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Materials and methods

Data collection.

Following procedures consistent with published approaches to Reddit content analysis (Brett et al., 2019; Chew et al., 2019; Lea et al., 2019; Sharma et al., 2017; Sowles et al., 2018), four subreddits were selected to be sampled based on relevancy to the objectives of this study, subscriber number, and community age: r/hydro, r/Hydroponics, r/Urbanfarming, and r/Aerogarden (Table 1). The content of those subreddits was systematically crawled (i.e., methodically browsed) using a custom code developed with Java (Oracle Corp., Redwood Shores, CA). Data were then filtered to leave only relevant OPs and comments from each subreddit by searching for the terms “suggest,” “help,” “advice,” “recommend,” “search,” “how to,” “advise,” and “wrong,” their inflectional forms, and typical misspellings, which were previously noted as being commonly used in the selected subreddits. The dataset timeframe ranged from June 2012 to June 2019, which provided enough data points for a robust analysis from recent threads. Data were imported into Microsoft Excel (Microsoft Corp., Redmond, WA) for processing.

Table 1.

Subreddit description as of 19 Nov. 2019 of four online communities from which original posts and primary comments/questions related to hydroponic indoor gardening were extracted and analyzed.

Topic selection.

Based on three books for hydroponic plant production (Jones, 2005, 2014; Resh, 2015) and readily available syllabi from college-level hydroponic courses at the University of Florida, Iowa State University, and The Ohio State University, a list of nine distinctive topics relevant to hydroponic indoor gardening was developed (Table 2). In addition, a tenth topic identified as “others” was included for OPs that could not be easily categorized, typically containing questions that were not included in any other topic description, or compound questions that pertained to two or more topics. Afterward, a list of specific search words for each topic was created, assuring no overlap among topic descriptions.

Table 2.

Search words and/or concepts used to classify original posts related to hydroponic indoor gardening from the r/hydro, r/Hydroponics, r/UrbanFarming, and r/Aerogarden subreddit communities.^z

Data processing and content analysis.

All data were processed using RapidMiner Studio software (version 9.1; RapidMiner, Boston, MA) following content analysis methods described by Krippendorff (2018). The specific data preparation included seven steps: 1) tokenization (i.e., converting individual characters into aggregate sets of characters) with removal of short (<4) and long (>25) symbols; 2) removing a list of English stop words (i.e., commonly used words in the English language such as articles) obtained from Ranks NL (Amsterdam, The Netherlands); 3) removing stop words from a customized list containing words that frequently appeared within OPs but were either non-topic specific (e.g., help, advice) or unrelated to a particular topic (e.g., water, awesome, excited); 4) stemming (i.e., reducing inflected words to their word stems); 5) transforming words to lowercase; 6) transforming words with misspellings, variant spellings, and synonyms to a common term; and 7) filtering the most frequent and infrequent words (e.g., those encountered in over 70% and less than 1.5% of documents, accordingly). Finally, the main topics appearing in the pre-processed reviews were extracted using Latent Dirichlet Allocation methods as implemented in the RapidMiner package with optimized topic density/word density parameters.

After completing the topic modeling, the discovered topics were manually revised for accuracy and interpretation. About 20% of all OPs were found to be misclassified because they either did not belong to a preselected subreddit or belonged to “others” and were wrongfully classified into another topic. In addition, as the topic revision was completed, commonly used jargon terms [e.g., nutes = nutrients, res = reservoir, and cukes = cucumber (Cucumis sativus)] were included as topic descriptors. Finally, posts that were unrelated to hydroponic indoor gardening were removed, and the number of OPs with no responses were counted. This task was accomplished by reading all OPs and identifying the topic descriptors present within each OP. Once manual classification was completed, topic classification was validated by randomly selecting 10% of OPs and reviewing all data using interrater agreement (i.e., degree of agreement among different raters for topic classification) and intrarater reliability (i.e., consistency of classification by a single rater) assessment, resulting in coefficients of 0.88 and 0.97, respectively, which suggest that the topic descriptors used in the manual classification were highly accurate (Tinsley and Weiss, 1975). The resulting dataset included 1617 OPs and 4891 primary comments/responses to OPs. Comments for each of the 10 topics were manually rated for accuracy using the following scale: 1 = completely inaccurate; 2 = mostly inaccurate; 3 = 50% accurate; 4 = mostly accurate; and 5 = completely accurate. An expert panel consisting of a senior master student with over two years of experience with hydroponic indoor gardening, and a university professor with over 15 years of experience in controlled environment agriculture rated comments based on the accuracy scale. Additionally, where rating was not obvious, information provided in the comments was corroborated using various scientific resources to ensure accurate rating (e.g., Jones, 2005, 2014; Lopez and Runkle, 2017). Examples of OPs with corresponding classification, topic descriptor, response accuracy rating, and rating rationale are shown in Table 3. Lastly, to identify relevant points of discussion about the most common knowledge gaps, the top three topics based on post frequency were subcategorized and inductively analyzed (Table 4).

