Combining Plants Based On Tolerances

Combining Plants Based On Tolerances

Plants may look good together in combination baskets, however the success of a mixed container depends on more than just aesthetics. Picking plants that grow well together is essential to avoid consumer dissatisfaction. You will also save employee time at retail answering why a given plant in the combo died and how the retailer should deal with the problem.

Consider these basic characteristics of spring annuals used in combinations: their ability to take up iron (iron efficiency), tolerance to light and heat, high or low salts and wilt. Selecting plants that have similar requirements in these areas will make you a better grower and more appreciated by your customers.

Iron Efficiency

The uptake of iron into the plant is a two-step process. The first step is the solubility of the iron in the root medium, which is affected by media-pH. In general, as the pH of the soil solution increases, the solubility of iron decreases.

The second step in the uptake of iron depends on the plant and how efficiently it takes up the soluble iron from the soil solution. At one extreme are “iron-inefficient” or Petunia Group plants. These are plants that are prone to iron deficiency. Iron-inefficient plants have limited ability to scavenge for iron when media-pH is high or fertilizer application rate is low. In order to maximize iron solubility and uptake, it is recommended to grow iron-inefficient plants at a pH range of 5.4 to 6.2.

Often, iron-inefficient plants are misdiagnosed as having a high-iron or high-fertilizer requirement. In most cases, they do not necessarily require excessive amounts of fertilizer or iron if the media-pH is maintained within the acceptable range. Iron-inefficient crops include bacopa, calibrachoa, diascia, dianthus, nemesia, pansies, petunias, snapdragons and vinca.

At the other extreme are iron-efficient or Geranium Group plants. These are plants that are prone to iron/manganese toxicity. The problem with iron-efficient plants is that they are very efficient at taking up the iron from the soil solution, especially when iron solubility is maximized by low media-pH or by high application rates of fertilizer. In order to minimize iron solubility and uptake, grow iron-efficient plants at a pH range of 6.0 to 6.6. Iron-efficient crops include seed and zonal geraniums, marigolds, New Guinea impatiens, pentas and lisianthus.

The third group of plants is the General Group, which contains plants that, under normal conditions, are not especially prone to either iron deficiency or iron toxicity. The recommended range for growing plants of the General Group is 5.8 to 6.2. Examples in the General Group include chrysanthemum, impatiens, ivy geraniums, osteospermum and poinsettia.

Figure 1. Examples of problems associated with growing
iron-inefficient and iron-efficient plants in the same pot.
Notice the chlorosis from iron deficiency in the scaevola.
In contrast, the zonal geranium looks normal. In this
case, the media-pH was 6.5, too high for the
iron-inefficient scaevola but fine for the iron-efficient
geranium. Also note that anything done to correct the
iron deficiency on the scaevola (supplemental iron
drenches or lower the media- pH) will put the geranium at
greater risk of micronutrient toxicity.

Growing combination baskets or pots that contain both iron-inefficient and iron-efficient plants can prove difficult. Notice there is an overlap in ideal pH ranges between the three groups, but that overlap is small (between 6.0 and 6.2) and can be difficult to maintain during production for the grower or through the summer for the customer. From a pH management standpoint, it is best to make combinations from plants in the Iron-Inefficient and General Group, or the Iron-Efficient Group and General Group, but not the extremes. 

Light/Heat Tolerance

Although common species used for hanging baskets can survive in most spring-summer conditions with proper care and adequate water, not all plants perform equally well. We recommend checking local trial garden results when selecting cultivars, for example, for Michigan State University and for University of Florida.

In general, southern exposures receive the highest light and temperature. Eastern and western exposures get about the same number of hours of sunlight but a western exposure has higher temperatures because it receives the sunlight in the afternoon.

Ivy and zonal geraniums, petunias and calibrachoa are considered high light/heat tolerant and therefore would grow well in the southern or western exposures of a house. Impatiens, New Guinea impatiens, Non-Stop begonias and fuchsia will tolerate an east or west exposure but may flower and perform better with the eastern exposure because of the lower temperatures. The northern exposure has the lowest temperatures but also receives the least amount of light. Impatiens and Non-Stop begonias will best tolerate conditions of very low light. 

Salt/Fertilizer Tolerance

The amount of fertilizer required to produce a pot or basket of any of the species listed in Table 1 is about the same. The difference is that some of the species can tolerate higher root-media nutrient levels than others. For example, geraniums, ivy geraniums and petunias are species that would be considered salt tolerant because they can tolerate high concentrations of salt in the media without root damage. In comparison, New Guinea impatiens and Non-Stop begonias would be considered salt sensitive because they will sustain root damage if the salt levels in the root media get too high.

Media-pH also plays a role in salt/fertilizer tolerance. When NPK fertilizer salts are high in the root media, high micronutrient concentrations are also likely if complete fertilizers were applied. For an iron-efficient crop such as zonal geranium, high root media nutrient levels (3 to 3.5 mS/cm with a saturated media extract test) will probably not cause a problem if the root media-pH is in the acceptable range (6 to 6.6). However, if the pH drops below 6.0, the likelihood of iron/manganese damage from high salts increases dramatically.

Figure 3. The foliage of impatiens is remarkably
tolerant to wilt and may recover even under a severe wilt.
However, under the severest wilt, the flowers of the
impatiens will drop off, leaving the plant green for about
one to two weeks until new buds form.

Wilt Tolerance

Basket plants differ in their ability to tolerate a lack of water. Impatiens and fuchsia can lose flowers and buds from wilting. Impatiens foliage is remarkably tolerant of wilting and will usually recover after watering. Fuchsia foliage is very sensitive to wilting and can turn yellow and drop. Ivy and zonal geraniums can tolerate dry conditions for several days without damage to the foliage or a reduction in flowering. Non-Stop begonias do not tolerate wilting but like the geraniums, a wilted appearance does not rapidly develop. Petunias will wilt rather dramatically and leaves may turn yellow, but flowering will continue.


Consumers purchase mixed containers as a mini, instant and lasting landscape. If you would not normally combine these plants together in a landscape setting or would not manage them the same in the greenhouse, the combination is unlikely to work in a container. Next month we will conclude this series looking at key information for the consumer.

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