I have several horseradish plants. After they flower, tiny "pods" appear that look like they should contain seeds. When I open a pod, or wait for it to naturally open, they look empty. I expected to see something like a mustard seed. Is it possible to collect the seeds?
You may or may not ever see a single seed! Sometimes they produce, but that is rare.
Here is an article that will help you to care for the plant:
https://www.gardeningknowhow.com/edible/herbs/horseradish/horseradish-plant-has-flowers.htm
I have lots of horseradish leaves and would like to know if they can go in a compost
Sure! There is nothing wrong with composting them.
Here is an article that will help you with composting:
https://www.gardeningknowhow.com/composting/basics/ultimate-beginners-guide-composting.htm
It started off great guns, new leaves, etc., but no new leaves in a while and 3 of the 4 still there are turning brown. It is in sun in a very deep container with ample drainage. I water when it feels dry to about 2" below the surface.
Unfortunately, your photo did not come through. I am unable to assess the damage. Also, I am unable to see where you are located, as the number you have provided is an invalid US zipcode.
The only guess that I have at the moment is overwatering. This is even more true if the container is much larger than the plant, since there won't be any roots to take up water through most of the container.
If this is the case, then the infections from overwatering will require a fungicide. This article will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
I dug my horseradish root and it has an orange center
Root discoloration can be caused by Verticillium dahliae and is not good news. Contact your extension agent to find out an exact diagnosis:
https://www.gardeningknowhow.com/plant-problems/disease/verticillium-wilt-treatment.htm
rowing season? The horseradish has been in place for 6 years or so. I have not seen anything related to bitter horseradish on websites.
Research shows not to harvest until late fall or early spring before new growth resumes. So harvesting time may have been an issue. I also read to avoid any green roots while harvesting or after peeling, that it could be bitter. Did you encounter any green tinge on your roots?
Another reference said once cut or grated, the oils in the root degrade and become bitter if not used immediately or vinegar added to stop the degradation.
Also older roots become woody and not useable. So there are several ways to get bitter horseradish.
https://wimastergardener.org/article/horseradish-armoracia-rusticana/
https://www.gardeningknowhow.com/edible/herbs/horseradish/harvesting-horseradish-root.htm
I just dug up my 2 year old Horseradish plants and was disappointed with the results I found. The roots are all split and deformed. There is not enough good root to do any processing after cleaning. I suspect it is a fungus of some kind. The two plants were planted in different planter boxes (4' X 4'). My Tomato plants last season (2020) suffered the same type of malady even though they were planted in different boxes at the other end of the garden area. Do you have any ideas what could be done to fix the soil in the garden boxes for future plantings? I have enclosed photos of the Horseradish Roots.
You could solarize your soil to destroy any disease-causing organisms. It also wouldn't hurt to get soil samples to see what nutrients may be missing. If you are in a hurry, you can always replace the soil with new. Make sure it is not soil from your garden, but lightweight soil for containers.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm