Ice plants are drying up and dieing
Thank you for your help. The picture shows ice plants that we planted in the spring of 2019. Up until this summer, these plants have done very well and were gradually spreading with lots of flowers. They face south and get lots of sun starting in the late morning or early afternoon depending on the season. They are planted on an incline that drains well. Earlier this summer we planted some additional ice plants in our front yard in an area that gets sun almost all day. The new plants are planted in a more typical flower bed that is mulched. The garden bed is also on an incline, but not as steep and is more flat at the top. The plants at the top of the new bed died rather quickly. The plants at the bottom of the bed have survived and seem to be doing ok. However, none of our ice plants are doing great this year. Even in the original location we lost one of the plants. The leaves first turn from green to greyish. They then dry up and turn brown. Finally the whole stem dries up and turn brown. On some of the plants this has eventually happened to the whole plant and it dies. For some of the more established plants, the drying out part has only happened on a few stems, but all of the plants are at least greyish at this point. I did sprinkle some plant-tone on the plants at one point during the early summer and I think I sprayed them once with some miracle grow bloom booster. I didn't use the plant-tone last year, but I think I sprayed the bloom booster on them once or twice during the summer last year. I tried putting a few of the dying plants in their own containers, but this doesn't seem to have worked. I included a picture of these people plants. The container on the right includes stems from one of the dying plants and they are still alive. Anyway, I decided to include this picture because it shows other stages of death and though it might be helpful.
Montgomery County Maryland
Ice Plant (Delosperma) can be a challenging perennial to grow in our area. As succulents, they prosper in sunny and dry conditions, needing minimal intervention throughout the year beyond initially siting them in a good location that has excellent drainage. Their natural habitat consists of rocky outcroppings where even grasses do not grow well to beach dunes and they are fairly intolerant of soil wetness in winter.
We cannot discern the exact cause of their decline here - and more than one issue is possible at the same time - but some of the graying-but-not-desiccated foliage seen in the close-up photo may be damaged by mites or thrips. Both are very small and difficult to get good photos of, but signs of their feeding are similar - fine stippling and loss of leaf chlorophyll that gives the afflicted tissues a silvery or bleached look. These pests often take advantage of stressed plants or those in hot, dry conditions. While those conditions are otherwise fine for growth and part of the normal habitat for these plants, here the added reflected heat from the stone mulch and radiated warmth from the stones overnight (when lower temperatures help heat-stressed plants "rest") may be encouraging outbreak populations. The best approach would be to lessen plant stress, but secondarily a treatment with horticultural oil or insecticidal soap may help. Check the label of any product you opt to try to make sure Ice Plant is not listed as sensitive to treatment.
As an example, some Ice Plant species or their close relatives can be invasive weeds on beach environments in other countries - the nutrient-poor soil with its very limited water-holding capacity suits them just fine. For species preferring lean soils, the use of too many nutrients can either damage roots outright or encourage lanky growth that might be more susceptible to pests and disease. For the future, you should not need to use either fertilizer for these plants.
A likely issue is insufficient drainage or another condition keeping the roots too wet. Suburban soils are often compacted, a conditions which impedes drainage, particularly if soils are clay-based as they often are in central MD. Sloped soils are not always as well-drained as they appear on the surface. At various times this past year we've experienced quite wet conditions, and while frequency of rainfall itself may not matter as much, the speed at which that water volume moves out of the surface soil does. When soils are too wet for species not adapted to wet environments, roots suffer from lack of oxygen and begin to die back or become infected with opportunistic pathogens in their weakened state. There is no treatment for this condition except to give the plants time (hopefully in drier conditions) to see they have enough vigor left to re-grow lost roots.
A full-sun or southerly exposure for Ice Plants is good, but the soil may need further improvement for drainage (raised beds can help with this) for them to succeed long-term. Plants up against the fence will also depend on the soil on the other side of the fence not being kept too wet, either through natural drainage patterns on the neighbor's land or through watering they need to do to keep their particular plantings healthy. Drainage can be easier to manipulate in container plantings than it would be in the ground, but container-grown plants run the risk of root stress via temperature fluctuations, as soil is much less insulated in a pot and is susceptible to extra hot or cold temperatures which can similarly damage or kill roots.
Gravel and stone can be a useful "mulch" to minimize the risk of stem rot for plants sensitive to touching wet soil, in addition to the unique aesthetic it provides. However, as stone mulches are usually placed on top of landscape fabric to keep them from sinking into the soil below, the fabric itself can risk impacting plant health. This may require further study in horticulture, but it is thought that landscape fabric, despite having tiny drainage holes, impedes moisture passing though the soil (and may prevent some evaporation) and impedes oxygen diffusion into the soil as well - both of which can harm root health. If landscape fabric is present underneath this stone, it's possible its use is contributing to problems with plant dieback. For the plants in more traditional bark mulch, perhaps the issue lay more with the underlying soil (or fabric again, if there was some) than it did with the bark.
No chemical treatments are practical or even effective on plants struggling with environmental stresses, even if they contract a disease like root or stem rot as a result. Instead, switching them out with other species better-suited to the conditions would be best. Sedum, while also a succulent, tends to be a bit more flexible in its tolerances. Otherwise, non-succulent species like Hardy Plumbago (Ceratostigma plumbaginoides), Moss Phlox (Phlox subulata), Pinks (Dianthus), Green-and-Gold (Chrysogonum virginianum), Heath Aster (Aster ericoides 'Snowflurry'), and Creeping Speedwell (Veronica - several species) would make good flowering groundcovers for sun.
This is so thoughtful. I have everything I need in what you shared to solve the problem or at least find a good alternative. Thank you very much.