Drainage Is More Important Than Moisture

Wildflower Garden, 10 July 2019

As I continue to plant my wildflower garden, I am continuously reminded that the success of my plants lies more in the drainage of my soil than the amount of moisture it has. To explain... the great diversity of plants I am adding run the spectrum of Florida plants that find their homes in scrub to moist flatwoods. These various habitats in nature vary in the types of soil they typically have. Scrub and sandhill occur in the most well-drained sands found in Florida while flatwoods can often have standing water for short periods of time in the summer rainy season. What all of them share, however, is moisture at the root depth. It's not as much about some of these plants needing moisture because they all do. What they need as much as anything is sufficient drainage so that the moisture gets to their roots and that lies in drainage.
Typical home landscapes build up organic matter beneath the lawn grass. Removing the turf beneath this lawn does not do much to reduce these soil organics. Many gardeners equate organic matter with fertility, but that is a mistake. Organics only supply plants with nutrients after they decompose and the amount of fertility they impart is purely dependent on how much nitrogen, potassium, phosphorus, etc. that they have inside before they decompose.Once plant or animal matter is deposited on the soil, it doesn't accumulate more fertility. Some might be quite fertile, but it may not be so. Woody organics, for example, are relatively low in fertility.
Organic matter does have two significant properties that always accompanies it when added to the soil. It holds moisture when wet and repels it when dried out. Here in Florida, surface organic matter gets dry during the months of low rainfall and during rain events water runs off the surface instead of soaking into the lower soil layers where the plant roots await it.  Many times over the years, I have watered my plants during a drought and seen the water stand on the surface. Eventually, some of it soaks in, but if I take a trowel to those areas, I find that it has only wet the upper few inches. The shallow surface roots have access to this water, but the deeper ones are in dry-powder soil. This is anathema to plants of all habitat types.
Once the rains come, this soil then gets water-logged as the organic matter stays wet. It captures the rain and holds it from going deeper into the aquifer and below the root zone. It's at this time of year that many of my established plants die. Their roots simply can't breathe. Roots need oxygen  as much as we do and when they don't get enough, they die and rot. Water is vital to a plant's success, but so is oxygen. It's a fine balance that must be maintained for a plant to function properly.
I am fortunate that my landscape seems to sit on top of relatively natural soil. When I first dug holes to see what I had, I found a layer of sand beneath the topsoil. Even though I am not sitting on top of former scrub or sandhill sands, the soil drains and the upper surface does not contain too many organics to prevent it from allowing the moisture to percolate into this deeper layer. Scrub and sandhill plants do not grow in dry sand, they do so in deep sand that has moisture at the root zone and sufficient oxygen to keep the roots healthy.
The summer rainy season is in full force here now and this would be the time when some of the plants I've added might succumb to "over watering." So far, they are doing well and it gives me hope that they will continue to do so over the next few months where an inch of rain on a daily basis is normal. The butterfly milkweed (Asclepias tuberosa), is a good example. In Florida, this plant is notoriously difficult in home landscapes. Our variety occurs in a variety of habitats, but all of them are characterized by extreme drainage. I have killed quite a few in my former landscapes, but it's because they didn't get enough moisture at their deep taproot, not because they were grown too wet. It was the organics that ultimately killed them. Sufficient moisture is necessary, but so is drainage.  Water from a cloudburst or from a hose has to reach the root zone, but there also has to be enough oxygen to allow those moist roots to breathe.
So far, so good. The next few months or a tropical depression may prove me wrong in believing I've made the right choices here, but for now the upland plants I've added are thriving alongside the ones that are more common to the somewhat drier flatwoods areas. Do not add organic material to your soil. Choose species native to the soil you have and work to improve its drainage - not the other way around.