The Monday Garden
Alder: The Nitrogen Fix

Alders can handle intermittent wetness, so do well along streams and marshes where flooding and drought are both likely. Alders are temperature specific and each variety’s range is limited by its tolerance of heat and cold. The babies can get started in deep shade but the adults want full sun or just a touch of shade. The wingless alder seeds can be spread short distances by wind but generally travel by water, buoyed up by internal air floatation sacs, hence the frequency of the alder family along water bodies.

Despite our nice array of native alders, the Euro settlers felt compelled to bring with them the European black alder (alnus glutinosa). I suspect that these settlers didn’t yet know enough about our native flora to know that we already had plenty of good alders. However, they were savvy farmers and knew that planting their alders from home would make nearby plants grow much bigger and faster – a good thing when you grow your own food. I also suspect that the early Euro settlers probably didn’t know why alders had this effect. Since Europeans didn’t isolate nitrogen as an element until the 1770’s, they probably didn’t learn about the workings of nitrogen-fixing plants until sometime later. The European black alder has naturalized throughout Northeastern USA and Southeastern Canada, north to Zone 3 and south to Zone 7, but generally is not at the top the “bad invasives” list. For example, in my home state of Connecticut, the European alder been declared to have “invasive tendencies” which means that, for us, it’s not the worse thug on the block but we should be careful where we put it as it can spread.

Now, nitrogen is mega-important to plant growth. On a fertilizer bag, you’ll see three numbers; the first one tells you how many pounds of nitrogen would be found in a 100 bag of the fertilizer (percentage by weight is another way of calculating with the numbers). (FYI: The other two numbers are potassium and phosphorus.) Plants need a bunch of things, like oxygen, that they can grab from the air, and trace minerals that are generally present in most soils. However, not everything that plants need to thrive is always present in a form that they can use.

The reason that most fertilizers contain the “Big Three” is that these elements are often in the shortest supply. In many environments, nitrogen becomes the “growth limiting factor”. This means that nitrogen is the first thing that the plants will run short on, which will then be the factor limiting further growth. (BTW: Natural limits on growth are not necessarily a bad thing, unless you’re a farmer trying to maximize per-acre production). There is plenty of nitrogen in the air but most plants can’t access it. The nitrogen fixers though, can take advantage of nitrogen in the air, often through a symbiotic relationship with bacteria-like micro-organisms. The nitrogen fixers include the pea and birch families.

In the case of alders, the alder benefits from the nitrogen that the mycorrrhizal fungi attached to its roots add to the soil, and the neighboring plants benefit for the extra nitrogen that’s quickly released from the alder’s falling leaves.

IDENTIFICATION: Once you’ve gotten to know one alder, you should be able to identify the family. Alders have distinctive leaves, flowers, seed-cones, bark and buds, so they are pretty easy to spot at any time of year. Like so many woody plants, they cross–breed and have individual variations, so you may see characteristics of more than one species on an individual plant.

LEAVES: Alder leaves are generally elliptical and mid-green with pronounced, straight parallel vines in V-shaped pairs that run right to the leaf edge. The leaves don’t have much color in autumn – usually they drop while green or turn brown and drop

Leaf characteristics that vary between alder species are whether:

• the leaf tip is pointed, round or missing,
• the leaf base is U or V shaped,
• the leaf edge’s teeth are fine or jagged,
• the leaf edge curls under or is wavy
• the leaf underside is lighter or hairy,
• the leaf top is shiny or dull.

The European alder is missing the leaf tip.

FLOWER AND SEEDS: Alders start next year’s flowers in summer and, as noted, hold the cones over the following winter. Also, alders start to bear when only a few years old. So it’ll be rare that you’ll see an alder total bereft of the distinctive cones or the birch-family trademark catkins. The pollen floats from tree to tree (and into your sinuses) via the wind in spring for most alders but in fall for the remarkable Seaside Alder (Alnus maritime) which, due to an altercation with the last glacier, is now only found in Maryland, Delaware and parts of Oklahoma.

Alders have separate male and female flowers on the same tree. The male flowers are long (up to 5”) reddish catkins; the females are much shorter (about ½” or less). The female flowers mature into cute little pine cone-like objects that contain tiny wingless nuts with built-in air sacs for floatation. The cones start out green, and ripen to brown in the fall. When ripe, the cones slowly release the nuts through fall and winter. To distinguish between alder species, pay attention to whether cones and catkins droop or stand erect.
BARK: Alder bark tends to be dark and smooth with light colored horizontal markings. The markings and color vary between species. The European alder has gummy twigs and young leaves. The speckled alder has prominent lenticels and the European alder bark tends to have warty patches.

BUDS The fat, blunt, reddish buds sit on top of short stubby stems and have a prominent “seam” where the two side covers (bud scales) meet. The Seaside alder’s bud scales don’t quite meet.

WILDLIFE: Alder seeds are eaten by small songbirds such as goldfinch and the stands provide cover for many birds and other small critters. The American deer like it too. Beavers and possum are said to savor the pollen rich (and therefore protein rich) catkins.