Tips and Good Cultural Practices for Producing Healthy Transplants in the Greenhouse

Author: Heather Stoven, Small Farms Program, Oregon State University

Publish Date: Spring 2015

For those who have access to a greenhouse, growing your own starts can give you a leg up on the growing season. However, growing in this environment often requires extra attentiveness and care due to this specialized setting. A few key best management practices as well as thoughtful preparation and planning can help lead to success.

Doing some research before you start regarding the particular requirements of your seeds can increase germination rates and help with space management and planning. Some things to consider are the temperature and light requirements for the seed to germinate as well as the length time before transplant. Bottom heat systems such as heat mats, hot water tubing systems and/or germination chambers can be a good way to achieve the needed germination temperatures.

Greenhouse air temperature regulation is also extremely important due to the short period of time before a greenhouse will heat up on a sunny day, or conversely cool down when the sun goes down. Side vents, fans, and shade cloth can provide assistance in cooling a house as day length and outside temperatures increase, however unless thermostat control systems are in place, it is important to be vigilant, since outside conditions can change quickly.

As starts are ready to transplant, hardening them off to acclimate them to the outside environment can help increase transplant success in the field. Gradually reducing watering and then placing the starts outside in a protected spot for three to six days before transplanting can help reduce plant stress and increase plant survival.

In addition to temperature regulation, the other most important aspect to greenhouse growing is proper water management. Irrigation systems can be set up to assist with keeping plants watered, but need to be well designed and monitored to ensure that the plants are not over or under-watered based on the changing outside weather conditions. Hand watering of edges of flats is often warranted to ensure the entire flat isn’t overwatered to compensate for a poor irrigation pattern or a few dry cells or pots. This can be time-consuming but pays in dividends when it comes to plant health. Plant stress from improper irrigation, especially over-watering, can contribute to pest problems in the greenhouse as well as create a good environment for pathogens. Damping off of seedlings (wilted seedlings often with a darkened lesion at the soil line) is caused by several fungi or water molds. Maintaining proper sanitation by using soilless mixes, sanitizing containers if reusing, properly irrigating, and using bottom heat to encourage rapid germination will help to reduce this problem.

Fungus gnats are also very common in greenhouse environments and like damping off, become problematic in wet conditions. Proper irrigation and sanitation also go a long way in the prevention of this problem. The ¼-inch translucent larvae may not be readily apparent when looking at seedlings from above, but when present in the soil, they feed on tender roots of seedlings, stunting growth and providing a route of entry for pathogens. Monitor for the adults, which are 1/8 inch long and mosquito-like with yellow sticky cards placed at the soil line, or for the larvae with raw potato chunks placed on the media surface (they will congregate below the potato). Biocontrol agents such as predatory mites, rove beetles or nematodes can be purchased and applied to effectively control fungus gnats.

Other insects and pathogens can often be avoided by sanitation techniques such as maintaining a clean floor free of weeds and debris, keeping a weed-free barrier around the greenhouse, disinfecting benches between crops, not reusing potting media and keeping hose nozzles off the floor.

Practicing the above sanitation techniques and best management practices for greenhouses can be a challenge but should not be overlooked due to the sensitivity of seedlings. Using good practices when establishing plants in a greenhouse can help to produce stronger, healthier transplants which will help give the crop a good start and prevent larger problems down the road.