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Hort Update March 10, 2017

Divide rhubarb as early in spring as possible or wait until fall

1. No hurry to begin spring careWait for turfgrass to become active for most care practices

Allow turfgrass to come out of dormancy and begin growth, and soil temperatures to warm, before starting most lawn care practices. From scheduling mowing and irrigation to controlling white grubs, refer to Management Calendar for Cool-Season Lawns, Nebraska Extension.

2. MowingBegin once grass is growing and tall enough to need mowing

Mowing is not needed prior to turfgrass greening up and starting to grow. The first mowing should take place when green grass is tall enough to require mowing. Mow at 2.5” to 3.5” for the entire growing season, returning clippings to the lawn. Do not remove more than 1/3 of the total canopy height at one time. Mowing at the shorter end of the recommended range will require more frequent mowing than mowing at the higher end. Mowing too infrequently – called scalping – accelerates growth rate, reduces quality and canopy density, and encourages weed invasion.

3. WateringOkay if warm and dry but not needed until turf green and growing

Watering is fine if conditions remain dry, warm and windy; however, until turfgrass is actively growing, watering may not be needed. As we begin the season, a reminder that more lawn problems arise from over-watering than under-watering. Lawns should be watered deeply with 0.5 to 1.0” of water (depending on soil type) only when wilt is observed. Common symptoms of minor drought include light blue-green color and lingering footprints. Automatic irrigation systems should be closely monitored, and be equipped with either a rainout or soil-moisture sensor to prevent irrigation when there is sufficient soil moisture.

4. FertilizingMight be okay to apply earlier this year with above average temps, but no rush; growth rate and greenness of turf should guide applications

Fertilizing should begin after growth begins. Once the spring growth surge begins to slow and turfgrass color is off is the time to begin fertilization. With above average temperatures this year, beginning fertilization in April instead of May may be needed this year. Fertilizer containing 50% each quick and slow release nitrogen sources are good choices.

5. CultivationBegin once turfgrass growth is active; April is okay but fall is ideal

Cultivation (power raking and core aeration) is best avoided until turf resumes active growth. It needs to be done prior to applying preemergence herbicides for crabgrass. Lawn aeration or thatch removal (dethatching) is permissible if soil compaction exists or thatch is greater than ¾” in depth. Cultivating through a preemergence herbicide barrier may reduce efficacy. This practice is preferred in the fall, but April is okay if soils are not too wet.

6. Weed ControlToo late for preemergence control of knotweed; too early for crabgrass

Preemergence weed control of knotweed and crabgrass - Prostrate knotweed is a summer annual weed; however, its seed breaks cold dormancy and germinates much earlier than other summer annual weeds. Germination begins with soil temperatures of 35 to 50 degrees F., stopping when soil temperatures reach 50 degrees F. During most years in Nebraska, germination likely begins between late February and early March. It is now too late to control knotweed with PRE products. Postemergence products containing Dicamba or triclopyr can be effective.

Crabgrass, also a summer annual, begins germination when soil temperatures reach 55 degrees F. at two to four inch depth. This typically does not occur until May in Nebraska making late April into early May the treatment window for PRE products.

Trees & Shrubs

7. Early BuddingIf frost occurs, blooming may be reduced or leafing delayed

Early budding of shade trees and shrubs is common with above average winter and spring temperatures. Swelling of buds, or actual opening of buds, increases the risk of low temperature injury to tree and buds. This creates concern and raises questions. Most temperate zone plants survive this well. If the buds are flower buds, the only loss for shade trees and shrubs is the loss of blooming for that year. If leaf buds are injured, this will result in delayed growth. However, otherwise healthy plants will develop secondary buds and do fine. This can be a stress for plants. Along with warm temperatures, if conditions remain dry, drought may be and added stress. We cannot do much about temperatures and swelling buds, but we can water plants in the absence of rain. Remind people most plants are not actively growing and so a lot of water is not needed; just enough to moisten soil 6 to 8 inches deep.

8. BagwormMonitor evergreens; hand pick old bags now; apply controls in June

Check evergreens, especially spruce, Juniper and Arborvitae, for overwintering bagworms. As many as 500 to 1000 eggs can overwinter in one female bagworm. Removing and destroying bagworms from now until May 1st can help reduce the bagworm population. Destroy bagworms by crushing or immersing in soapy water. If bags containing eggs are discarded on the ground, eggs may still hatch and larvae return to the tree.

It is too early to apply insecticidal products to evergreens for bagworms. Wait until after egg hatch. Products are most effective in reducing damage if applied during the early stages of bagworm development. Insecticides, as well as Bacillus thuringiensis, are best applied from mid-to late- June. They can be applied up until about mid-August, but increased damage will occur the later they are first applied.

Bagworm, Nebraska Extension

9. Pine BlightIdentify disease; apply fungicides at correct time for each disease

Fairly moist weather the last few growing seasons has promoted an increase in pine diseases like Dothistroma needle blight and Sphareopsis tip blight. Now is the time to check pine trees for these two evergreens. If present, timing of fungicide applications is most important. For Dothistroma, the first application is made as needle are emerging, about mid-May, and repeated after new growth has occurred, about mid- to late-June. If the blight is Sphareopsis, the first application is recommended during the 3rd week of April and a second 10 – 14 days later.

