Soil amendment for Washingtonia Filifera

[Advective]

Looking to plant a W. Filifera in 8A North Texas with heavy clay. I'd like to amend the soil closer to native. Palm Springs looks to be mostly sand.

Based on the soil triangle, my plan is to mix 60% sand, 20% compost (silt substitute) and 20% clay to get a soil bordering Sandy Loam and Sandy Clay Loam. Still a lot of clay but I have to work some of it back in.

Here are my questions:

1. What is the minimum depth and circumference I can get away with for the amendment, assuming a 3 foot wide trunk?

2. Is compost the right substitute for silt? I couldn't find a silt soil for sale.

3. If I use a 20% compost mix, will the amended area "sink"? Should I mound it to a degree?

[Pip]

Washingtonia are very adaptable. They can become a weed in many location. Seedlings pop up in alsorts of pkaces and grow with little help in sandy or clay soils.

When I amend/improve the soil I'm working with I like to do the whole garden bed I'm working in. Organics such as compost and manures are great to work into sandy or clay soil before planting out while gypsum will help clay soil become more friable.

[Kostas]

What i would do is dig an area to 20-30cm depth(that area could be just a meter or less in circumference, despite trunk size, Washingtonia are fine with growing in pure clay, the amendments will just give it a boost in growth rate and help with preventing rot in a colder than ideal climate), fill back with sand/organics and clay and mound the area 10-20cm above grade with mostly organics and sand. I would plant the Washingtonia with the root initiation zone 5cm deep and let it grow. Soil will sink some(thats why you create a small mound) but Washingtonia grow so so fast that they only let the soil sink little as they anchor it and create soil structure fast with a huge root system and a web of surface roots

[TexasColdHardyPalms]

Mix in a bag potting soil and call it a day. There are filifera's growing all over the place here without issue as long as they aren't subjected to standing water for long periods of time.

[Ben in Norcal]

I wouldn't amend the soil at all.

[_Keith]

I wouldn't amend the soil at all.

I am with Ben. Just keep it well mulched and let nature do the rest.

[Advective]

Thanks for the input. The amendment idea came about by a Palm Talk post detailing Filiferas that survived about -5ºF / -20ºC one winter in New Mexico. Since it can potentially go down to low single digits or even subzero in my area, my thinking was to replicate the NM conditions as much as I can. Hence, the sandy soil (and I would also wrap the center crown area on ice days to keep the bud dry).

If low single digits or subzero survival is possible in clay, I'll leave it as-is.

[Ben in Norcal]

I wouldn't bank on survival at those temps regardless. That's 20 degrees below what I'd consider their floor.

[Brad Mondel]

Filifera has been known to survive -11*F but this was with mature, established specimens and also this caused major defoliation. If you don't want it to defoliate then i'd protect it starting at 20*F.

Edited February 22, 2015 by ArchAngeL01

[TexasColdHardyPalms]

There are mature filifera all over the Dallas fort Worth area. The only year that I saw any foliage damage on the local filifera was 2010. Even then the damage was minor. That same year most of the robusta population died, especially the large ones, and almost all located out in the open, unprotected.