The Berry DisPatch

Newsletter of the Wisconsin Berry Growers Association

A Contemporary Trilogy:

I. Mulch To Do About Nothing

II. To Bee or Not to Bee

III. Weed Like You To Die

April, 2003

By Brian R. Smith

Extension Fruit Specialist

University of Wisconsin-River Falls

Mulch to do About Nothing

This has been a very strange fall and winter for weather in many areas of Wisconsin. Many growers are worried about winter injury. I know it is the first time I have seen so many brown leaves on my plants and a frost depth of 6.5 feet at River Falls! What I am seeing is that some areas have in fact sustained considerable injury but most areas it is only relatively minor-they only look bad from a distance. Most of the crowns I have cut into from around the state had some browning inside but they appeared very capable of making a recovery, especially if the roots looked good. To help with a speedy recovery, try to prevent as much further stress as possible; adequate pest control, proper watering, etc. Most of the time I do not suggest fertilizing in the spring because you can get soft fruit and more susceptibility to disease. However, a low dose of calcium nitrate or ammonium nitrate(10-15 lbs/A, actual) , and some Solubor may help in the recovery process.

Another closely related topic to winter injury is the proper timing of winter mulch removal. It is dependent on several factors, making it a more complex decision than some will admit. Research reports from Minnesota and Vermont would suggest mid-March as the best time for removal. Researchers in Illinois indicated removal should occur once the soil temperatures at a 3-4” depth reaches 40ºF. Whichever guideline a grower uses, there is little doubt that the earlier plants are uncovered the better IF temperatures are not expected to fall below 15-20ºF at night. That is a big IF. As I write this article on March 31, I note that the record low for this date is -1ºF. Early removal will allow for some flower bud initiation and /or developmental completion yet this spring and also less stunting because of less dependence on plant carbohydrate reserves accumulated last fall. This can all collectively result in up to 25% greater yields. Early mulch removal will also result in earlier fruit maturity but a greater number of nights involving frost control. Try to get a feel for what the weather is going to do. If it is late March or early April and the long-term forecast indicates temperatures normal or above- then uncover.

There is a small group of strawberry growers out there that either do not have great enough water pumping capacity and/or sufficient water availability for adequate frost protection. One of the options in this situation is to delay mulch removal. Taking this route can end up being a very dangerous, slippery slope, but it can be beneficial under the correct circumstances. Remove the straw when the plants start appearing whitish-yellow. Things can turn from bad to worse in a big hurry at this point, especially since daily highs are probably already near 70ºF. Two days too late and you may have such a significant yield or stand loss that the original purpose for delaying removal is defeated. Expect a 20 – 35% yield loss with this method. Advantages include a 3-10 day later bloom and at least 65% of your crop saved, versus the 100% potential loss alternative. Worst case scenario would be a very late freeze and you not only lose your crop anyway but may also lose plant stand from stress resulting from the late mulch removal.

Bottom line is, delayed mulch removal is a poor substitute for having a good irrigation system for frost control. If we consider a typical $5,000.00 irrigation system investment per acre (financial burden somewhat lessened by depreciation over 15 – 20 years), you can easily justify the high cost since even with the “delayed mulch scenario” the entire irrigation system can be paid for entirely in only two years! How?

If we assume delaying mulch removal results in about 30% yield losses (about 3,000 lb/A annually), then 3,000 lb/A @ .90/lb means you just lost $2,700/A/yr. A more optimistic viewpoint would suggest gains of 3,000 lbs. and $2,700/A if a grower practices early mulch removal and has the irrigation system to back that up with sufficient watering capacity for frost control.

Sometimes the problem of insufficient water is as simple as digging a large enough holding pond if your well is too small. An acre-inch is 27,154 gallons, so if you are watering for frost control at the rate of 0.25 in(acre inches) per hour, just do the math. If you had 5 acres of strawberries and you frost controlled for 4hrs/night for 10 nights and your sprinkler irrigation system is only 70% efficient, you would need about 1.93 million gallons on hand. Even a small well pumping continuously at 10gpm could accumulate sufficient water in about 5 months to handle this situation, even assuming evaporative losses.

Growers with insufficient water can also help protect their crop by using row covers. Alone, a row cover is nearly worthless for frost protection, giving only 1-3º of control, but it does slow the rate of temperature descent. When row covers are used in conjunction with irrigation however, the same degree of control is achieved with up to 40% less water. You can irrigate right over the row cover and obtain excellent frost protection.

