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I am planning a xeric garden and the plants I want to use all require “infertile, fast-draining soil.” I live in northern New Jersey and have some underlying clay areas but after many years of composting, mulching and amending,I suspect that my soil
is too fertile for these plants. Do I need to make it LESS fertile? Is that even possible?

Your website says that, even for xeric plants, you should add lots of compost,organic matter and mulch. Isn’t that making it even more fertile? I guess I’m confused.

Drought Smart Plants reply:

Keep in mind, my recommendations are based on the fact that I have a very sandy gravel soil already, so the addition of organic matter will only contribute to the water holding ability. I don’t have a problem with drainage, but I do have such poor and infertile (and acid) soil that I need to add more organic matter all the time.

Your conditions may vary.

Depending on the type of plants you’re going to use, some require more or less water; the organic matter in your soil (depending on the source) will provide more moisture retention.

The plants that absolutely require very fast drainage, such as Thymus and some of the more finicky hardy succulents like Rosularia and Orostachys, as well as many alpine plants will do better with the addition of grit or even sharp gravel. The danger of adding too much smaller sand to an already clay soil is that you can actually bind up the pore space even more, resulting in the opposite of what you want.

If you have added lots of manure based compost over the years, the salts can build up in a poorly drained soil, creating a toxic condition. Many plants, even the toughest of xeric plants, can’t live in a salty environment.

You don’t want too much Nitrogen (from manure type compost) in your soil because it will cause a lot of green growth, making the plant very susceptible to drought and wilting in dry and hot conditions, the exact opposite of what you want.

If you’ve used leaves, straw or other organic matter (not manure), this won’t be a problem. The lean or infertile part of the requirement tends to refer to the lack of nutrients, such as what is found in manure, so using leaf mold, well rotted compost and then mulching with natural materials won’t be a source of Nitrogen.

I usually recommend adding some type of aggregate to most soils for xeric gardens, depending on what is locally available.

This can be lava rock or pumice (extremely expensive unless you live near a volcano), sharp gravel (not rounded pebbles) or even larger sand, such as the size of turkey grit or chipped granite or marble.

The main problem with these products is their sharpness, making it a little hard to work with without gloves. This will add drainage, and comply with the infertile requirement.

Based on your comments about your soil, I’m assuming your climate is cool in the winter too? If you have cold snowy winters, cool springs taking a long time to warm up, and hot baking summers, your plants will need the addition of sand or gravel. This will help the soil warm up faster in the spring, drain off faster, and contribute to healthier root growth first thing after the plants come out of dormancy.

One suggestion that often helps when gardening on clay soils is to make raised beds to eliminate the issue of water pooling or puddling.

If you have access to rock, make retaining walls to get the plants up off the base of clay. I don’t recommend using treated lumber for these type of beds, I prefer native stone, or even some of the concrete blocks that interlock.

You can add some grit or gravel to your good compost based soil at a rate of about 1/3 gravel/sand to 2/3 of the soil, and plant your xeric plants in this. It won’t have to be really deep; probably around 30cm (one foot) deep is adequate.

Please don’t hesitate to ask me some more questions if I haven’t answered you completely.