Question about nitrogen

seachelleleeJanuary 18, 2010

Hello,

I have three Madame Pericats that are planted near a concrete slab in my yard. I just planted them last spring, so I think they're still working to get established, but they don't look as healthy as I'd like them to - not a lot of leaves and some yellowing. I tested the soil and the PH is 6.5 and I need to add nitrogen. When should I add nitrogen? I'm afraid if I add it now, it will inhibit blooming this spring. Also, can I work to amend the soil now by adding sulfer, or should I wait till after blooming? Thanks for any help!

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rhodyman(SE PA, USDA Z6)

If the leaves are uniformly yellow, then it is caused by your low nitrogen numbers. If the leaves are yellow with green veins, then your nitrogen isn't too bad, but you have an low-acidity/phosphorus problem.

When to fertilize: For most garden situations the old rule "once before they bloom and once after they bloom" is still a sensible approach. Actually the fertilizer timing has nothing to do with the time the plant flowers, it simply means once in the early spring, probably April then again in June. Never fertilize after mid-summer. Over-fertilizing is worse than not fertilizing at all. Established azaleas often do well with no fertilizer at all. Nutrients are slowly released by any organic mulch that you use, so rely on this as the primary source of nutrients. Excess nutrients may promote larger than normal populations of azalea pests like lace bugs and azalea whiteflies. It´s very easy to burn up the fine roots. Fertilizing after late June in a northern climate promotes tender growth in the fall, which doesn't harden off before the first frosts of winter. This gets killed by the frost. This growth may have the buds for next year's flowers on it, which would also get killed by the frost. Research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved.

For acidity, flowers of (powdered) sulfur or iron sulfate are best. Do not use aluminum sulfate since aluminum builds up in the soil and is toxic to most plants eventually. For flower bud production and hardiness, super phosphate is best. Around the base of each plant I use a tablespoon of dry sulfur and a tablespoon of dry super phosphate when a plant shows signs of problems.

For a general fertilizer for rhododendrons and azaleas, Holly-tone is preferred by many growers. It is an organic 4-6-4 fertilizer with powdered sulfur, minor elements such as magnesium, iron and calcium, and trace elements also. Try to use an organic product since chemical fertilizers reduce the number of mycorrhizae in the soil and the mycorrhizae are what make phosphorus available to the plant. When fertilizing only fertilize once in the spring and at half the rate on the package. Some people fertilize once before blooming and once after blooming, but only fertilize at half the rate on the package.

Here is a link that might be useful: How to care for rhododendrons and azaleas.

    Bookmark   January 19, 2010 at 2:42PM
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seachellelee

Hi Rhodyman,

Thanks for your reply. I think I've read "once before they bloom, once after they bloom" before - it was probably one of your previous posts! I was just worried about adding nitrogen too early in the season. Is there a chance that any developing flower buds will turn into leaves if I do this? And I was thinking of adding blood meal, since it's organic and high in nitrogen. According to my soil test, my P & K levels are fine - just need nitrogen. Another booboo I might have made was adding mushroom compost to the soil when I planted these azaleas. I thought it would be good as an organic additive, but I did not realize it would throw off the PH. Thanks!

    Bookmark   January 19, 2010 at 8:02PM
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rhodyman(SE PA, USDA Z6)

Flower buds are flower buds and foliage buds are foliage buds. That was established before they hardened off. THe risk in applying nitrogen too early is that it my make the plant break dormancy too early and the tender growth could be hit by a late frost and killed.

Another advantage of organic fertilizers like Hollytone is that they are fairly benign and have a very slow release.

Mushrooms are heavy feeders and mushroom soil is depleted. We have lots of mushroom houses near us. We never use it on acidic beds. Farmers us it in their fields.

Check to see if the yellow leaves show green veins. If they do it is not a nitrogen problem.

    Bookmark   January 20, 2010 at 9:34AM
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