To Bee or Not to Bee

The need for strawberry pollination has been discussed periodically over the years. To obtain a uniform, fully formed fruit, each pistil of the flower must be pollinated and eventual fertilization of the egg within must occur to get seed(achene) development and the accompanying flesh ( fruit) expansion around each achene. Please see illustrations on Page 11.

Those pistils not pollinated result in “blanks” and no flesh develops in that area (the developing embryo in the seed produces phytohormones like auxins and gibberellins causing flesh development around each achene).

Pollen distribution (pollination) in strawberries occurs primarily from gravity (70-80%) and to a lesser extent (15 –25%) from pollinators like honey bees and maybe 5% from wind . “King” flowers may have over 550 pistils that must be pollinated if a full-size uniform fruit is to form.

Pollination from honey bees has been shown to be responsible for up to 20% increases in yield and earlier fruit maturity. However, conclusions such as these may still be underestimating the importance of using supplemental honey bees for pollination.

Some cultivars(‘Cavendish’) produce very little pollen no matter what the environmental conditions and others always produce abundant pollen (‘Honeoye’). However, in the early spring, when conditions are least favorable for pollen production and release, the most important floret(“king”) of all is the first to bloom. The king floret which develops into the king fruit may be responsible for 10 – 20% of total yield but 15 – 35% of marketable yield. The actual fertilization process, pollen germination, and subsequent growth down the interior of the pistil and eventual ovule fertilization within the ovary of the pistil will also be affected by typically cold, early spring conditions. Also bees typically do not forage much at temperatures under 55º and or dense cloud cover. Additionally, it is estimated that each flower requires 15-20 bee visits for adequate pollination. This all would indicate overwhelming need for bee assistance to carry pollen from the “have” flowers to the “have not” flowers even with a self-fertile crop like strawberries. The guideline is 2 - 5 hives or colonies/A with the higher numbers under adverse conditions.

Remember, a hive can bee defined using many criteria. I have rented hives that turned out to have no more then 5,000 bees ; a strong hive should contain 40 –60,000 bees! It is critical to not introduce bees into the field until the strawberries are starting to bloom (so they don’t fly or work plants elsewhere) but yet sufficiently early so that as many king flowers get pollinated. If the first kings are opening get the bees in! Bee colonies should be at 500 ft intervals – and as close as possible to the field, normally within 2-300 feet. Bee conservative! Strawberry flowers do not contain very much nectar so they are not that attractive to bees. Growers must, therefore, consider placing more hives per acre and closer to the crop than they might have to with raspberries or melons.

Weed like You to Die-Sinbar Revisited

The new supplemental Sinbar label on strawberries allows for applications in newly planted strawberry fields. The label indicates 2 – 3 ounces of Sinbar/A can be applied after transplanting but before new runner plants start to root. If strawberries are allowed to develop new foliage prior to application, then irrigation or rainfall in the amounts of 0.5-1 in. must occur immediately thereafter in order to wash sinbar off the foliage and prevent phytotoxicity. Since Sinbar activity improves with migration to the weed seed germination zone, standard practice should automatically involve irrigation of 0.5-1.0 in. after any Sinbar application-during dormant or active growth.

Sinbar use during early establishment after planting is a risk. If you are applying 3 oz. to a sandy loam soil with less than 2% organic matter and a slight miscalculation occurs in calibration, massive injury can result. If a grower plans on using Sinbar the first year just before freeze-up, then no more than 5oz/A could be applied due to the total 8 oz/A restriction annually.

The label indicates Sinbar should not be used on soils with less than 0.5% organic matter. I believe a grower should seriously consider the consequences of applying it to soils with less than 1.5% organic matter. The recommendation is a maximum of 4 oz./A/application on medium and fine soils with less than 2% organic matter.

Although we now have two herbicides for use postplant (Dacthal and Sinbar) in spring of the establishment year, Sinbar cannot be applied in the spring on established bearing beds if you missed your fall application prior to freeze up due to a 110- day preharvest interval. I doubt if there is a grower anywhere in our climate that could spray Sinbar on post mulch removal and yet avoid winter injury and stay outside the 110 day window. The 110-day PHI would also preclude the use of Sinbar for first year strawberries in the “waiting bed” technique in which junebearing strawberries are sequentially planted throughout most of the summer and harvested the first year. The other option is to apply over the straw in late winter but this results in too many complications, such as reduced herbicide activity, uneven applications, run-off etc.