STARTING GARDEN PLANTS IN YOUR HOBBY GREENHOUSE

Is it a good idea?

Many people with hobby greenhouses also grow plants outside when weather conditions permit. You can get a head start on the outdoor growing season by starting outside plants in your greenhouse. Of course, it takes space in your greenhouse to do this, but you may find that the benefits may make it worthwhile for you. Following some of the procedures outlined here may help you better accomplish your goals and get a little earlier start.

Plants destined to be used outdoors are handled differently in the greenhouse than the same plant grown to maturity in the greenhouse. This is mainly because the needs of the plants to be moved outside are different from those of plants remaining in the greenhouse. The differences will be highlighted as we look at the procedures and objectives that follow.

How early can outside plants be started in the greenhouse?

There are a few factors to consider when developing your plan to grow plants that will ultimately be transplanted outdoors. First, the earlier plants are started in the greenhouse, the larger they will be when it is time to put them outside. Though this may seem to be a good idea on the surface, the larger the plant is at transplant time the less room there is for error. The older the plant, the less tolerant it is to the changes demanded of it by the transplanting process. In addition, the larger the plant, the more space it will require in the greenhouse and the more rooting medium it will need to keep it growing and developing properly. Consider whether you are able to dedicate the room in your greenhouse for growing transplants.

What kind of growing system should I use?

Although the production system used for starting the transplants should be compatible with what you are otherwise doing in the greenhouse, you must consider the needs of the plant and the environment it will be growing in when it is moved outside. The less the change in the plant root environment at transplant time, the less the need for plant adjustment after transplant. In other words, the less the plant roots are disturbed, the less the interruption in the growth process resulting from the transplanting.

Ideally, all plant roots should be inside the medium being used to grow the plants. Roots that are outside the medium, either in water or fertilizer solution, will not perform when they are eventually transplanted into the soil or other growing media. Roots develop according to the medium in which they are growing. If the medium around the root is changed upon transplanting, that root may not be able to function in the new medium. The roots do not adapt. Instead, the plant will replace the portion of the transplanted plant’s root system that cannot function in the new environment. While this is happening, other growth processes stop. This check in growth is often referred to as transplant shock.

How large an outdoor plant can I grow?

Plants larger than the typical bedding plant size can be grown in your greenhouse for setting out in the spring. To get a better jump on the growth and development of the outside plant, however, you must do things in the correct way. We already know that larger plants require more greenhouse space and more growing medium for their roots. With these two things and a little extra care at transplant time, you can have larger plants to move outside in the spring. Larger, older plants are less pliable and break more easily than their younger bedding-plant–size counterparts. As a result, extra care is needed when handling the plant at transplant time.

Commercial bedding plant growers do not grow larger plants for spring transplanting because the increased media and space requirements would raise their costs and reduce their overall production and financial return. On a small scale, however, these factors are small prices to pay if you want to get a real jump on the outside growing season.

What media should I use?

A soilless media mix is the best choice because there are many available. It is usually free of disease and insects. This is important, since young plants can be easily killed or damaged by soil-borne insects or diseases.

The starting medium should be placed into a tray or containers where each plant has its own growing space. The old way of filling a tray with medium and then broadcasting seed on it works, but does not deliver the best results. Giving each plant its own rooting volume will provide a quicker-growing plant and allow for a minimum of root disturbance upon transplanting; thus, the plant will suffer less of a setback at transplant time.

If more than a half-dozen plants are going to be started, it is good to plan on transplanting the young plants into an intermediate temporary growing phase in the greenhouse. You will seed more plants than you want and select the best plants to be transplanted into larger quarters in the greenhouse. It also allows you to occupy less of your greenhouse space with the outside starts for the first couple of weeks they are in the greenhouse.

How can I prepare the plants?

Upon transplant, plants go from a controlled, comfortable environment in the greenhouse to an environment with variable temperatures and windy conditions. They go from conditions that encourage and support rapid plant growth and development to conditions that are, at best, marginal for existence. To maximize the success of transplanted plants, the growth of the plant can be slowed prior to transplant in preparation for the stressful conditions it will experience.

This intentional growth slowdown should occur two to three weeks before the transplant time. To achieve this, one of the first things to do is to stop providing fertilizer to the plants. You want to have the plants use most of the nutrients present in the medium, then slow its growth because of the lack of some of the ingredients, such as nitrogen, that it needs. The gradual restriction of water availability will also slow the growth of the plant. Over a period of a week to ten days, let the plants get a little drier between waterings. This will slow plant growth, including cell elongation, in the plant.

