“Gardening in a Bag” Offers an Alternative
for Growing Bedding Plants in the
Landscape
Clydette M. Alsup-Egbers
Associate Professor,
Horticulture
Missouri State University
"There is no question that when
the soil is poor, Gardening in a Bag
yields superior results and the
plants are given time to adapt to
native soil."
Highlights of Our First Gardening in a Bag Studies
More Gardening in a Bag Pictures
Gardening is America’s number one leisure activity but shallow or rocky soils make it difficult for some homeowners to establish ornamental gardens. How many of us have declared that the soil in our region is so much more challenging than everyone else's?
Homeowners in the Missouri Ozarks often use shovels, picks or pry bars to create
garden beds, but such activity is not always possible, especially among people
who are elderly or handicapped or for people whose busy lifestyles don’t allow
time for intensive soil-breaking work.
Alternatives
Alternatives to preparing a plot of soil for a garden include containers or raised beds. Container gardening works well for many people but has several drawbacks. The potting substrate in containers often dries out quickly, requiring frequent watering; and containers may restrict plant roots which can affect plant growth and performance. Root zone temperatures can become abnormally high in containers, resulting in damage to root systems. Containers have a relatively small diameter, limiting the number and size of plants that can be grown in them. While containers come in a wide variety of shapes, sizes and materials, they can be expensive.
The
use of raised beds or berms eliminates many of the problems associated with
gardening in containers, but can still be laborious and expensive to build.
Bag
culture offers a third alternative to traditional gardening. Bag culture has
been used for greenhouse production of cut flowers and for strawberries,
tomatoes and other vegetable crops. Vertical bags with pre-made holes along
their length are marketed as alternatives to hanging baskets for ornamentals,
and are available for growing tomatoes and other crops in greenhouses. Grow bags
are successfully used to produce transplantable woody nursery crops.
Our Initial Research on Gardening in a Bag
Dr.
Pamela Trewatha and I searched the literature extensively and found one report
on growing crops outdoors in bags, but no reports were found on research
focusing on the use of bags in landscape gardening. We therefore conducted a
study of whether “Gardening in a Bag” is a feasible method for growing annual
bedding plants in outdoor garden situations. We found that Gardening in a Bag is
a promising method for growing bedding plants, especially for gardeners wanting
to contain growth of ornamentals to a small space.
Our
study began in 2002 when we compared the growth and appearance of 25 cultivars
of bedding plants using “Gardening in a Bag” vs. “in the ground” planting
methods. We grew one or more cultivars of the following plant species in the
ground and in bags of generic topsoil purchased from a garden center: Purple
Knight Alternanthera, dianthus, gazania,
marigold, ornamental pepper,
peek-a-boo plant (Spilanthes
oleracea), petunia, salvia,
verbena and vinca (Catharanthus
roseus). Using the same
methods in 2003, we narrowed the plant
selection to Wave petunias, dianthus, vinca and rose moss.
Plants were placed either in ground beds or into bags of topsoil placed on top
of the ground in our research plots. Long X-shaped cuts were made on the bottoms
of the bags to drain excess water. Tops of bags were slit and three plants were
planted to the level of their root ball in each bag. All plants were mulched
with enough sawdust (2003) or cypress mulch (2003) to completely cover the bags
of soil so they would not be visible. The plants were fertilized after planting
and again in the summer with a soluble, general purpose fertilizer. We watered
the plants as needed to prevent drought stress.
We
measured height and spread of each plant, counted numbers of flowers per plant,
and rated the visual appearance of the plants about once a month during the
growing season. Factors considered when ranking the plants included compact
habit, branching habit, foliage color and number of flowers.
What We Discovered
Growth responses to the two planting methods varied by species and sometimes by
cultivar within species.
(click here to see highlights of the results)
Some
of the plants were taller when grown in the ground than when grown in bags while
others were shorter. Plant spread was greater for some cultivars grown in bags,
but some cultivars were wider when grown in the ground. Most of the cultivars of
petunias, which tend to sprawl, grew wider when planted in the ground than when
planted in bags, but their height and appearance were not affected by planting
method.
Visual ratings were similar for 14 of the cultivars regardless of planting
method.
Some
plants were considered more attractive in bags than in the ground but planting
method did not affect their height or spread. The opposite was also true for a
few plants—they were considered more attractive in the ground than in bags but
height and spread were not affected by planting method.
In
some cases, ground vs. bag culture affected growth, flower number or visual
quality early or late in the growing season but not at other times during the
season.
In the cases where the bedding plant species grew better in the ground than their counterparts grown in bags, we believe the plants in the ground may have had greater access to water and nutrient resources than the plants whose root systems were confined to the bags. Soil temperature also may have affected growth of the plants. Temperatures were slightly lower in the soil than in the bags throughout the growing season, a factor that would favor plants preferring cooler growing conditions. Species that prefer cooler growing environments, such as dianthus, tended to do better planted in soil than bags.
