Comparison of Cover Crops for Aisle Weed
Suppression in Black Plastic Growing Systems

On the land I rent two miles from my farm, I grow all my annuals on black plastic.  Although I have qualms about all the plastic, it is the best way I can grow a large amount of flowers with minimum weeding, the soil is warmed quickly in the spring, and my flowers are kept clean of soil splashing. The one-acre plot is plowed each spring and I hire a neighboring farmer with a mulch layer to lay the plastic with drip tape underneath. Everything looks incredible until about June, when the weeds strike!

We have tried various methods of weed control: rototilling aisles, mowing weeds, string-trimming weeds, hoeing close to plastic edges. Because I grow with minimal chemical input, herbicide on the aisles is not an option for me. The worst weed in my field is crabgrass, which loves the heat of late summer. Once established in the aisles, it tends to creep over the edges of the plastic. And since it grows very low to the ground, it can produce viable seed below my mowing height…my problem keeps getting worse!

This land is also heavy clay with low organic matter content. I began to wonder if I could kill two birds with one stone by planting cover crops in the aisles, to both out-compete the crabgrass and to improve the organic matter content of my soil. Ideally, a quick to germinate, but low-growing cover crop would not allow the crabgrass to germinate, and stay low enough to minimize aisle mowing.

To test this, we planted three different cover crops and a plain weed control using the following method:

Land was plowed and twelve 350-foot rows of black plastic were laid on May 14, 2011. 

On May 26, we lightly rototilled all12 rows (to disturb any weeds that had germinated), and we planted our cover crops. We used a push-style drop spreader to seed 3 rows each of the following: oats (Avena sativa), summer alfalfa (Medicago sativa), and white clover (Trifolium repens). Each crop was seeded at its appropriate rate. For the 3 control plots of weeds, we did nothing after the tilling.

We also took soil samples of each of the four test plots, making a representative sample compiled of 15 cores from each three row plot. These samples were sent to Penn State for analysis.

That night, we had a severe hailstorm which flooded my field and shot holes through the plastic.

On May 28, we reseeded the oats. Surprisingly, the smaller alfalfa and clover seeds did not wash away. Enough had already started to germinate that we decided not to overseed these rows.

On June 23, we push-mowed the aisles for the first time, mowing the alfalfa, clover and weed plots. The oats were still low and did not need mowing. 

We continued to mow as needed (when weeds/crops were 8-12” tall) and recorded each mowing date, time spent mowing, and any observations.
At the end of the season, we again took representative soil samples of the test plots and sent them to Penn State for analysis.

Maintenance and Mowing Results

All plots required 6-7 mowings from June 23-September 1. Mowing the 12 aisles took about 1½ hours each time (or at least the times when my mower was running properly!). We did end up hoeing or weeding the edges of almost all rows of plastic sometime throughout the season; the crabgrass that had germinated was creeping onto the plastic. This was especially problematic for direct-seeded crops that were small enough to be sensitive to the crowding and, conversely, were not big enough to shade out the crabgrass.

Initially, the oats germinated quickly and evenly, requiring one less early-season mowing than the other crops. The oats seemed to crowd out all early weed seeds and they did not grow as quickly as the early weeds (mostly hairy galinsoga). The oats died out by late July, but weeds did not germinate at this point, and the soil was somewhat bare for the month of August. 

The white clover was slow to establish, but made a very nice carpet by the end of the season. (We actually left this in the ground to continue over the winter.) Once established, it did outcompete the weeds and created a low carpet. The clover stayed low enough that we were able to skip the final mowing on September 1. On a side note, the creeping clover did root through the edges of the black plastic. This made my most hated job of lifting out the plastic even more difficult than normal.

Annual alfalfa grew tall in mid-summer but seemed to die out later. Crabgrass was very thick in the alfalfa rows in late summer. The alfalfa established slowly, allowing the weeds to germinate, and then died early allowing the weeds to overtake. 
The control weed plots were comprised largely of crabgrass with some hairy galinsoga mixed in.

 Oats6 Did not mow first mowing, second also thin
 Clover6Did not mow last mowing
 Annual alfalfa7Very tall and thick midsummer

Soil Improvement Results

In addition to decreasing time spent mowing aisles, we hoped to increase organic matter via cover crops. Soil was sampled May 26 and October 11. Results are below.

 Annual Alfalfa2.32.7+0.4

Oats most significantly increased organic matter. This may be due to early biomass production and subsequent breakdown over late summer months. This surprised me because these aisles were more bare than the others from August-September. The same reasoning can be applied to the alfalfa. Clover provided the least significant increase in organic matter, however, if tests were taken again in spring after plowing, results may be different.


Oats planted in aisles give good suppression of early weed seed germination. As long as soil is not disturbed after this, few weeds germinate late summer, even after oats have died out.  The oats also most significantly improved soil organic matter content. Oats appear to be the best (and cheapest) crop to plant in aisles.

White clover is also slow to germinate but spreads nicely and prevents weeds from becoming aggressive. In long seasons, it may be more effective in eliminating the need to mow late summer through fall. This may be a viable option for multiple-year plastic or permanent aisle systems.

Summer alfalfa is slow to germinate but does provide a good biomass. It dies down midsummer and the already germinated crabgrass takes over. I would not recommend using this unless biomass is the only goal.

Because this is a low-organic matter soil, and leaving it bare/allowing weeds to grow actually depleted the organic matter further, I would recommend planting a crop in the aisles for soil health. In addition, any weed suppression will decrease weed seed populations for subsequent years. All cover crops required the same or fewer number of mowings as the weed control, and all cover crops increased organic matter content. Although all crops gave better results than the control, oats were the best choice and the one I’ll be using next year.

Michelle Elston

Roots Cut Flower Farm

Michelle Elston Roots Cut Flower Farm, Carlisle, Pennsylvania