SRW Tip#1: Preparing Good Rose Soil: Clay soil preparation

» Mark_Whitelaw - Clay soil preparation

Hi David! Welcome to Rose Garden!

With a name as famous to rosedom as yours, it's no wonder you're growing roses! Glad you stopped in for a visit.

Your questions are good ones, and subjects we haven't touched on for a while. Let's take each of them and explore the possibilities.

Is it OK if I simply relaced 2 - 3 cu. ft. of the clay with top soil... rather than amend the clay.

It would be okay if your clay soil was reasonably loose. But I don't recommend the procedure for a number of reasons.

First, 2 - 3 cu. ft. of soil isn't very much soil - essentially, only about a small wheel barrow full. If your soil is clay-based (as are many of the soils here in my part of the country), you will need to replace the soil for 2 - 3 ft. down and for the width and length of your rose beds. Since you are planting four good-sized roses, this could mean a cubic yard of soil for each rose... or roughly 108 cubic feet of soil! That's a lot of bags of soil!

Another problem with simply removing the existing soil and importing a soil blend is that you may be creating a "bowl effect" from which water cannot drain. Furthermore, because the top soil may quickly appear dry, you may be tempted to irrigate more and exacerbate the problem. Planting roses into well-draining soil which is placed inside a poor-draining hole (the bottom and sides of which are composed of compacted clay soil) will not solve your drainage problem. It would be as if you were planting a house plant in a container without a drainage hole in the bottom... like a water bucket. Water will drain through the good "top soil" very well, but will collect in the bottom of the hole, possibly taking hours or even days to drain away.

Still another problem can arise. Since the top soil drains well, the soil and roots near the surface will dry while the roots and soil in the bottom of the bed will remain wet and displace the air in the soil, eventually fouling the soil and killing the plant.

If your clay-based soil is extremely heavy, I would recommend you either dig out the bed and install a drainage system, or elevate the bed.

Of these two options, I recommend the first - the drainage system. If you elect the in-ground drainage system method, you will need to dig out the soil for about 32 in. and install perforated drainage pipe or several inches of coarse (1 in.) river rock in the bottom of the bed. You can then fill in with either an amended mix of your native soil or an imported soil blend.

The native soil, amended to include those elements missing from your soil test, is preferred. Amended native soil will allow your roses to adapt to their microenvironment more quickly than using an imported soil blend. Otherwise, when the roots grow beyond the imported soil and extend into the native soil, their growing conditions will change. Many of my clients who opted for the imported soil method have had roses do very well for the first few years, only to have the roses undergo stressful conditions as the roots penetrated into the native soil.

Going back to your bed options, if you opt to raise your rose bed, you will want to loosen the soil underneath the elevated bed's location for the first foot or so below the ground level. Then, after creating your elevated bed, you can either use amended native soil or bring in imported bedding soil. Again, I recommend the amended native soil option.

So, you ask, how do you determine if your clay soil will suitably drain for growing roses?

The answer is to perform a "percolation test" of your intended bed's location.

The American Rose Society suggests a method of performing this test. (They "borrowed" the method from the Ortho Company - with their permission, of course.)

Use a 2 - 3 lb. coffee can (or a can of similar size), the top and bottom of which have been removed. Push the edge of the can into the soil to a depth of about 1 in., and fill the can with water. Measure the length of time it takes for the can to completely drain of water. If it takes more than an hour for the water to drain, you must take appropriate steps to improve the drainage of the soil.

Once you have amended the soil, you must also take a measurement to see if the soil will retain moisture sufficient to do your roses any good. (For some folks who live in sandy soil, this can be a real problem! Water drains so fast, the roots don't have a chance to take it up!) Again, thanks to the Ortho Company and the ARS, there is another test you can perform called a "water retention test."

Fill a 1 quart (about 8 in.) plant container (the bottom of which should have drainage holes) with your amended soil (or soil blend). Saturate the soil mix with one quart of water and elevate it above a clear glass container or jar (like a Mason jar used in canning fruits and vegetables). A funnel placed in the mouth of the container or jar will make this task more easily accomplished.

Measure the amount of water that drains from the saturated pot after 15 minutes, 30 minutes, and again after one hour. Knowing that you poured in one quart of water (which equates to "100% drainage"), and knowing how much water drains in a quarter hour, half hour and whole hour gives you a "water retention factor." What you're looking for is a range of 50% - 60% water retention after one hour.

If your reach the 50% factor in a quarter hour or half hour, you must amend your soil mix to retain more water. (In your case, add more native clay soil.)

If you have less than 50% drainage after one hour, you must amend your soil blend to improve the drainage... add more soil loosening agents like aged pecan shells, gravel, etc. But, under no circumstances, should you add "brick" sand to your bedding mix as a loosening agent. This eventually settles to the bottom of your planting bed, creates a hard pan, and reduces drainage.

Apart from the tonic you mention in another article... What do you suggest this [monthly] feeding should be?

Because of our rose growing conditions here in Texas, I prefer to feed my roses with a semi-monthly application of fish emulsion and seaweed. The product I use is Bio-Form™ although there are several products available which have a preblended formulation of these ingredients. When the heat of our summer begins (usually around the end of June or beginning of July), I discontinue feeding until my late summer light pruning and fertilization in preparation for fall flush. This usually occurs around the end of August.

Hope that helps a bit. Sorry for the length of this response, but your questions were good ones and deserved a suitable reply.

Have a great gardening day, and come back often!

-- posted by Mark_Whitelaw