Table 3.

Examples of original posts and comments/responses related to hydroponic indoor gardening posted on the r/hydro, r/Hydroponics, r/UrbanFarming, and r/Aerogarden subreddit communities with a corresponding topic descriptor and classification, comment accuracy rating, and rating rationale. The accuracy rating was based on a scale where 1 = completely inaccurate; 2 = mostly inaccurate; 3 = 50% accurate; 4 = mostly accurate; and 5 = completely accurate.

Table 4.

Search words/concepts used to subcategorize original posts related to hydroponic indoor gardening from the r/hydro, r/Hydroponics, r/UrbanFarming, and r/Aerogarden subreddit communities pertaining to the three topics with the highest frequency of questions.

Data analyses.

Sampled data used for the identification of each topic and subcategories for the top three topics were quantified with Microsoft Excel and graphed using SigmaPlot (version 13.0; Systat Software, San Jose, CA). A one-way analysis of variance was conducted to test the hypothesis that the mean accuracy rating differed among topics. Group means were compared using Tukey’s honestly significant difference test at P < 0.05 with statistical analysis software (JMP version 12; SAS Institute, Cary, NC).

Results

OP classification.

Questions made about the topics of production systems (19%), plant lighting (17%), and root-zone environment (14%) accounted for 50% of the total OPs (Fig. 1). The other 50% of questions were distributed among all other seven topics, which included plant disorders (13%), cultural practices (10%), diseases (6%), plant material (4%), environmental conditions (3%), pests (2%), and “others” (12%). Except for the topic plant lighting, all topics had at least 10% of unanswered OPs (Table 5). The subcategories and corresponding number of posts for each of the three topics with the most OPs (in parentheses) were: 1) production systems, with setup (115), general design (104), and substrate (21); 2) plant lighting, with lamp type (85), lamp intensity (71), and lamp setup (63); and 3) root-zone environment, with nutrient solution formulation (118), water temperature (47), and pH balance (29) (Table 4, Fig. 2).

Table 5.

Summary statistics and characterization of response (resp.) accuracy to original posts (OPs) related to hydroponic indoor gardening from the r/hydro, r/Hydroponics, r/UrbanFarming, and r/Aerogarden subreddit communities. Response accuracy was determined based on an accuracy scale rating, where 1 = completely inaccurate; 2 = mostly inaccurate; 3 = 50% accurate; 4 = mostly accurate; and 5 = completely accurate.

Comment ratings.

There were no significant differences among the means of the accuracy rating in response to OPs across topics, which ranged from 2.3 to 3.5 (Table 5). The three topics with the highest percentage of completely accurate responses, regardless of sample size, were diseases (40%), root-zone environment (20%), and environmental conditions (17%). In contrast, the topics with the lowest percentage of completely accurate responses were pests (0%), plant disorders (3%), and others (4%). The three topics with the highest percentage of comments that were completely inaccurate were plant material (43%), plant disorders (23%), and others (22%). The topics with the lowest percentage for completely inaccurate responses were production system (4%), cultural practices (4%), and plant lighting (7%). In general, 14% to 50% of comments for OPs were rated as mostly inaccurate, 14% to 36% as 50% accurate, and 7% to 28% as mostly accurate.

Discussion

Based on our results, 50% of the most common knowledge gaps among indoor gardeners are related to production systems, plant lighting, and root-zone environment (Fig. 1). However, it is interesting to note that, regardless of the number of OPs, these three topics had the lowest percentages of unanswered questions, ranging from 5% to 14%. In addition, among the responses provided to OPs, accuracy of responses provided to questions related to the topic of production system were equally distributed in the scale from 2 to 4 (i.e., mostly inaccurate to mostly accurate) (Table 5). Similarly, over 25% of responses for questions related to plant lighting and root-zone environment were rated within 2 to 3 in the accuracy scale. This suggests that, although research-based information is available for indoor gardeners, misinformation is likely being disseminated in these subreddits.

Hobbyist gardeners struggle to discern between reliable or unreliable information available online (Jones, 2014), some of which typically lacks a scientific basis. Even if gardeners can identify reliable information, the terms and concepts used in research-based literature tend to be considered overly technical (Schmitz, 2004). For example, unstandardized units of measurement (e.g., a mix of U.S. customary units and metric units) and varying terminology (e.g., references to essential element, nutrient, and salts) found in the scientific literature can be confusing to nonhorticulturists, such as indoor gardeners (Jones, 2014). Therefore, although there is a vast amount of scientific information available related to hydroponic use for gardening purposes, the information may not be easy to interpret by people with no formal training in plant production.