For identification and fungicides see:

10. Pruning summer blooming shrubsThinning or renovation pruning can be done now

Summer blooming shrubs can be pruned just before they begin growth in spring. These shrubs develop flower buds during the current growing season, or on new growth, Pruning in spring will not result in the loss of blooming as it would with spring flowering shrubs. Shrub pruning can involve thinning of larger, older stems or complete renovation; pruning the shrub near the ground if it has become overgrown and has a lot of dead stems. It is fine to wait and prune summer blooming shrubs after they bloom in summer as well.

Pruning Shrubs, Nebraska Extension

11. Pruning evergreens - timingSpruce, yew, arborvitae and juniper prune now; wait on pines

Pruning evergreens timing. Prune evergreen shrubs, such as juniper and yew, in late March to mid-April before new growth begins. Light pruning may also be done in late June or early July. Prune Japanese Yew, Spruce and Fir between April and August, but only if needed. Prune pines just after they have finished new growth (candling), typically mid-June to mid-July.

Pruning Evergreens, Nebraska Extension

Landscape Ornamentals

12. Bulb (tulip & daffodil) cold weather protection & recoveryTemperatures below 20 may cause flower & foliage damage

Early growth of spring bulbs may be occurring following warm periods this winter. Leaf and bulb growth can be damaged when normal spring hard freezes occur. Tulips and daffodils can survive freezing temperatures in the upper twenties without much damage, but colder temperatures can damage the plant's flowers and foliage. Freeze damaged foliage turns white and limp. Little long term damage is likely to the plants. Do not cut back damaged foliage until it dies back on its own.

Continued growth of bulb shoots may be slowed by applying 3-4 inches of wood chip mulch to keep the soil cold.

Fruits & Vegetables

13. Fruit tree flower bud cold toleranceReturning cold temperatures increase risk of bud death/damage

During winter when fruit flower buds are fully dormant, they can tolerate very cold temperatures with little or no damage. Recent warm late winter temperatures have caused tree buds to begin to lose their winter dormancy, so much so that some cultivars of silver maple are in full bloom in eastern Nebraska. Reduced cold hardiness makes buds much more easily damaged when cold temperatures return as nighttime freezes or a multiple-day cold snap.

Potential damage to flower buds on tree fruits is a serious concern, since a significant crop reduction will occur if many buds are killed. The stage of bud development determines how susceptible any give fruit crop is when freezes occur.

14. When to divide rhubarbDivide as early in spring as possible or wait until fall

When a rhubarb crown is 6 to 10 years old, it may be dug up and divided. This should be done as early in the spring as possible, or wait until September to divide. Insert a shovel about 6 inches into the ground next to the base of the plant and lift out the entire crown. Break the crown into fist-sized pieces, each with at least one bud and a large root piece. Pull away the dark brown sheaths left from last years' stalks. Replant new divisions as soon as possible. If planting is delayed due to weather conditions, store them in the refrigerator. Rehydrate the divisions before planting by soaking in water for at least two hours, or preferably, overnight. Allow new divisions one good growing season to reestablish before harvesting.

Rhubarb, Cornell University

15. Nebraska average last spring freeze datesGuidelines for when to plant cold-sensitive plants

map of average last spring freeze dates

Nebraska average last spring freeze (32° F) dates indicate that half of all final spring freezes will occur before the dates shown and half will occur after, based on 47 years of data from 1949-1995. This year warmer than normal early spring temperatures is causing early growth in many plants and making frost predictions difficult.

In southeastern Nebraska that average last spring freeze date is approximately April 30 and May 21 in the northwest corner of Nebraska's panhandle. These dates are guidelines only. Freezing temperatures may occur after the dates listed below. Also remember that local microclimate conditions can significantly affect the occurrence of frost in your landscape. These dates can be used as guidelines for gardeners planting early spring crops. Frost sensitive plants will not tolerate freezing temperatures and must be protected if freezing temperatures occurs after planting.

16. Early season vegetable gardeningCold tolerant vegetable planting can begin soon

Early warmer temperatures this spring mean many gardeners are anxious to start planting their vegetable garden. Gardeners need to be aware of the last average spring frost date for their area and either wait until after that date to plant cold sensitive crops or have a plan to provide cold temperature protection if it is needed. However, many vegetables are very cold tolerant and can be planted long before the spring frost date. The following is a partial listing of cold tolerant vegetables, with the potential spring planting dates for eastern Nebraska.

  • March 15th - Asparagus crowns, collards, onion sets, garden peas, radishes, spinach and turnips
  • March 30th - Leeks, mustard, potatoes and swiss chard
  • April 5th - Beets, cabbage, carrots, bibb lettuce and leaf lettuce

Making use of these cool season vegetables can provide an early vegetable harvest with extra sweet flavor. Some vegetables, like carrots and radishes, taste better when grown in cool weather and tend to get bitter or hot if growing temperatures are too warm. Most other vegetables including beans, cucumbers, eggplant, muskmelons, okra, peppers, pumpkins, squash, sweet corn, sweet potatoes, tomatoes and watermelon should not be planted until after the spring frost date, unless extra cold protection is provided.

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