A third step that can be taken to lessen shock before plants are transplanted is to lower their environmental temperature. Because you will probably have other plants in the greenhouse that you do not want to expose to the lowered temperature, you will need to use a method other than lowering the greenhouse heating thermostat. One way to accomplish this is to move the plants to a location outside the greenhouse for at least part of the day so that plants can begin to acclimate.

On warmer, sunny days the plants can be set outside for increasing lengths of time. They could be set in a cool sun porch if one is available. Make sure they get light during the day! The plants can also be moved to a cooler night location, such as a garage. The key is to not forget to put the plants back where they can get adequate light during the day.

How can I prepare the outside location?

The soil in gardens is slow to warm up in the spring. Cold temperatures at night slow the warming process, as does excess water in the form of early spring rains. Water warms much more slowly than soil does, so water-drenched soil will warm even more slowly. There are some things you can do to allow the soil to warm up more quickly.

Providing drainage in areas to be planted will remove excess water. That can include trenching for the removal of surface water, or it may include mounding the soil up where the plants will be planted so that it will dry more quickly and warm faster. This is more likely to be done where the soil has a greater clay content. The use of permanent underground drainage tile may be considered if the presence of excess water delays transplanting each spring.

The application of black plastic sheeting can accelerate the warming of the soil. Black plastic landscape sheeting can be purchased for fairly large areas. If you have a small area, you can use a few black garbage bags. Slit them down both sides and spread them out over the area to be warmed. Cover the edges of the plastic with soil and weigh down the middle in a couple of places with a stone or other heavy object. You need to prevent the wind from getting under the plastic in order to keep it in place. The black plastic attracts heat and traps it under the plastic where it warms the soil. Light does not get through, so weeds do not get a head start.

At transplant time you can leave the plastic over the soil for a while. Cut holes in the plastic where the plants will go. The newly transplanted plants will benefit from the additional heat in the soil. Once things warm up, however, the plastic should be removed or covered with mulch. If the plastic is not removed, provision must be made for water to reach the roots. This can be accomplished by poking holes in the plastic or running drip irrigation lines under the plastic before it is laid down. Wood chip or bark mulch is preferable because the material will eventually decompose and add organic matter to the soil. The mulch should be deep enough so that the mulch material does not dry out and blow off the plastic, leaving exposed plastic where excessive heat can build up in the soil during the summer.

What are the precautions at transplant time and thereafter?

Once the soil is warm enough and the temperatures outside are sufficiently warm most of the time, transplant time has arrived. Dig a hole a little bigger around and just barely deeper than the rooting medium around the plant roots. Set the plant into the hole, supporting the medium in the palm of your hand as you move the plant from container to planting hole. This reduces the chances of the plant and its roots being damaged while it is being moved.

Place soil, soilless mix, or a mixture of the two into the hole until it is half-full. Pour warm water into the hole. This reduces the chances that large air spaces will be left that might allow part of the root system of the plant to dehydrate. Continue to fill the hole, making sure to place a little soil or soil mixture over the medium in which the plant was grown. This will lessen the chances that the plant will dry out before it becomes established. Gently press the soil around the plant to ensure that any large air pockets are eliminated. A little more water can be added at this point, if it is needed. Many people leave a little depression around the plant that can be filled with water a few times following the transplant process to help the plant become established in its new home.

If the plant is large enough to be blown over by the wind, a support stake should be added after the plant has been transplanted. The stake should not go down through the media that was around the plant roots in the greenhouse. Drive the stake into the soil at an angle so that the underground part of the stake is a little ways from the plant and the top of the stake is close to the plant stem, about one-third of the way from the top of the plant. Loosely tie the plant stem to the stake with a twist-tie.

Be prepared to cover the newly transplanted plants at night when the temperatures get low. This is especially important to do if there is a risk of frost. The cover will hold some of the heat in and protect the plants from frost damage even when air temperatures are two or three degrees below freezing for up to a few hours. Uncover the plants after the air temperature gets above freezing but as early in the morning as possible.

Is this worth doing?

Whether or not you start your own outside plants in your greenhouse is your decision. Doing so will give you opportunities not available to those who don’t have their own greenhouse. By starting plants and transplanting them, you will get earlier blooms, enjoying garden colour before you’d normally see blooms on garden plants planted as bedding-size plants from commercial growers. If you are starting and transplanting vegetable garden plants, they are ready to harvest and enjoy earlier in the outdoor growing season than would otherwise be possible. It is yet another way to use your greenhouse and look forward to spring!