots and infections progress upward. Early blight begins as ¼ to ½ inch brown circular spots with concentric rings in their centers. Septoria begins as small, 1/16 to 1/8 inch, water soaked circular spots with dark brown margins and gray or tan centers that enlarge to about 1/4 inch. The use of disease resistant varieties and avoiding overcrowded plants and overhead irrigation will reduce disease severity. The use of fungicides labeled for use on vegetables, such as chlorothalonil or Mancozeb, will help prevent new infections.

Bacterial spot and speck leaf lesions typically have a yellow halo around them. Bacterial spot leaf spots are brown while speck lesions are black and slightly raised. Symptoms on green fruit are often most noticeable. Some strains of bacterial spot & speck are resistant to copper fungicides, otherwise copper fungicides provide fair control. Copper fungicide can be tank mixed with mancozeb for enhanced control. During the growing season avoid overhead irrigation and overcrowded plants. In fall, good sanitation is essential to prepare for the next season. A 3-4 year crop rotation plan is recommended. 

Leaf and Fruit Diseases of Tomatoes, Nebraska Extension   


8. Squash vine borerSudden wilting/death of cucurbit vines or plants

When squash plants like zucchini, summer squash and acorn squash suddenly collapse and the base of the stem is mushy with holes, the likely cause is squash vine borer. Adults are ½ inch long moths that look more like a thick wasp than a moth. They have an orange abdomen with black dots and are day fliers, unlike most moths which fly at night. Eggs are laid from about mid-June into July.  After hatching, caterpillars bore into the stem where their feeding causes plant wilting and eventual death. Once caterpillars are inside the stem, insecticides will not work. During egg laying, synthetic pyrethroids can be applied to the base of stems. Just prior to egg laying, about mid-June, a physical barrier such as steel wool or tin foil can be placed around plant bases or plants covered with row covers.

At this time of year, if plants look good but holes are found in stems, gardeners can use a knife to cut the stem with the grain of the stalk to find the borers. Use the point of the knife to pierce them. Once borers are removed, cover the cut area with soil to encourage new roots to form.  If plants are completely destroyed, remove and destroy the dead material which will also remove any caterpillars actively feeding.  Because caterpillars pupate in the soil where they emerge as adults the following year, rotate new plantings to a different area the next season. Second plantings of summer squash can be done up to mid-July.

Squash Vine Borers, Universiy of Minnesota Extension


9. Biting insectsChiggers, ticks, mosquitoes

If clients ask questions about biting insects like chiggers, ticks and mosquitoes, below are one page fact sheets about management they can be provided with or referred to:


10. White grub managementStill time for preventative applications

Control depends on proper timing of the application and moving the insecticide into the root zone where grubs feed. Preventive control applications are made from mid to late June. They can work in early July. Curative or rescue treatments are made in August or September.

Preventive - Most of the preventively-applied insecticides are systemic in nature and will be taken up by the plant and translocated to roots. The following products are effective against young grubs. Labels: c = commercial product, h = homeowner product.

  • Chlorantraniliprole – Acelepryn(c), Scotts GrubEx (h)
  • Clothianidin – Arena (c)
  • Halofenozide - Mach 2 (c)
  • Imidacloprid – Merit (c), Bonide Grub Beater (h), BioAdvanced Season Long Grub Control + fertilizer (h)
  • Thiamethoxam – Meridian (c)

Curative (rescue treatments) – Trichlorfon (BioAdvanced Grub and Insect Control) provide the best control due to their higher kill rate against mature white grubs. These products must be watered in for acceptable control. Moving the insecticide into the root zone involves applying ½ inch of water immediately after application. Do not use products that contain ONLY bifenthin, deltamethrin, cyfluthrin, permethrin, lambda-cyhalothrin or gamma-cyhalothrin for soil-applied grub control. These chemicals bind with organic matter at the soil surface and will not move down into the soil to provide effective grub control.

Thatch plays an important role in reducing the efficacy of turf insecticides applied for white grub control. If the thatch layer exceeds ½ inch, light aeration and increased post-treatment irrigation will enhance insecticide penetration and should improve white grub control. In problem areas, such as those with thick thatch layers, repeated irrigations may be necessary every three to four days to continue moving the insecticide into the soil. When white grubs are deeper in soil, curative treatments are more effective if a retreatment irrigation of 1/2 inch is applied 48 hours before the insecticide application. This will encourage grubs to move closer to the soil surface and enhance the level of white grub control.


11. Yellow nutsedge controlApple-green, grass-like plant with waxy blades and a triangular-shaped stem

Yellow nutsedge is the apple-green, waxy, grass-like weed that grows faster than turfgrass and is now becoming more noticeable in lawns. While the ideal window for controlling yellow nutsedge is past, it can still be controlled. Ideally it is best to apply a post-emergent product with the active ingredient of either Halosulfuron, Sulfentrazone or imazosulfuron when yellow nutsedge plants are small. Read and follow label instructions on herbicides as some may have temperature restrictions or require surfactants. By this time of year, plants have developed underground tubers that may begin growth once the parent plant is killed, and repeat applications are usually needed. The sooner these new yellow nutsedge plants are killed, before they develop tubers, the better. It is likely, the area will need to be treated again in subsequent years. Doing so prior to June 21, prior to plants developing new tubers, is best.

Lawn Care Pro Series: Yellow Nutsedge Control, Nebraska Extension Turfgrass iNfo


Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Nebraska Extension is implied. Use of commercial and trade names does not imply approval or constitute endorsement by Nebraska Extension. Nor does it imply discrimination against other similar products.