1012 
as their methods were crude, and where 
the necessity was greatest the orchards 
were badly located and the task was al- 
most hopeless from the start. Frost 
warnings were issued by the Weather 
Bureau during that time, although very 
little attention was paid to them, as fore- 
knowledge of frost is of practically no 
benefit to the horticulturist unless he is 
prepared to protect his crop from threat- 
ened injury. 
In 1907 Mr. P. J. O’Gara, one of the 
scientific assistants in the Bureau of 
Plant Industry, was sent to the Rogue 
River valley to study the pear blight, 
which was making inroads among the 
pear and Spitzenburg apple trees in that 
section of the country. He quickly real- 
ized that the fruit growers were losing 
much more fruit by spring frosts than 
they were willing to acknowledge, and 
being familiar with orchard-heating 
methods in California, he soon induced a 
number of orchardists to adopt similar 
methods in the Rogue River valley. The 
plan was so successful the first year that 
it was tried the next on a fairly large 
scale and with even greater success. In 
the meanwhile a few orchardists in other 
important sections had taken up this 
work, and by the spring of 1910 the move- 
ment had obtained large proportions in 
four important fruit centers, viz.: Rogue 
River valley, Yakima valley, Lewiston or- 
chard district, and the Boise orchard dis- 
trict. 
ROGUE RIVER VALLEY 
* When Frost May Be Expected and 
Where Frost Is Likely to Occur 
The conditions obtaining in the Med- 
ford district are thus described by Mr. 
P. J. O’Gara: 
In the spring it is found that during 
the day, that is between sunrise and sun- 
set, the wind blows mostly from northerly 
quarters. These winds are not moisture 
laden as a rule, the relative humidity 
often being as low as twenty-five per cent 
at a temperature of seventy degrees Fah- 
renheit. During the night when frosts 
are likely to occur the winds die down 
altogether, or change to a southerly quar- 
* Office of Pathologist, Bulletin No. 5. 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
ter. The winds from the south are very 
dry, and the relative humidity is often 
much lower during the period in which 
the winds come from the south. If the 
winds continue to blow from the north- 
west or westerly quarters, frosts rarely 
occur, because these winds tend to raise 
the dewpoint, or, in other words, bring 
in air with a larger percentage of water 
vapor present. While the water vapor 
content of the atmosphere is high, dam- 
aging frosts cannot occur. It is only 
when the dewpoint temperature  ap- 
proaches the freezing point or is below it 
that we may expect a serious freeze. As 
a rule it is only on the valley floor that 
serious injury may be caused by low 
temperatures during the blooming period 
or some time thereafter. Even on the 
valley floor where there may be some 
slight elevation no frosts occur, while 
serious injury may result only a few 
feet below. The hillsides surrounding the 
valley usually escape frosts altogether, 
and the average variation in temperature 
in favor of the lands lying above the val- 
ley floor is from five to six degrees; 
therefore, even though a heavy frost may 
occur on the valley floor, the temperature 
may not go to freezing on the uplands. 
During the past season some records 
were made by observing temperatures on 
and near the ground, as well as on the 
roof of the Garnett-Corey building, which 
is fifty feet above the street level. While 
temperatures ranged as low as twenty- 
three to twenty-five degrees on the ground 
and four feet above it, the temperature on 
the roof was from thirty-two to thirty- 
five degrees. There is at times, therefore, 
a difference of twelve degrees or more 
between the temperature on the ground 
and at a height of fifty feet above when 
taken on the valley floor. Under usual 
conditions we are quite safe in saying 
that there may be little danger to the 
crops on the higher lands surrounding 
the main floor of the valley. 
The experience of the season of 1911 
indicates that a heavy rain followed by 
a cold wave gives practically the same 
temperature on valley floor and hillsides, 
and also that under certain conditions 
