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  4. Alyssum


 1. Soil and pH Needs

Alyssum are tolerant of a wide range of soil conditions, but prefer a rich soil with good drainage. 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 5.8- 6.2 for optimum growth; 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.


2. Fertility

Alyssum are moderate feeders during their life cycle. 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 high pH soils (alkaline soils) additional iron or iron sulfate may be beneficial to reduce chlorosis (yellowing) of the foliage.  Yellow foliage on alyssum may also be caused by a nitrogen deficiency within the plant. Both can be corrected with the proper supplemental fertilizers.


 3. Disease

Alyssum are relatively disease free, but may be susceptible to downy mildew. Downy mildew is usually found as a gray to white fungus lightly covering the underside of the foliage. Older plants will turn yellow, be stunted and die from the top down. Irrigating correctly can prevent most of the fungal diseases. Provide water early in the day to allow for plants to thoroughly dry, and utilize drip irrigation when possible.

Downy mildew symptoms on the leaf underside


 4. Insects

Aphids and whitefly may harm Alyssum. Aphids are tiny soft bodied insects that cluster on the growing tips of leaves and flower stems, sucking plant juices. The plants become stunted and deformed. Both adult whiteflies and larvae extract food from the plant. This affects the plant’s processes and results in reduced growth. Whiteflies may also transmit viruses. Healthy, vigorous plants are less susceptible to pest damage than unhealthy plants.  

Aphid infestation


Adult whitefly and larvae


5. Tips for Success

  • Alyssum grows best in full sun in cool weather.
  • Alyssum do not like prolonged dry periods.
  • Shearing spent flowers will prolong bloom time and help produce a fuller plant and more flowers. Shearing is easier then deadheading with this plant
  • Remove weeds from the plantings that compete with the alyssum for moisture, nutrients and light.