Tuesday, December 7, 2010

Rhododendrons - Part 2

Rhododendron 'David Waldman'

This is probably my favorite Rhododendron (azalea?). Can't remember which this one is and since I can't see the stamens, I'll just leave it as a pretty flower. It is named after David Waldman who owned Roslyn Nursery before he and Harriet retired.

Today's rhododendron information is on planting them and keeping them happy. According to Harold Greer who is a recognized expert on Rhododendrons, there are 3 things all rhododendrons want: A constant supply of water, well-drained soil (not a contradiction here - they need ample water and don't want to dry out, but standing water will quickly do them in, especially during hot weather), and an acid soil with a pH of 5-6 that is coarse enough for the roots to have access to oxygen.

A little more explanation on these. The water thing. Rhododendrons are one of those plants that draw water all year 'round. In places like here where the ground freezes, this presents a problem. A good mulch helps, but they also have their own defense. Remember the picture of curled leaves I posted yesterday? That's why they do it. To keep from losing water in their leaves that they aren't going to be able to replace from the frozen soil. There are treatments for the leaves that help seal them from moisture loss, but I've never tried them and find that the plants take care of themselves. In more severe climates, especially where the rhododendron will be exposed to a lot of wind, a screen of burlap is a good idea. A couple of bamboo stakes and some burlap will do the trick. Don't close the top. The plant still needs sun and whatever rain it might get on warmer days.

So how do you know if your soil will have standing water that could kill your rhododendron? You do a test that we pretty much do with anything we plant. Dig the hole and then fill it with water. If the water doesn't disappear in a few minutes, you may not have good enough drainage. (If it does drain, just fill it again. This way when the new plant is settled in, all of the surrounding soil will be nice and moist.) You need to find a happy medium between soil that doesn't drain and soil that drains too fast. You don't want stagnant, standing water, but neither do you want the plant to dry so quickly that you will have to spend all of your time watering it. If your soil is not optimum, did a much bigger hole than you need and then amend the soil before replacing it when you plant the rhododendron. The old time formula is one third bark, one third peat moss and one third garden loam. A nice coarse mix is what you're looking for, up to one third air spaces - space for water to get in to the roots and space for the extra water to drain on through and not sit and rot the roots. The late Hank Shannen (hope I spelled that right) from Rare Find Nursery, recommended cutting the root ball about a third of the way up from the bottom twice (in an X) and then spreading the root ball out sort of flat. Be sure there is plenty of new soil underneath so all of the roots are in contact with the soil. They tend to be really root bound when you buy then and this gets their roots spreading into the surrounding soil faster and helps to be sure they'll be absorbing water as well as they can and not strangling themselves as the rootbound rootball often tends to just grow in on itself and not get out where it needs to be.
Another trick I almost forgot, and this is good for any plants you buy, when you bring them home, submerge the root ball in water until air bubbles stop coming up. This makes sure it is thoroughly wet before it gets plants.

After you get the rhododendron planted, you should give it a good mulch. Eventually it will provide its own mulch as it sheds leaves, but in the beginning, you'll have to provide the mulch. Things that don't work are peat moss - dries too fast and acts like a thatched roof to shed water and see that none gets to the plant - and black plastic. Sure, plastic keeps the weeds away, but no water gets in and it will just smother the plant. Shredded leaves work well - just run the lawn mower over them. If you have lots of leaves, you can try black bag composting. Put the damp, shredded leaves in black garbage bags and leave them in an out of the way place for 6 months to a year. When finished, you will have soft, wonderful compost, all ready to mulch your plants. We also use pine needles for a lot of the things that like an acid soil. We have plenty of pine trees in our woods, so collecting them is time consuming, but wonderful for the plants. I just try to take a bit from here and a bit from there, so as not to take too much away from the pines.

Watering: Drip irrigation is wonderful and the plants love it, but if that's not an option, a hose or watering can is fine. You don't have to worry about burning the leaves on these like you do on some more delicate plants, so water them when it is convenient. Actually a summer afternoon shower that cools them off somewhat will be as much appreciated by them as it would be by you.

Just make sure to water deeply so that you don't let the rootball dry out. If that happens, sometimes it is very difficult to get it to absorb water again, so be sure to water deep and often and as long as you have amended the soil so it drains well, you really won't be able to overwater. This is especially important on newly planted ones. Our established plantings rarely get anything not provided by mother nature. There are just too many of them and once the roots get out into the surrounding soil in a few years, they can pretty much take care of themselves except during extreme drought.

Fertilizing: We don't fertilize, depending instead on compost and mulch and that seems to make healthy plants for us. Some people like to give their rhododendrons a dose of fertilizer in the spring. Either works. Whatever you like.

Rhododendron aurtrinum (azalea)
More tomorrow, probably on diseases and other problems and some insects that might bother them. I'll say now that we've never had problems with anything on rhododendrons, but better to know what might happen and to know what to look for and how to fix it.


Sylvia (England) said...

A great post Jane. My soil is about neutral so I don't grow Rhododendrons but I am tempted to try one just to see if they would be happy! One other tip, here in some areas of the UK, our tap water is very hard, alkaline so we need to use rain water. If not other water is available then tap water is better than no water!

Look forward to the next post. Best wishes Sylvia

Hoot Owl Hollow Nursery said...

I know of a woman who waters hers with vinegar water. I think a better solution would to be to use Lime Sulphur as I do on my blueberries to be sure the soil is acid enough.