In contrast to the low percentage of unanswered questions for the top three topics, the topics pests, diseases, and “others” had 32%, 25%, and 40% of unanswered questions, respectively (Fig. 1). As previously described, the topic “others” included complex questions that were difficult to classify based on the topic descriptors (Table 2). Respondents typically struggled with answering multiple questions within one OP, or questions that relied on visual aid for proper identification. Most responses in the topic “others” requested additional background information and/or high-resolution photographs, and if OPs were not updated, questions usually remained unanswered. The ability and willingness to respond to OPs by Reddit users seems to depend on question type and on whether adequate background information is provided in the OP. Thus, clear and concise questions within OPs should help improve response rate and accuracy.

The most common questions within the topic of production system were about setups (43%) and general design (39%) (Fig. 2). In addition, most OPs were related to do-it-yourself (DIY) systems built and designed by indoor gardeners (data not shown). In contrast, fewer OPs asked questions about commercial countertop systems. While indoor gardeners may rely on different system setups or designs, the cost difference between a DIY system built with raw materials vs. a prefabricated commercial system can be large, with DIY systems typically being less expensive. Our data indicate that some of the most common types of systems used by do-it-yourselfers (also known as “DIYers”) are deep-water culture (DWC), nutrient film technique (NFT), ebb-and-flood, and vertical planters; aeroponic systems are used less frequently (data not shown). Correspondingly, DWC, NFT, and ebb-and-flood systems are frequently recommended for hobbyist gardeners (Jones, 2014; Resh, 2015), and closed-loop hydroponics in particular offer advantages when there are space limitations, which is a general consideration among indoor gardeners (Solis-Toapanta et al., 2020). It is likely that the lack of preference for aeroponics can be attributed to the limited information available for these systems, as they have been commercially available for significantly less time than most other hydroponic systems (Gopinath et al., 2017).

Over 85% of OPs related to plant lighting were indicative of knowledge gaps related to lamp type, intensity needed for sole-source lighting, or lamp setup (Fig. 2). Although most lamp fixtures have been designed for human use, recent developments with light-emitting diode (LED) technology have enabled companies to design “horticultural-grade” lamps (Both et al., 2017). However, the widespread use of LED lamps has contributed to the significant confusion about plant responses to light quantity and quality. For example, claims about light sources that can “optimize” indoor plant production may be disseminating misleading information. That is because a general conclusion from plant lighting research with LEDs indicates that plant responses to the different light properties are highly dependent on many biological and environmental factors and thus, an “optimal” recipe for all crops and production purposes is unlikely to exist (Bugbee, 2016; Mitchell and Sheibani, 2020).

Many questions in the OPs were related to LED use, with fewer mentions of high-pressure sodium or fluorescent lamps (data not shown). In addition, one of the most recurring remarks within OPs was the search for a low-cost lamp that would provide enough light for sole-source lighting. A large portion of the OPs suggest there is considerable confusion about the minimum amount of light required to produce both leafy greens and fruiting crops indoors. Considering that most indoor gardeners want inexpensive lamps, off-the-shelf LED lamp fixtures could be a suitable sole-source lighting option, as they have been shown to provide sufficient light to maintain pick-and-eat leafy greens within indoor environments (Paz et al., 2019; Solis-Toapanta and Gómez, 2019). However, fruiting crops require more light than leafy greens, and thus, such crops may need a different light source capable of providing the minimum light threshold for production (Dorais et al., 2017). Nonetheless, to our knowledge, no research has evaluated light requirements for indoor gardening of fruiting crops; thus, specific recommendations are still unknown.

Additionally, indoor gardeners struggle to correctly interpret and make use of appropriate light units for plant production and typically rely on photometric units such as lumens, lux, and foot-candles (Both et al., 2017; Halleck, 2018). Nonetheless, peer-reviewed and outreach materials that explain the importance of measuring light used by plants (e.g., photosynthetic photon flux, daily light integral) are readily available to the general public (Runkle, 2015, 2018). It was interesting to note that correct terms for the topic of plant lighting, such as the ones given in the previous example, were used fewer than 10 times in the entire dataset (data not shown).

Questions in OPs about lamp setups primarily asked about the number of lamps needed to provide enough light, or about the most suitable spacing between plants and lamps. Although these questions belong to the subcategory of lamp setup, it is likely that the difficulty experienced by indoor gardeners originates from the existing misinformation about appropriate lamp types and intensities. As previously described, there is much confusion surrounding sole-source lighting, which often leads to mistakes that could result in unsuccessful indoor gardening experiences. Although there are many websites, outreach articles, and research-based information about plant lighting, the main questions identified in this study illustrate how indoor gardeners struggle to find and make use of consistent and accurate information about this topic.