Some
of the plant species that are native to subtropical or tropical regions may not
have been affected by the warmer bag temperatures, since their growth and
appearance were not affected by ground vs. bag conditions.
Disease Control
In
2002, our research plots suffered some flooding damage in July. Verbena growing
in bags survived while verbena in the soil died of a root rot disease.
Waterlogged soil impairs the performance of roots and allows root-rotting fungi
to attack plant root systems more easily. Bags are essentially raised beds,
which drain quicker than soil at ground level. Raised bed production helps
control root diseases in raspberries and azaleas, and improves relations between
plant roots and beneficial soil fungi (mycorrhizae).
Could We See a Difference?
Even
though height or spread of plants grown in the ground was often different from
height or spread of plants in bags, the difference was not always visually
apparent. In many instances, visual appearance ratings were high on plants in
bags even though the plants were smaller than the same species grown in the
ground. Compact plants may be preferred in cases when gardening space is
limited, to avoid overcrowding of plants, or to provide the appearance of dense
flowering.
Since
visual quality was generally unaffected by growth in the bag vs. the soil, the
labor-saving benefits of growing bedding plants in a bag may make it a useful
means for bedding plant displays in landscapes where soil quality is poor or
people lack time to prepare the soil.
Topsoil or Potting Mix?
In 2005, we began a study to learn if some plant species perform better in a soilless mix, or a blend, or with compost added to soil or media. We planted flowers in 100% potting mix, 100% topsoil, or various formulations that contained composted poultry litter. We discovered that plants in bags with the compost didn’t survive well, but that was because the litter wasn’t as composted as it needed to be. Basically, plants in those bags ‘burned up.’
We left the bags in the research plots that winter so that the poultry litter could finish composting. The next spring we planted right back into the bags. This time around, plants in bags containing the compost tended to perform better than plants in bags with just topsoil or just potting mix. In fact, plants in the 100% potting media looked the worst by season’s end.
Pulling Out the Plastic Afterward?
I’ve had numerous questions from people who hear about my Gardening in a Bag on whether it’s hard to get the plastic bags of soil or media out of the flower beds. The answer is yes, although I’m not convinced one would ever have to get them out of the garden. But the questions are valid, and in 2008, we looked at different media/soil combinations in plastic bags vs. the same combinations in paper bags. Plants growing in any of the media/soil formulations did better than plants in the ground, but plastic vs. paper didn’t usually matter. What does matter is that the paper will break down in the soil and then no worries about whether to remove it.
I was lucky enough a few years ago to meet the renowned plantsman Tony Avent (Plant Delights Nursery) in North Carolina. Tony is a walking encyclopedia of herbaceous plants. He is an avid proponent of improving the soil before planting, and focusing always, always, always on the health of the soil. I briefly alluded to my concept of Gardening in a Bag, and he was not enthusiastic. He repeated his beliefs on how people should be educated in taking the time to make sure their soil is healthy, not in using some instant gratification method.
I'll agree to disagree with Tony, because I think Gardening in a Bag CAN improve the health of the soil. Over time beneficial soil microbes are going to transform the bag’s soil/media/other rooting substrate components into organic matter-slash-healthier soil. Plus, if one plants into a paper bag filled with media/soil, then the paper will biodegrade.
Conclusions
I have not looked at the total data throughout
the years of these studies to come to any statistically validated conclusions,
but my gut instinct and visual impressions say that in most cases where you grow
an annual flowering plant, plants grow better in the soilless mixes than in
topsoil. But if you grow a perennial or something that needs high levels of
nutrients or a heavier rooting substrate, then mixes containing soil and/or
compost are a better bet. If you are going to use the same bags in subsequent
years, then the soilless mixes probably aren’t the best answer.
Highlights from Our First “Gardening in a Bag”
Experiment
Planting Method Had No Effect on Plant Growth and Appearance
Purple Knight Alternanthera
Chilly Chili Ornamental Pepper
Durango Red Marigold
Petunia—Tidal Wave Silver, Wave Lavender
Peek-a-Boo Plant
Vinca—Big Ruby, Mediterranean Punch
Planting Method Affected Height or Spread, but not Visual Ratings
Dianthus—Amazon Neon Cherry, Corona Cherry Magic
Petunias—Blue Wave, Clear Waterfall Mix, Easy Wave Pink, Wave Lavender, Madness
Magenta, Stars and Stripes
More
Attractive Planted in Bags than in the Ground
Orange Cream Gazania
Durango Tangerine Marigold
Margarita Scarlet Rose Moss
More
Attractive in the Ground than in Bags
Marigold—Bonanza Harmony, Durango Bolero, Durango Yellow
Petunia—Purple Wave, Old Glory
Blue
Ribbon Salvia
Quartz Waterfall Mix Verbena
Icy
Pacifica Pink Vinca