102 
GARDENING IN THE SOUTHWEST 
Lettuce 
Long Day 
Plants 
Short Day 
Plants 
Plants 
Indifferent 
to Length 
of Day 
Latitude 
Control 
A plant that needs a long day for flowering is Lettuce. One com¬ 
monly hears, "The days are getting warmer, so the Lettuce will 
shoot,” but it has been determined that unless the length of day is 
more than twelve hours, the same Lettuce cannot be kept from 
blossoming, even at a much lower temperature. 
In both Lettuce and Salvia, usefulness is increased by a short day, 
because in one case we want the plant to bloom and in the other 
we want it not to. By planting Salvia close to the east side of a 
building, the heavy shading the plants get in the afternoon will 
shorten their day length and hasten their flowering somewhat. 
Chrysanthemums may be brought into flower much earlier by 
the exclusion of the sunlight for several hours of each day. It has 
been suggested that paper cups may be used for this purpose or 
other protective shade. 
Asters will flower on a short day, but on a funny, shortened 
stem, so the whole plant looks like a blossoming Hyacinth. Petu¬ 
nias, Nasturtiums and Sweet Peas will bloom very sparingly on a 
short day, but reserve their greatest efforts for the long ones. 
Certain plants pay no attention to day length. Marigolds, Snap¬ 
dragons, Roses and Calendulas are notable examples in the garden. 
In these plants blossoming begins as soon as the plant grows big 
enough and continues until cut off by cold weather. In the tropics 
many plants have the characteristic of flowering continuously. 
Since the length of day varies according to the latitude, from the 
twelve-hour day at the equator to the twenty-four hour midsum¬ 
mer day at the poles, it is evident that plants with definite require¬ 
ments in day length will vary greatly in their time of flowering, 
depending on where they are being grown. In general, the plants 
of the very cold regions have to be those that respond to the very 
long days by rushing through their life history at a rate unheard 
of in the regions nearer the tropics. Short-day plants have no place 
in these conditions because cold becomes a limiting factor before 
short days arrive. Just the opposite is the case in the subtropics 
where the days follow one another with little change in tempera¬ 
ture or day-length and there the plants wander through their life 
history, growing to great size, it is true, on account of favorable 
