1. Soil and pH Needs
Lobelias like rich, loose and fertile soil which drains well. However, Lobelias do not perform well in wet, poorly drained soils. Heavy clay soils will retard root growth and starve the plant of oxygen, which can lead to root diseases. Soil should be amended with fully composted organic matter prior to planting and in the case of clay soils the addition of Canadian sphagnum peat moss and well-rotted compost are good sources of organic matter that will aid in aeration. If possible, incorporate two to three inches of organic matter into the top six inches of soil. If adding organic matter, be aware that organic matter that has not been fully composted will tie up nitrogen and sulfur leading to nutrient deficiencies and poor growth. Soil pH should be 5.5 - 6.2 for optimum nutrient uptake; and should be tested prior to planting but after adding any amendments. This can be done quite easily with a portable pH tester available for about $100. Mix one part soil with two parts distilled water, stir well and wait thirty minutes, then follow the instructions for using the pH meter. If pH is too low, the addition of lime is warranted. However, the rate will vary depending on how much you need to raise the pH and the type of soil you are dealing with. Clay soils or those with a lot of organic material have a high buffer capacity and require more lime than sandy soils, which have a low buffering capacity. A good soil testing lab can determine the lime requirement index (LRI) of your particular soil and can recommend types and rates of lime to use. The frequency of watering will depend on the type of soil, weather conditions and the amount of mulch. Mulch will not only reduce soil water evaporation but will also reduce splashing of water onto the lower leaves, moderate soil temperatures and reduce weed competition. Allow the soil to dry slightly between watering; if Lobelia is kept too wet it will lead to problems with root rot.
Lobelias are moderate feeders in the landscape; and can have a landscape life of three – four months. An application of a balanced controlled-release fertilizer (CRF) such as 10-10-10 or 14-14-14 with a 3-4 month formulation applied to the bed at planting will keep the plants well fed, depending on the amount of irrigation required and the average daily temperature. In areas of high irrigation and high temperatures it may be better to use a formulation with a slower release rate since higher temperatures will cause the fertilizer to be released quickly. Fertilizers that are not CRF are not recommended since the fertilizer will be leached out of the root zone long before the crop needs the nutrients. In high pH soils (alkaline soils) additional iron or iron sulfate may be beneficial to reduce chlorosis (yellowing) of the foliage. Yellow foliage on Lobelia may also be caused by a nitrogen deficiency within the plant. Both can be corrected with the proper supplemental fertilizers.
Water correctly to prevent most of the fungal diseases that can cause problems for Lobelia such as: botrytis and pythium. Botrytis will develop as tan spots on the foliage. Established plants that are infected will rot at the crown, and the infected tissue will be covered with dusty gray fungal growth. Pythium rot will develop as shiny, blackened areas on the stems and petioles of established plants. These areas often occur at or just above the soil line and plants will ultimately collapse and die. The best preventative is to use drip irrigation. At the very least, direct water from the hose onto the soil or mulch, and not on the plants themselves.
You may find thrips or spider mites on Lobelia. Thrips cause damage to the plants by piercing or sucking out the cells on the leaf surface. This causes silvery grey spots on the foliage. At high infection levels leaves may wither. Thrips are also a primary vector for virus transmission. Spider mites feed on the underside of the leaves. This feeding causes yellow spots to form which results in decreased plant growth and production. Remember that healthy, vigorous plants are less susceptible to pest damage than unhealthy plants.
5. Tips for Success
- Lobelia does not like "wet-feet"; do not keep the soil wet. Allow the soil surface to dry before watering again.
- The Techno® series is more heat tolerant and will reliably bloom during warm temperatures.
- Remove weeds from the plantings that compete with Begonia for moisture, nutrients and light.
Information provided by Syngenta Flowers