Irrigation for a Small Greenhouse

Another rainy day project in the garden!

Several years ago, with the help of a friend, I installed a greenhouse in a corner of my garden. This is a typical backyard structure, just ten-by-twelve feet, not an expansive “plant factory” like those on the annual open greenhouse event hosted by local nurseries for mostly flowers.


My greenhouse, although small, has ample bench and shelf space for an ambitious program of propagating and maintaining plants. Plants will assume the primary role of growing, and a greenhouse can easily exclude deer and gophers, but operating a greenhouse still requires investments of the gardener’s time for planning, planting, monitoring and controlling climate, diseases and the smaller pests.

Best intentions aside, other priorities can leave the greenhouse unattended. When that has happened to me, my plants have tended to expire.

In addition, my greenhouse developed another problem. I had installed a sprinkling and misting system to irrigate the plants, using an inexpensive battery-operated timer. It worked fine until the battery died when the irrigation valves were open. The timer did not have the power to close the valves, so they ran for days, unobserved. This resulted in soggy plants, a huge water bill and attendance in a water conservation workshop.

I turned off the system in favor of watering by hand, but because the greenhouse is away from the house, it was easy for me to neglect the watering schedule. Except for some succulents, my plants did not do well under that plan. A clear need existed for a reliable automatic irrigation system.

Working with another gardening friend, we are installing such a system, with the longer-term objective of starting seeds, growing new plants to garden-ready size, and maintaining surplus plants for giving to other gardeners. (I could also grow plants for sale, but the small-scale nursery business would seem uneconomic.)

The irrigation system includes a professional grade controller, capable of scheduling four irrigation valves, and two valves plus capped connections for two future valves. Wires running the length of the greenhouse support flexible tubing over plants, which will be on tables or shelves (not on the ground). The tubing will have multiple emitters that will either spray or drip water on the plants. Spray is best for groups of seedlings or small plants while drip is more effective for larger plants.

Some gardening friends use their greenhouses to maintain collections of succulent plants that require warm, dry conditions, or orchids or other tropicals that require warm and moist conditions. Greenhouses in public gardens often focus on one or the other of these plant groups.

When planning a greenhouse for the home garden, a good first step would be to decide which plants are to be grown, and what climate those plants will need: cool/moderate/warm, or dry/tropical. To learn about these options, search the Internet for “types of greenhouses.”

My greenhouse is intended to moderate temperatures, year-round, and to control moisture levels through scheduled spray and drip irrigation. This growing environment will support a wide range of plants, excluding only those that require extreme conditions. Plants that require a winter chill, for example, grow poorly in the coastal Monterey Bay area, whether in a greenhouse or in the ground.

The home gardener can use a greenhouse to extend the growing season, protect against larger pests, and control the environment for either specialty plants or general gardening. Greenhouse management can be an engaging and satisfying form of gardening; it can also be more expensive and time-consuming than conventional gardening. Enter with eyes wide open.


Watering Roses in Summer

Q. Dear Mr. Karwin: I can’t find any guidance in my various gardening books on how much water one should give roses after they have stopped blooming (most of mine have), especially between now the beginning of the rainy season. Any suggestions? Many thanks.

August 2013

A. Roses should be watered even after blooming to keep them healthy and growing. This is important during hot summer weather, when the plants could be heat-stressed. Be sure to let them dry out between watering sessions, particularly for roses in containers.

Here is independent advice (unfortunately I lost track of the source):

Summer Watering Tips

Roses like infrequent, deep watering as opposed to watering a little bit every day. They prefer a good deep soak and then like to be dried out before receiving another deep watering.

How do you know if your roses need water in the first place? The leaves may droop and lack the suppleness they normally have.  (Don’t confuse this with the drooping that often occurs when temperatures exceed 90 degrees).

How will you know if you’ve watered too much? The foliage may feel spongy and may turn yellow. If watering from overhead, do so early enough in the day so the foliage has time to dry out before nightfall.  Spraying the leaves with water will often wash away any disease causing spores before they have an opportunity to take hold. So don’t hesitate to do this on a hot, dry day. Your roses will thank you for it!

Best wishes,

Irrigating with Soft Water

Q. I have a master gardener type question. In an effort to save water here in our Opal Cliff Drive home (we are part of Soquel Creek Water District, very hard water), we have to have a water softener which helps a lot. People keep on talking about using your water inside to water plants etc. I’ve always been told that soft water cannot be used for watering plants, lawns etc.

Also, we asked a water storage person who puts together rain catchment systems and were told you have to have a certain amount of space for the canisters, piping etc.. Our tiny tiny beach area yard has little space and can’t do that.

The other day in the San Jose Mercury/News the profiled a woman who is in the water business who says no space is needed and you can use soft water on your plants, that it is the best. I don’t want to kill my plants. I think everyone has different situations and so far we’ve been able to conserve our water usage by almost 25%, lost all we can do.

So you can see the concern.

March 2014

A. I asked Golden Love, a friend who installs gray water system, for an expert opinion. He wrote “When setting up gray water systems, we ask people to change their water softeners to citrus based model. Sodium used in the softeners is harmful to soil and plant life.”

Here is a link to his website, Love’s Gardens.