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Watch that Nitrogen!

Published in the six Nashoba Publishing papers on Friday, 20 September 2002

It's too late to fertilize most plants — except grass. Perennial lawn grass keeps growing for a month or more yet; whereas most of the weeds, being annual, don't. So if you fertilize your lawn now, you help the grass get ahead of the weeds. (See my recipes for organic fertilizer.)

The problem with fertilizing now is nitrogen. This element (N in the NPK formula) is essential to growth of leaves and stems — exactly what plants don't need this late. Tender new growth won't survive frost, and its freezing will weaken the whole plant.*

It's hard to resist picking up bargain perennials on sale at garden centers (who don't want the work of overwintering them). And it seems natural to feed a plant when you put it in the ground. Still, give them greensand for the potassium (K) they need for cold hardiness, and plenty of rock phosphate (P) for root development — but no nitrogen until spring.

Spring bulbs go in the ground now; don't put compost in the planting hole, it encourages fungal diseases. Bulbs mostly like lean soil anyway. Compost is safe as a top dressing; blend in rock phosphate; or if you use the traditional bonemeal go easy (2 tsp. per bulb): although it's mainly used for its phosphorous content — up to 30% — it does have 1 to 2% nitrogen. In any case, the first year's bloom does not depend on any fertilizer you add now; right after they flower is when bulbs really need it, to feed up for the next year.

Bearded iris have come out of their summer dormancy, showing lots of new shoots. Even established iris like phosphorus and potassium in fall, but not nitrogen: it will decrease bloom next spring. The time for a complete NPK mix is after plants set flower buds, and before they go dormant (six weeks after bloom). Even then, the nitrogen should be much less than the P and the K, and don't apply the fertilizer over the rhizomes — or you'll encourage bacterial soft rot, which turns rhizomes to smelly mush.

Two small pails hold the fertilizer I mix from separate organic ingredients: one for the back yard, one in front, so they're handy. Too handy: yesterday I had just a minute to spend, and so transplanted three little sedums with the existing mix — including nitrogen. I'd better go throw the rest on the lawn before I do that again.

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© Copyright 2002 Catherine Holmes Clark

* After this was published, I read another reason not to add nitrogen too late: the temperature of the soil needs to be no less than 50° for nitrogen to be incorporated into it. Lower, and the nitrogen will just run off.