Management of soil clay - Ask Extension
Spring is approaching and I started preparing a small bed for seasonal flowers – 2 feet by 5 feet. While digging out dead plants and weeds, I disco...
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Management of soil clay #925170
Asked February 27, 2026, 4:33 PM EST
Spring is approaching and I started preparing a small bed for seasonal flowers – 2 feet by 5 feet. While digging out dead plants and weeds, I discovered compact clay the size of footballs, (numerous footballs) 6 to 10 inches below the surface.
• How should this excessive amount of clay be managed?
o Dig out and remove from the flower bed?
o Broken into smaller pieces and mixed with the soil?
o Try to find flowers that thrive in clay soil, but not sure what that would be?
o Add worms to the soil? If so, what kind of worm would you recommend and what do I need to add to feed the worms?
I also understand it is important to mulch when planting. In the past I have used pine bark mulch. However, over time this becomes very hard and prevents water from moving into the soil. Should the much be removed when this occurs or should the much be worked into the soil?
Your insight and guidance would be greatly appreciated.
Joni
Baltimore County Maryland
Expert Response
Clay soil has several positive attributes, such as good nutrient retention and moisture retention, though when it gets compacted, that's when the moisture retention can be problematic (and dry compacted clay can also be hard to re-moisten). Avoid removing soil when it needs improving, especially since layering one soil type atop (or next to) another can create percolation problems that may result in the root zone staying wetter or drier than the surrounding soil.
Mixing an amendment into the clay as-is can help, such as using compost when planting. You can either mix some compost into each planting hole (blend it with the existing soil), or add it before planting as a top-dressing that is either manually worked in to the top few soil inches or tilled in, though the latter approach can temporarily damage soil structure and make the soil more vulnerable to added compaction. For an area that small, tilling with a machine would not be practical anyway. A garden fork or broadfork can be a useful tool for breaking apart some of the surface soil clods and letting the compost sift down into the cracks; soil life will move it more thoroughly into the rest of the soil from there. Earthworms may already be present, but few (if any) are native here in Maryland, so we don't recommend adding any more.
A hardpan layer of soil 6 to 10 inches below annual plants won't necessarily impact them as much as perennials or shrubs grown in that area instead; their root systems don't often need great depth for the plants to fare well. (Many window box containers and other pots annuals are grown in are not that deep, and they can grow normally in them.)
We don't have a list of clay-tolerant annuals, but can provide a few ideas of annuals that are generally resilient as long as they aren't over-watered. (To be fair, many plants don't do well if kept too consistently wet.) You don't mention the other site conditions (how much summer sun does the area get? do deer browse in the area? does that garden bed skew dry or wet?), but if we assume a sunny or mostly-sunny area with adequate drainage, here are a few potential candidates:
Biodegradable mulch not only adds organic matter to the soil as it decomposes (just top-dress as needed to renew any needed depth every 1-3 years), but it also helps to suppress weed seed germination, moderate the soil temperature, and reduce evaporation of moisture from the soil surface.
Miri
Mixing an amendment into the clay as-is can help, such as using compost when planting. You can either mix some compost into each planting hole (blend it with the existing soil), or add it before planting as a top-dressing that is either manually worked in to the top few soil inches or tilled in, though the latter approach can temporarily damage soil structure and make the soil more vulnerable to added compaction. For an area that small, tilling with a machine would not be practical anyway. A garden fork or broadfork can be a useful tool for breaking apart some of the surface soil clods and letting the compost sift down into the cracks; soil life will move it more thoroughly into the rest of the soil from there. Earthworms may already be present, but few (if any) are native here in Maryland, so we don't recommend adding any more.
A hardpan layer of soil 6 to 10 inches below annual plants won't necessarily impact them as much as perennials or shrubs grown in that area instead; their root systems don't often need great depth for the plants to fare well. (Many window box containers and other pots annuals are grown in are not that deep, and they can grow normally in them.)
We don't have a list of clay-tolerant annuals, but can provide a few ideas of annuals that are generally resilient as long as they aren't over-watered. (To be fair, many plants don't do well if kept too consistently wet.) You don't mention the other site conditions (how much summer sun does the area get? do deer browse in the area? does that garden bed skew dry or wet?), but if we assume a sunny or mostly-sunny area with adequate drainage, here are a few potential candidates:
- Lantana (Lantana camara)
- Ageratum (Ageratum houstonianum)
- Globe Flower / Globe Amaranth (Gomphrena globosa and other species)
- Marigold (Tagetes species)
- Canna Lily (Canna species/hybrids) - can be overwintered if dug out and stored as a tuber around the time of the first autumn frost; occasionally they survive in the ground as perennials in Maryland; many varieties are quite tall, but some are compact
- Petunia (hundreds of cultivars) - just make sure they aren't overwatered
- Mealycup sage (Salvia farinacea)
- Nasturtium (Nasturtium)
- Annual Lobelia (Lobelia erinus) - best in cool spring and autumn weather...can peter-out and decline when it gets hot
Biodegradable mulch not only adds organic matter to the soil as it decomposes (just top-dress as needed to renew any needed depth every 1-3 years), but it also helps to suppress weed seed germination, moderate the soil temperature, and reduce evaporation of moisture from the soil surface.
Miri
