Native Grass Restoration and Management
Site Preparation in Late Summer and Early Fall
Early fall is the time of year that you prepare for native grass germination and plant installation. This can be part of a new native seeding project, an overseeding, or natural reseeding in an established stand.
Background
Surface Organic Matter - Native grass seedlings are rather weak, compared with those of european weeds. Most species of native grass generate little foliage during their first year, and invest a great deal of energy developing a perennial root system. As a result, seedlings of many long-lived native grasses, like Festuca or Nassella, are not capable of growing through thick mulch. A major goal of fall maintenance is to reduce the surface organic matter as much as possible, without creating an erosion hazard. If you can’t reduce the amount of material, you can, at least, reduce its depth.
Weed Seeds - Almost all sites have an abundant supply of weed seeds. If the site is left unmanaged, it will quickly become overrun with weeds. A freshly tilled or graded site will most likely be invaded by broadleaf weeds. In the Bay Area, two of the most important pioneer broadleaf weeds are bristly ox-tongue (Picris echioides) and mustard (Brassica nigra). Thistles tend to invade more established sites. A weedy grassland will mostly contain seeds of annual grasses, such as annual ryegrass (Lolium multiflorum), wild oats (Avena fatua), ripgut brome (Bromus diandrus) and foxtail (Hordeum murinum).
On sites with overhead irrigation and a budget for additional site preparation, it is possible to perform “grow and kill” procedures to lower the weed seed population. For most sites, however, you have to live with the weed seeds, and use ongoing management to reduce the weed population.
Soil Fertility and Structure - This is one of the most important factors in the establishment of any native vegetation, but it is the area in which we generally do the least. The reason is that it is very expensive to modify soil, particularly when large acreage is involved. Most restoration projects in the Bay Area are on slopes or in riparian zones, where it is difficult to import materials and almost impossible to use tilling equipment.
As it happens, native plants benefit from a side effect of poor soil conditions. This is not to say that natives don’t appreciate good soil structure and fertility. They do. It’s just that vigorous weed growth requires deep soil with decent fertility. If the soil is thin or has very low fertility, the weeds don’t grow well, and the natives have an advantage. An example of this can be found along roadsides. If you want to find native plants, look for them on the cutslope. If you look instead on the other side of the road, on the fill slope, you will only find weeds.
The take away message is to have a soil structure that is just good enough for acceptable native plant growth. If you make the soil too deep and loose or too fertile, the weeds will have a great advantage and the native seeding will fail.
Goals
Three goals for site preparation are:
1. Reduce the depth of surface organic matter. Reduce the quantity, if possible.
2. Reduce the weed seed population as much as possible.
3. Improve soil conditions where necessary and where feasible.
Methods
Surface Organic Matter - For seeding, it is best to have a thin layer of mulch. Whether the material is straw or last year’s weeds, you should be able to see the soil through it. Mowing is the most widely used method to reduced surface organic matter. Mow the weeds to make the mulch as thin as practical within your time and budget.
On steep slopes, use weed trimmers. On level or nearly level ground, you can use high weed mowers and tractor mounted mowers. Mowers are preferable to weed trimmers, because they shred the material into smaller pieces, which are more easily decomposed. Weed trimmers cut the material at the base, and it falls generally intact. Any mowing is better than no mowing, so use a weed trimmer it that’s all you have available. The native seed may be seeded and raked directly into the mulch. Make sure to add and rake the seed while before the first major rain of fall, so that the seed will fall to the bottom of the mulch and, hopefully, into the soil.
On the other hand, you may be restoring a construction site with freshly graded subsoil. In this case, you need to add organic matter. A good suggestion is to add a surface layer of compost. It should be at least 1/4” thick. 1/2” to 1” thick is better. If the soil surface is not compacted and crusted, the compost may be laid right on top. If the soil surface is crusted and sealed, then lightly till or roughen the surface before adding the compost, so that it will lock in place. The native seed may be seeded and raked directly into the compost.
Make sure it is well composted material. If the material is incompletely composted, it will create severely low nitrogen conditions, and only make the situation worse for a few years.
Weed Seed Reduction - The only system for weed seed reduction in the fall is the “grow and kill” method. It can only be performed if you have an overhead irrigation system, a good water supply, enough time, an adequate budget, and some luck. The idea is to irrigate the field and cause the weeds to germinate. When you have enough germination, you kill the weeds by tilling or spraying. Then you irrigate, and start another round of germination, etc., etc. If you get good germination and you are diligent, you will kill many of the weeds. In general, the results are fair to middling.
The fall heat (or the lack of heat) is a major variable in how many weeds germinate. It is almost impossible to keep a field moist and to get seedlings to germinate when the weather is hot and dry. The irrigation timing and quantity also have strong effects on the germination rates. It takes some experience with irrigation systems and seed germination to get good results. Also, weed seeds are buried in the soil. If you till to kill the weeds, each tilling will bring up more weed seeds. No matter how hard you try, you won’t get them all, but “grow and kill” should help.
Improve Soil Conditions - In most restoration projects, there is simply no budget or opportunity to improve soil conditions, so a restoration professional can probably disregard this section. We talk about soil tests and fertility monitoring, but only rarely do we have the opportunity to do anything about it. This is really for native plant landscaping near homes and businesses.
If the soil is severely low in organic matter and lacks structure, water and air will not flow through it properly. In this case, consider tilling compost into the top 6” of soil. This will aid in air flow and drainage, and will aid in plant establishment. In the long run, the native grass roots will add the organic matter and develop the soil structure.
If fertility, particularly nitrogen, is only on the low side, avoid adding fertilizer. Only add nitrogen if the level is severely low. Remember that the weeds will take advantage of the nitrogen faster and better than the natives. Your goal is to have slow growth. Low fertility suppresses weeds. Vigorous, lush growth will also surely doom your project.
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