Among the subcategories identified within the topic of root-zone environment, most OPs were related to nutrient solution formulation (56%), water temperature (23%), and pH balance (14%) (Fig. 2). Nutrient solution formulation is one of the most misunderstood aspects of growing plants hydroponically by indoor gardeners, considering that most literature is limited to providing nutrient solution recipes for crops commonly grown commercially (Jones, 2014; Solis-Toapanta et al., 2020). Few hobbyist gardeners understand the complex relationship between the composition, concentration, and rate at which plants use the elements present in nutrient solutions, particularly in closed-loop hydroponic systems. For example, scientists tend to recommend adjusting the concentration of nutrients within hydroponic nutrient solutions depending on the different plant growth stages (e.g., seedling, vegetative, or reproductive) (Bugbee, 2004). Nonetheless, hydroponic plants can be grown with an array of solution formulations such as those of Hoagland and Arnon (1950), Muckle (1993), and Yuste and Gostincar (1999), among others. Although there is substantial research-based information about nutrient solution management in hydroponics (Bugbee, 2004; Christie, 2014; Trejo-Téllez and Gómez-Merino, 2012), inexperienced indoor gardeners might find the task of formulating their own nutrient solution daunting, and they could benefit from pre-mixed commercial formulations for general use. Nonetheless, it is important to consider that using commercially available nutrient solutions may not be infallible, as additional factors such as solution temperature and water pH may affect the success rate of hydroponic production. Considering that most indoor gardeners prefer closed-loop hydroponic systems that require constant monitoring and handling, coupled with the presumption that they struggle with understanding nutrient solution formulations, it is likely that challenges with proper solution management contribute to unsuccessful indoor gardening experiences with hydroponics.

According to Jones (2014), when growing plants hydroponically, nutrient solution temperature should be maintained similar to ambient temperature to ensure optimal plant growth (assuming temperature ranges are not limiting). However, the most frequent advice provided in response to OPs encouraged Reddit users to cool down the nutrient solution, regardless of air temperature, an action that could affect overall plant health and productivity (Jones, 2014). Finally, although most OPs within the topic of root-zone environment mentioned pH, there seems to be a significant lack of understanding about the importance of pH management in hydroponic production. Most of the available literature that refers to optimal pH ranges originates from studies conducted to determine nutrient availability (Argo and Fisher, 2003; Jones, 2001; Dickson and Fisher, 2019). However, the effects of water quality on solution pH, electrical conductivity, and alkalinity is an important consideration for hydroponic indoor gardeners, particularly when periodically replenishing systems with tap water. Therefore, it is important for nutrient solution formulations to be adaptable to a wide array of water qualities, as they will directly influence the gardening experience.

A possible explanation to the high frequency of OPs asking questions about indoor plant production may be related to the limited accessibility to research-based information, because scientific findings relevant to hydroponics and controlled-environment plant production have generally been published in journals with a fee-based subscription. However, with the rapid changes in academic publishing and science communication, open access journals continue to become popular because they enable scientists to increase their reader base and communicate research-based findings with larger audiences (Osterrieder, 2013; Piwowar et al., 2018). According to Archambault et al. (2014), open access articles published in peer-reviewed journals will soon overcome paywalled literature. Therefore, to ensure that appropriate audiences (e.g., indoor gardeners and product suppliers) benefit from research findings relevant to indoor gardening, open access journals should be considered as the primary outlet for peer-reviewed publications in this area. In addition, close collaboration between researchers and extension professionals is critical to facilitate the delivery of outreach materials to lay audiences interested in indoor gardening.

In conclusion, the widespread use of social media can provide a quick and responsive network to gather and exchange information among science communicators and the general public. Data mining tools enabled the identification of common knowledge gaps among online communities discussing hydroponic indoor gardening. Results from this study indicate that most posts from four subreddits focused on hydroponic indoor gardening relate to confusions regarding the design and implementation of appropriate hydroponic production systems. In addition, misinformation about plant lighting seems to be a major part of discussions about growing plants indoors. There is also limited understanding about nutrient solution management, particularly about fertilizer formulation, pH balance, and on the impact that solution temperature has on plant growth and development. In general, there were no differences among response accuracy for all analyzed topics. However, regardless of topic, responses for most OPs had less than 50% accuracy, which demonstrates that misinformation can be disseminated in social media. Based on the frequency and type of questions posted on Reddit about hydroponic plant production indoors, there is an opportunity to address knowledge gaps with targeted open access research and outreach efforts that can help foster positive experiences by indoor gardeners.