Weeds can surprise you with the amount of competition they create in the springtime, especially when they've been protected under snow or plastic and row covers. Here we'll discuss control of some of our common winter annual weed problems, and also two perennials.
Clockwise, from upper left, common chickweed, Canada thistle, shepherd's purse, henbit. Source: http://plantscience.psu.edu/research/centers/turf/extension/plant-id.
Troublesome Early Season Weeds
Common chickweed
(not to be confused with mouse ear chickweed, a perennial) germinates mainly in the fall, though it will germinate anytime weather is cool and moisture is sufficient.
It can even germinate and grow under the snow, which explains those "Where'd that come from?" moments in the spring. It also tolerates shade better than most weeds. As the plant grows, it roots at its nodes, thus potentially forming a large mat of a plant that can produce over 10,000 seeds. Seeds can survive in the soil for over 10 years. It flowers and sets seed in the spring and early summer, and the seed is capable of germinating immediately. The plant only needs 5 weeks of growing conditions to progress from emergence to seed set. Typically there is only one generation per year, but two are possible. Chickweed does not tolerate is drought, so it is rarely a problem in non-irrigated row middles in the summer.
Henbit
(not to be confused with purple dead nettle) is in the mint family, and also roots at its nodes.
It has a similar germination and flowering pattern as common chickweed. One plant can produce 2000 seeds, and its seeds remain viable for 25 to 40 years. Its seedlings are easily controlled by tillage, but timing is critical.
Shepherd's purse
germinates in early fall, later summer, or early spring in the Northeast, and produces seed in late spring and early summer.
Plants flower and produce seeds in late spring and early summer. Seeds are produced in heart-shaped pods, shaped like purses shepherd’s used long ago. One plant can produce as many as 38,500 seeds, which remain viable in the soil for up to 35 years.
Field pansy
looks a lot like Johnny jump-ups, though the flowers are less showy.
Seeds germinate in late summer and early fall, and the plant flowers in spring. A really healthy plant can produce 46,000 seeds. Yikes.
Dandelion
is a pervasive problem because of its windblown seeds and its large taproot that allows the plant to re-sprout several times if broken off.
The flowers can continue to mature seeds even once the plants are pulled. If dandelion plants are recently established, shallow tillage can be effective, but if the weeds are established, hand-pulling and tillage have little effect. Plants can be mowed or weed-whacked close to the ground before the dandelions bloom if in-between rows of plastic. 2,4-D at spring dormancy, or Roundup with a wick applicator as long as the 14-day PHI is observed, and the applicator doesn’t touch any of the plants, are the best options. Chateau is an effective pre-emergence material.
Canada thistle
has both vertical roots for food storage, and horizontal roots that allow it to spread.
Shoots that emerge in the spring flower and produce wind-blown seed, while shoots that are produced in the fall make food for the winter. Mowing it or using a burndown herbicide is more effective in the spring than at other times of the year, as food reserves are already low then. Frequent tillage, repeated as soon as the plants re-sprout, is also effective, but tillage used infrequently only multiplies it.
In matted-row plantings, weeds are most likely to be a problem where there is bare soil, so you'll have problems with them where the plants are weak or along the edge of a row. In plasticulture plantings, they may be popping out of the planting holes, or appear between the rows of mulch and be especially problematic along the edges of the rows.
Management Strategies
So, what can you do about these weeds in the spring? In matted-row plantings, much of the control of winter annuals can be accomplished simply from the use of straw mulch, as winter annuals are usually not especially good competitors. It's important when removing the mulch to only pull it back the minimum distance necessary to expose the strawberry plants, and possibly come back and tuck some back under the plants later, which will also keep the berries clean. If the mulch has been blown off during this windy weather, it's usually worth it to bring some straw out to fill in the gaps.
In plasticulture, the sole control method of weed control in planting holes is hand-pulling. If cultivating, cultivate shallowly to avoid bringing up more weed seeds. Do not apply any herbicides over plastic-mulched beds.
Here are some notes on herbicides, with their greatest utility being in matted-row plantings. Be sure to watch days-to-harvest limitations.
Chateau (flumioxazin)
is a pre-emergent herbicide that has both pre-emergent activity and some burndown activity. This means that it can only be used when the plants are dormant in late fall or very early spring before the strawberry plants begin to grow again, or with a hooded sprayer between the rows if the plants have begun to grow. It can be tank-mixed with 2,4-D or Gramoxone to give better emerged weed control. It has good pre-emergent activity against all of the weeds mentioned here except for Canada thistle. The sprayer must be cleaned out according to label instructions in order to avoid damage to subsequent crops on which the sprayer is used.
Dacthal (DCPA)
is an older pre-emergent material and has short-lived efficacy against chickweed and henbit, but not any of the other weeds mentioned. It can be applied in spring prior to bloom, but won't have any activity against weeds that are already emerged.
Devrinol (napropamide)
is useful in preventing grass seeds from germinating from straw mulch, and is best applied in fall before straw mulch is added. It can be applied in the spring prior to bloom also, but it is a pre-emergent material, so it will only have effect on germinating weed seeds, not established seeds. Of the weeds mentioned, it is effective against germinating chickweed and dandelion, but only fair for the rest. It breaks down quickly in sunlight.
Prowl H2O (pendimethalin)
is a pre-emergent material that can be used with a shielded sprayer between the rows of strawberries in both matted-row and plasticulture plantings. It has a 35-day PHI. Its efficacy can be reduced with heavy amounts of rain fall. It is weak against all of the species mentioned here.
Sinbar
has a 110-day pre-harvest interval on strawberries, so it cannot be used in the spring.
Stinger (clopryalid)
is a post-emergent material that is good against thistles, but not the other weeds mentioned here. It can be used as a spot-treatment for thistle patches and is best applied in two applications of 1/3 pint, the first at least 30 days before harvest in the spring and the second after harvest.
Formula 40 or Amine 4 (2,4-D)
is a post-emergent material that is good against dandelion, but has little effect on the other weeds listed. It can be applied in early spring to dandelions that are poking through straw mulch.
Roundup or Touchdown (glyphosate)
is best applied with a wick or sponge applicator to emerged weeds, and is effective against all of the weeds listed. Be sure not to get any of the material on the strawberry plants. If you do get any on the plants, they will yellow and grow small leaves that could be mistaken for a micronutrient deficiency or cyclamen mites. The damage often doesn’t show up until a week or two after the application, so sometimes the connection to the Round made. It has a 14-day PHI.
Source : psu.edu