Plant Science for May 7, 1998

Below is a listing of NDSU field day tours and other special events. Exact starting times will be given at a later date via this newsletter or other media channels.

To those interested: the NDSU Distribution Center now has a supply of newly revised circulars on Proso millet, tame mustard and safflower. These have been revised and updated this spring.

NDSU Extension Distribution Center, Morrill Hall 7, Box 5655, Fargo, ND 58105, phone (701) 231-7883;
fax (701) 231-7044 or e-mail: slane@ndsuext.nodak.edu.

These publications also are available from your local NDSU Extension County office.

Flax is an alternative cool season oilseed crop and should be planted early to maximize yield potential. How late can flax be planted and still get fairly good yields? Below are research results from flax seeding studies conducted over 9 years at the North Central Research/Extension Center at Minot, ND.

Seeding period

Average*
Yield
bu/A

%Early
seeded
date

1^st (Early-May)

2^nd (Mid-May)

3^rd (Late-May)

4^th (Early-June)

5^th (Mid-June)

6^th (Late-June)

100

Early June planted flax was 8 bu/A less than early May planted flax. These data suggest that all flax should be planted between late April to May 15 to ensure maximum yield potential. Good seedbed preparation, adequate shallow seed placement to moisture, weed control and fertility management also will contribute to high flax yields. Yield goals of 30+ bu/A are not unrealistic for well managed flax production.

Growing Degree Days (GDD) correlates plant development with heat units (daily temperature extremes). Based on plant emergence and historic temperature trends GDD can be used to predict the time when a crop will reach a certain developmental stage. The actual stage of a crop is best determined by visual evaluation, but GDD provides a very close estimate of developmental stage.

Using heat units to predict plant development functions on the premises that the actual number of GDD for a crop to reach maturity remains relatively constant across environments even though calendar days may change substantially. For example, when wheat is planted late early plant development generally occurs under higher temperatures than an earlier planted crop. Consequently the late planted crop will reach maturity in fewer calendar days than the early planted crop. Other factors influence the growth and development of crops that are not taken in to account when using GDD to predict development these include; fertility level, day length, and available moisture.

Growing degree days are based on daily high and low temperatures and are calculated for one day as follows: (high temp. + low temp.)/2 - minimum base temp = GDD in degrees Fahrenheit.

When calculating GDD there are minimum and maximum base temperatures for each crop that are used when temperatures reach extremes. When the temperature falls below the minimum base temperature or exceeds the maximum base temperature for the crop in question then the minimum or maximum base temperature is used in the calculation. For example if you are calculating the GDD for corn when the high temperature was 78^o F and the low temperature was 45^o F then the GDD would be (50 + 78)/2 - 50 = 14. The low temperature of 45^o is not used because it is below the minimum base temperature of 50^o F for corn. Table 1. gives minimum optimum and maximum base temperatures for several cereal crops.

Table 1. Minimum base, optimum and maximum growth temperatures in Fahrenheit of several crops for use when calculating growing degree days.

Crop	Growth Temperatures (F)
Crop	Base	Optimum	Maximum
Wheat	32	76	90
Barley	32	70	86
Oats	32	70	86
Corn	50	86	108

There is good agreement of what minimum base temperatures should be used when calculating GDD for the cereal crops, however, there is often disagreement of what maximum base temperatures are appropriate. Use of GDD is based on a model that assumes developmental rate increases linearly to a maximum temperature and then remains constant as illustrated in Figure 1 by the solid line. Developmental rate in any crop does not increase linearly and remain constant but will increase to an optimum then the rate will decrease as illustrated in Figure 1 by the dotted line. The disagreement occurs over the question, should the optimum temperature for development be used as the maximum temperature when calculating GDD or should the temperature at which development rate falls to near zero. I prefer to use optimum growth rates as the maximum when calculating GDD, as is commonly done with corn at 86^o F as the maximum temp.

Figure 1. Growing degree day based model and actual crop developmental rate relative to temperature.

When calculating GDD in North Dakota the maximum temperature will make little difference in many years. If the daily temperature rises above 90^o F three or four days during the entire growing season and does not significantly exceed the optimum temperature the rest of the season the total accumulated GDD will differ little with method of calculation.

In small grains early growth is the most sensitive to high temperatures and 70^o F should always be used as the maximum temperature until the crop has reached the two leaf stage. Following the two leaf stage the maximum temperature will be higher.

When using GDD to determine crop growth stage accumulation should start the day after planting. Small grains require about 180 GDD for germination and emergence. A wheat plant requires about 140 GDD for each leaf whether accumulated in four days or ten days, and about 2400 GDD to reach maturity, which can range from 83-100 calendar days.

This is important since the number of GDD that have accumulated for your crop will be different from the total accumulation for the growing season. The North Dakota Agricultural Weather Network (http://www.ext.nodak.edu/weather) provides daily temperatures that can be used to determine the accumulated GDD. GDD listed for wheat are for the entire season and should be adjusted for planting date. The maximum temp used by NDAWN is 95^o F for wheat and 86^o F for corn.

Growing degree days calculated for crops should not be confused with insect degree days (DD or IDD). Insect degree days start accumulating when the minimum temperature for the insect in question is reached.

Several questions have come concerning the use of systemic insecticides on horticultural plantings like vegetables and flowers. While systemics offer the benefit of carrying the insecticide throughout the plant, and thus offering a measure of protection from destructive insects, their wholesale or indiscriminate use can be dangerous, especially if the plant material is for human consumption, or to attract bees, butterflies, or hummingbirds.

Bees, butterflies, and hummingbirds attracted to flowering plants could be killed by the systemic pesticide. The butterfly larvae would be especially vulnerable since they feed on leaves which typically have the highest concentration of the pesticide. Hummingbirds, as well as other species could also be affected when they feed on the insects made sick from ingesting the pesticide-laden foliage. And certainly, any plant intended for human consumption should never have any systemic pesticide applied, no matter how early in the season it happens to be. If a pesticide is called for, use the least toxic, and one that has no systemic activity.

If plants are accidentally sprayed with systemic pesticides, the best action to take is to remove the flowers as they open so that pollinating insects or hummingbirds will not be attracted. If any plants intended for human consumption have been sprayed, remove them from the garden immediately, before they can flower and set fruit.

Before any pesticide use, read label instructions completely, and assume nothing! Just because a crop is not going to be taken off a vegetable planting for 12 or more weeks, and the label says to reapply every 4, doesn’t mean the crop is home free. It is not worth taking a chance with you or your family’s health.