Origin of a greenhouse weed problem

At first, it would seem possible that a growing medium could be the source of weed seed.  Hanging baskets are potted up in February and in a few weeks some weeds may emerge.  There are no weeds under the benches, none growing on the greenhouse floors (at least not until the floor warms up) and there are none growing outside the houses on snow covered ground, so where did the weed seed come from?   Weed problems that show up in winter actually started the previous summer thru fall.

Where does the weed seed come from? 
Weeds growing on the floors, under the benches, near vents, in containers or around the perimeter of the greenhouses produce seed from spring through fall, unless they are destroyed prior to going to seed.  Once they go to seed, the greenhouses are contaminated, and seed will find its way into crops the following winter / early spring creating weed problems.

Weed seed can also travel into the greenhouse by clinging to containers, flats, soilless media packaging, and other supplies that are stored in or around weeds.  Reusing containers from plants that had weeds may have weed seed clinging to the pot or if the containers were not cleaned, then weed seed could be found within growing medium residues.  Even if the pots are soaked in a bleach solution prior to planting, any remaining weed seed may still be viable.

In some cases, weeds and weed seed can arrive with liners or plugs. The weeds or their seed may not be visible, but they may be there.  Keep in mind, liners or plugs shipped to a grower in February were stuck or sown weeks earlier in a warmer climate where weeds could be actively growing.  Some weeds, such as Oxalis, have seed capsules that shoot seeds up to 20 feet. Before shipping, suppliers will 'clean-up' the weeds growing in their trays before they are packed, but weed seeds are scattered among the trays only to germinate after transplanting.

Where does the weed seed settle inside the greenhouse? 
Weed seed can be dispersed within a greenhouse through air movement, clinging to the glazing through static charge and some weeds can expel their seed long distances.  So, there are many places within the greenhouse where weed seed can be lurking.  Places to look include: greenhouse glazing (weed seed can adhere to it), on top of or inside fan jet tubes, inside the housing of exhaust fans or air inlets, on top of greenhouse support structures, potting benches, potting machines, greenhouse benches, greenhouse floors, etc.

How Does a Weed Problem Begin? 
Wherever weed seed is lurking, it often finds its way into the growing medium.  For example, if it is on the potting bench or clinging to containers or the growing medium package, it can accidentally become incorporated into the growing medium.  If weed seed is in or on fanjet tubes, exhaust fans, air inlets or horizontal air flow fans, the weed seed will be dispersed onto the crops when fans are turned on.  If weed seed is on the greenhouse glazing or structure, water will condense forming drops that contain weed seed that drip on to the crop below.

A weed problem can begin as a minor issue with a few weeds emerging here and there.  Often these weeds are not seen or ignored, especially in hanging baskets and stock plants.  They eventually produce more seed that will spread to surrounding plants, germinate and result in a significant weed infestation.

If there has been a history of weed problems in a greenhouse, now is the time to start looking for potential sources and sanitize the greenhouse to prevent weed problems in spring crops.

Photo Credit: Premier Tech Horticulture.

Source: Greenhouse Grower

Source: Greenhouse Grower