It was early summer a few years ago and my spinach and Swiss chard were ready to pick, when I noticed what looked like long whitish-grey drips on the surface of the leaves. I was puzzled, but removed the pieces from the garden and didn’t think about it again until they reappeared. Without much thought, I concluded that the drips must be caused by bird feces.
The following year, I took the University of Vermont (UVM) Extension Master Gardener course (uvm.edu/extension/mastergardener), which encouraged students to become more curious about what is happening in the garden. A closer look and a quick internet search of local Extension resources led me to the culprit: leaf miners.
When tunneling the leaf miner larvae eat the part of the plant between the outer layers of the leaves of spinach, Swiss chard and beets. The damage looks like white, opaque or grey trails through the leaf surface.
They also can be identified by the tiny, white, elongated eggs stacked along the underside of leaves. Leaf miners overwinter in the soil nearby and can become an annual aesthetic problem if not addressed.
To prevent leaf miner damage, you can install row covers over susceptible plants for the entire season. A row cover is a lightweight fabric that can be laid over your vegetable garden like a thin blanket. It allows sunlight and water through while shielding your garden from insects and may offer frost protection depending on the thickness.
The fabric can be cut easily and laid directly over plants or propped up with purchased or homemade low frames made of thick wire, plastic tubing or wood. Be sure to seal the edges with rocks or garden pins, making sure that there are no openings. It is important to rotate crops so overwintering insect populations are not trapped under the covers in areas where susceptible crops were planted the previous year.
If you lay the row cover directly over your plants, keep the fabric loose to give the plants room to grow. You can purchase row covers from your local garden store and many seed companies. These can be reused for a couple of years before they need to be replaced.
Row covers also can be useful in protecting broccoli from the imported cabbageworm. These garden pests do the most damage in their larval stage, a 1 1/2-inch green caterpillar with a small yellow line on top and yellow dots on the sides.
They chew through cole crops including cabbage, broccoli and kale. It is very possible that you will not notice them until you find the camouflaged worms on the interior stems of your broccoli when you are preparing dinner.
Depending on the pest you are attempting to control, you most likely will need to install the row covers as soon as you plant your seedlings. If you are using row covers to manage insects such as cucumber beetles on squash or other plants that require pollination for fruit production, you will need to temporarily remove the covers during flowering.
If row covers aren’t your style, try destroying infected leaves, reducing weeds and rotating crops for leaf miners. Or hand pick larvae for cabbage loopers.
I found that row covers made my crops significantly more enjoyable to eat. No more picking through my spinach salad or suspiciously eyeing my fresh broccoli. The tradeoffs were that it was harder to work in the garden due to needing to remove, recover and reseal the row covers every time.
It also was not as pleasant to look out at the garden because the plants were covered with white fabric. However, to eat flawless spinach and wormless broccoli made the row covers totally worth the extra effort.
For answers to your questions about row covers, pests and other gardening topics, call the UVM Master Gardener Helpline at (802) 656-5421 or (800) 639-2230 (toll-free). Volunteers are on hand to take calls on Mondays and Thursdays from 9 a.m. to noon, with expanded hours beginning in mid-June.
Bonnie Kirn Donahue is a UVM Extension Master Gardener and landscape designer from central Vermont.